How to Install a Hardwired Smoke Alarm – Part 2

By |Last updated on |Smoke Alarms|98 Comments

The hardwired smoke alarms were never connected to an AC power circuit. A new metal junction box is installed and the existing smoke alarms are wired to a continuous non-switched circuit for power. The new smoke alarms are then wired and installed. This project is continued from How to Install a Hardwired Smoke Alarm – Part 1.

How to Install a Hardwired Smoke Alarm

The smoke alarms in my house are 10 years old and will be replaced with new Kidde/Firex® Dual Sensor (Ionization and Photoelectric) 120VAC powered hardwired alarms with battery backup. I chose the Consumer Reports #1 rated Kidde PI2010 alarm that sells for about $30. See the PI2010 User Manual for detailed installation and operation instructions:

Install a Kidde/FIREX Smoke Alarm Model # PI2010

Install a Kidde/FIREX Smoke Alarm Model # PI2010

The alarm includes the 9 volt battery – a nice touch to get started.

Update: Kidde PI2010 and PI9010 Recall

The U.S. Consumer Product Safety Commission issued a recall notice on March 21, 2018 for units manufactured between 2016 Sep. 10 through 2017 Oct. 13. See Kidde Recalls Dual Sensor Smoke Alarms Due to Risk of Failure to Alert Consumers to a Fire and Product Safety Recalls at for detailed instructions to check if your unit is affected and obtaining a replacement.

Hardwired Smoke Alarm Wiring Diagram

In How to Install a Hardwired Smoke Alarm – Part 1 I described how the smoke alarm in the finished basement was disabled and improperly concealed in a ceiling crawl space in violation of the Building Codes. The scope of work for this project is to:

  1. Remove the old smoke detector and ceiling electrical box.
  2. Wire and install a new smoke detectors in both the finished and unfinished sections of the basement.

120VAC power for the smoke alarms will provided by a new branch circuit from an existing junction box on a continuous (i.e. non-switched) circuit. The new work is indicated by the green box in this wiring diagram:

Hardwired Smoke Detector Wiring Diagram with NM-B 14/3 cable

Hardwired Smoke Detector Wiring Diagram with NM-B 14/3 cable

The smoke detectors are hardwired as follows:

  • 120VAC power is provided by the hot (black) and neutral (white) wires.
  • Interconnect alarm signaling is carried by the red wire. If one alarm is triggered, it will signal the other alarms to sound.
  • The neutral (bare copper) wire is not used by the smoke alarms and no connection is required. However, I connect the ground wires between the different cable runs as a matter of personal preference. A continuous ground wire is your friend in safety.

If you’re not comfortable and experienced working with 120VAC wiring, I strongly recommend hiring a licensed electrician.

Smoke Alarm Placement

The National Fire Protection Association (NFPA) and the Kidde PI2010 Smoke Alarm Users Manual provide detailed guidance on where to install smoke alarms. In brief, smoke alarms should be installed:

  • On every level of the home, including the basement.
  • On the ceiling if possible, at least 4 inches away from the wall.
  • Outside of every bedroom on older homes to comply with the old rules.
  • New homes must have a smoke alarm in every bedroom and all smoke alarms must be interconnected.

Do not install smoke alarms:

  • Near windows, doors and heating/cooling vents because drafts can interfere with the detector.
  • In the attic.
  • In crawlspaces.

Smoke Alarm Installation Tools

The following tools were needed to wire and install the smoke alarms, moving clockwise from the top these are:

  • Drywall jab saw
  • Cordless drill/driver
  • Needle nose pliers with cutter
  • Wire stripper / cutter
  • Utility knife
  • Non-contact voltage detector
Hardwired Smoke Alarm Installation Tools

Hardwired Smoke Alarm Installation Tools

Smoke Alarm Wiring: NM-B 14/3 versus NM-B 14/2 Cable

Before I begin wiring the new smoke alarm, I’ll take a moment to explain the two types of electrical cable that I’ll be using.

NM-B 14/3 wire means Non-Metallic 14 gauge, 3 conductor plus ground. The ‘B‘ designates the application – for exposed or concealed work in normally dry locations such as inside the home. The conductors are:

  • Hot (black) or line side.
  • Neutral (white).
  • Red – used here for alarm interconnect signaling, so if one alarm goes off, they all sound.

plus the bare copper ground wire. The smoke detectors are ungrounded and the ground wire is not used.

NM-B 14/2 is a 14 gauge, 2 conductor plus ground wire. Two conductors are:

  • Hot (black) or line side.
  • Neutral (white).

I used NM-B 14/2 wire to run a new AC branch feeder circuit to power the alarms (see the junction box at the far right of the above wiring diagram).

Smoke Detector Wiring: NM-B 14-3 versus NM-B 14/2 Cable

Smoke Detector Wiring: NM-B 14-3 versus NM-B 14/2 Cable

In this project, I’ll use the terms “wire” and “cable” somewhat interchangeably. Strictly speaking, “cable” refers to the entire NM-B wire bundle with the outer insulation jack where “wire” refers to an individual conductor within the cable.

Unfinished Basement Smoke Alarm Installation

I began by removing the old smoke alarm and ceiling box from the crawlspace above the finished basement; recall that I determined after careful analysis there is no AC power on the alarm circuits. The new smoke alarm will be located nearby in the unfinished area of the basement where the gas furnace, gas water heater and my workshop are located. The new position is only about 5 feet away but in a much more accessible location. I’ll reuse the existing NM-B 14/3 cable that interconnects to the the other smoke alarms in the home:

Basement Smoke Alarm Installation: Relocate the Alarm Wiring

Basement Smoke Alarm Installation: Relocate the Alarm Wiring

Smoke Alarm Ceiling Junction Box Wiring

An octagon ceiling box is installed on the floor joist to wire in the new smoke alarm. The existing NM 14/3 alarm wire from the upper floors is brought in on the left and new cables are brought in on the right side: NM-B 14/2 for power and NM-B 14/3 for the second smoke alarm to be installed in the finished basement area. Remember the red wire in the NM-B 14/3 cable is needed for the smoke alarm signaling interconnection.

Note: The smoke alarm trim plate will not fit a 4″ square junction box, you must use an octagonal or rectangular ceiling box. I chose the deep octagon box because it can handle four cables without exceeding the box fill capacity per the National Electrical Code (NEC):

Smoke Alarm Octogonal Ceiling Box Wiring

Smoke Alarm Octogonal Ceiling Box Wiring

The above photo corresponds to these cables in the wiring diagram:

Smoke Detector Octogonal Wiring Diagram

Smoke Detector Octogonal Wiring Diagram

5/8 inch of insulation is stripped from the wires, then the wires are matched by color, right-twisted (clockwise) together and secured by a wire nut to the Kidde PI2010 AC Quick Connect Harness. A #10-32 green ground screw and ground wire pigtail is fastened to the junction box, then twisted and nutted with the other ground wires. Metal junction boxes must be grounded per NEC 250.148 (C).

Firex PI2010 Smoke Alarm: Junction Box Electrical Wiring

Firex PI2010 Smoke Alarm: Junction Box Electrical Wiring

The wires are tucked into the ceiling box with the AC Quick Connect hanging down as shown:

Octogonal Ceiling Box Smoke Alarm Wiring

Octogonal Ceiling Box Smoke Alarm Wiring

The AC Quick Connect pigtail is inserted through the trim plate, the trim plate fastened to the octagonal ceiling box with two screws (included with the ceiling box) and the AC Quick Connect plugged into the smoke alarm.

Kidde PI2010 Smoke Alarm Trim Plate and AC Quick Connect Installation

Kidde PI2010 Smoke Alarm Trim Plate and AC Quick Connect Installation

The smoke alarm is mounted to the trim plate by twisting on until it ratchets into place.

120VAC Branch Circuit Smoke Alarm Wiring

A new run of NM-B 14/2 cable is installed between the octagonal ceiling box and a convenient junction box on the wall about 10 feet away. The junction box is on a circuit that is connected at the main electrical panel. It’s extremely important the smoke alarms are on a continuous non-switched circuit and not to a light switch.

The new circuit for the non-switched (or full time) AC power is highlighted in the wiring diagram:

Smoke Detector AC Branch Circuit Wiring Diagram

Smoke Detector AC Branch Circuit Wiring Diagram

Electrical Safety: Be certain to shutoff the electricity at circuit breaker or fuse box. Confirm the circuit is dead with the voltage detector to prevent shock, burns, fire and/or death.

The new NM-B 14/2 cable run for the smoke alarm power feed is routed to an existing junction box and wired. Remember to secure your new cable with insulated cable staples within 1 foot of the junction box and every 4 or 5 feet along the floor joists or 2×4 wall studs:

Smoke Alarm AC Power Feed: Branch Circuit Wiring

Smoke Alarm AC Power Feed: Branch Circuit Wiring

If you don’t have a convenient junction box to splice into, you can splice into an existing circuit as explained in this project.

Jumping ahead for perspective, I’ve installed the 2nd smoke detector in the next photo. The green AC power indicator glows when the alarm is on 120VAC power. For now, the circuit breaker and AC power must remain shutoff.

Kidde PI2010 Smoke Alarm: Green LED AC Power Indicator

Kidde PI2010 Smoke Alarm: Green LED AC Power Indicator

Smoke Alarm on Unswitched Branch Circuit also Used for Power and Lighting

Update in response to a reader question about how to wire a smoke alarm on branch circuit with a light. The benefit is if the light isn’t working it indicates the smoke alarm also isn’t powered. Otherwise you may not notice the smoke alarm has lost AC power unless you stare at the little green alarm LED.

The NFPA 72®: National Fire Alarm and Signaling Code Handbook, 2013 Edition states in article 29.6.3(4) AC Primary Power Source:

“AC primary (main) power shall be supplied either from a dedicated branch circuit or the unswitched portion of a branch circuit also used for power and lighting.”

The following wiring diagram shows how to wire a light switch with interconnected alarms:

Interconnected Smoke Alarm on Unswitched Branch Circuit also Used for Power and Lighting

Interconnected Smoke Alarm on Unswitched Branch Circuit also Used for Power and Lighting

The circuit operates as follows:

  • A common feed from the circuit breaker (left side of diagram) powers the smoke alarm(s) and light switch branch circuit. This smoke alarm is on the unswitched portion of the circuit.
  • Two branch circuits leave the primary junction box: NM-B 14/3 cable for downstream interconnected alarms and a NM-B 14/2 cable for the light switch.
  • The toggle switch only controls the light.
  • The smoke alarm(s) are always powered because it’s on the unswitched portion of the circuit.
  • If the light isn’t working, it’s either a bad bulb or the circuit breaker has tripped. A tripped breaker means the alarms(s) aren’t powered.

For an “end of run” circuit with no other interconnected alarms, the NM-B 14/3 cable is simple omitted:

Non-Interconnected Smoke Alarm on Unswitched Branch Circuit also Used for Power and Lighting

Non-Interconnected Smoke Alarm on Unswitched Branch Circuit also Used for Power and Lighting

The branch circuit may serve an electrical outlet instead of a light switch. If the TV, radio, desk lamp, etc. isn’t working indicates the smoke alarms have lost power.

The second smoke detector is installed in a finished basement room in How to Install a Hardwired Smoke Alarm – Part 3.

Thanks for reading,

Bob Jackson

Copyright © 2018   Reproduction strictly prohibited.


  1. Siera January 17, 2013 at 7:30 pm - Reply

    The other morning my smoke detectors started going off and i could not get them to shut off. I cut the black wire in one of the alarms and now my bathroom lighting (which is on the same circuit breaker) is not working. what do i need to do to reconnect the wire and get my bathroom lights back working?

    • BobJackson January 18, 2013 at 8:44 am - Reply

      Uh oh! You’ve cut a 120 volt AC wire on the circuit that powers the smoke detector and bathroom lights. You’re lucky you weren’t shocked or worse! What you’ve got now is an exposed line-side live wire that is a shock and fire hazard. I strongly recommend that you call licensed electrician to repair the wiring. It shouldn’t be an expensive as I believe the electrician can repair the cut wires with a new pigtail connection in the ceiling box. Also ask the electrician to troubleshoot the original smoke detector problem.

    • Todd March 28, 2015 at 2:00 pm - Reply

      NUB here and on my phone so I’m not sure if I’m posting a new comment or replying to one.

      I’ve got 3 smoke detectors all installed and wired as described here. They all have the power indicator in and test independently, but testing one does not set the other two off.

      I think the problem is: when I hooked them to the new breaker in the panel, I ran 14/3 wire and hooked both the red and black to the hot terminals.

      Would this have fried the interconnectivity of the detectors? I have since disconnected the red in the panel and capped it with a wire nut and tape.


      • Bob Jackson March 29, 2015 at 12:02 pm - Reply

        Connecting the red signaling interconnect wire to the 120VAC hot wire probably did damage the smoke detector. The Kidde Intelligent Interconnect (I/O) System Technical Bulletin states the alarm signaling interconnect red wire carries 9 volts DC:

        “During a smoke alarm event, all interconnected models which are equipped with smoke sensing
        capabilities, will transmit or respond to a constant 9V DC (+/-2V) voltage on the interconnect line.
        (Interconnect signal return path and reference is the AC neutral)”

  2. Brian Krenicky February 19, 2013 at 12:31 pm - Reply

    Hey Bob

    I am remodeling a home that tennets have destroyed in Bangor, PA. It was built in the late 1800’s and I was contracted to restore it. Since it has wood lath and horse hair plaster which has been destroyed in so many rooms, I had to gut the entire home and start over. It even had all the piping for gas lighting for every room which was not operable.

    Someone has remodeled at some point but as usual, i found lots of code violations. I am surprised that thier was no fires. In any event, I am installing fire/smoke alarms and i was researching code requirements and came across your page. I understood already that because i had to gut the house it falls under the new constructions codes and not existiing structure codes.

    I first want to say your pages are awesome and very through. I love the schematics, pictures, and not to mentions your explanations are very detailed as well. Second, i just want to say thankyou for taking the time and putting together these pages. I am not sure if you got many comments on them but when i see good work i like to present credit where it is due.

    You have answered all of my questions and I want to say thank you. I guess that is all for now. Talk with you later.


    • BobJackson February 19, 2013 at 6:06 pm - Reply

      Thanks for sharing your story and the compliments. That’s an unusual restoration job! Reminds me of an older house my wife once wanted to buy. The house was in a great location on the bay, but I thought it was a money pit and would be washed away with the first hurricane.
      Good luck!

      • Daniel November 8, 2017 at 8:53 am - Reply

        Hello I have a question about installing these. Do you have to tie into a breaker box or can you tie into a outlet for power to kidde

        • Bob Jackson November 9, 2017 at 8:24 am - Reply

          The NFPA 72®: National Fire Alarm and Signaling Code Handbook, 2013 Edition states in article 29.6.3(4) AC Primary Power Source:

          “AC primary (main) power shall be supplied either from a dedicated branch circuit or the unswitched portion of a branch circuit also used for power and lighting.”

          Which in simpler terms means all smoke alarms are wired to a single, continuous (non-switched) power line, which is not protected by a ground fault interrupter (GFCI outlet protected branch circuit or breaker in the panel box).

          So to your question, you can either tie the smoke alarm branch circuit to the circuit that powers the wall receptacles or a dedicated branch circuit back to the circuit breaker panel box. The part in NFPA 29.6.3(4) about the “unswitched portion” of a branch circuit also used for lighting is to alert you that if the lights aren’t working the smoke alarms aren’t A/C powered, too.

  3. chris May 19, 2013 at 2:56 pm - Reply

    I am going through a similar experience. My 4 hardwired 2nd floor detectors have no power. I have one in the hall outside of three bedrooms. The hallway unit has three lines of 14/3 in the box. two bedrooms have two lines of 14/3 and one bedroom has one line of 14/3. There is no voltage what so ever. Did someone forget to install a hot line? Where would be the best place to get power to the Hallway unit.

    • BobJackson May 19, 2013 at 4:52 pm - Reply

      What I would so is:
      * Check the circuit breakers to see if any are tripped. If so, reset it and check for power at the smoke alarms.
      * Check for tripped GFCI wall outlets, the type with a Set/Reset button on the face. Reset the GFCI outlet and check for alarm power. Consider re-wiring the smoke alarm so it’s not on the GFCI outlet.
      * Turn on all light switches and check the smoke alarms for power. Someone may have incorrectly wired it to switch-controlled circuit.

      If still no power:
      * Trace the NM-B 14/3 cables in the attic or crawlspace. Look for damage to the cables – breaks, kinks, repairs, rodent bite marks, etc. – or an unconnected cable. Make necessary repairs depending on what you find, if anything.

      If you still can’t trace all the cables and find the problem, I would be very cautious about connecting the smoke alarm circuit to a convenient junction box for power because a hidden break or unconnected NM-B 14/3 cable could short and/or start a fire when energized. At this point it’s best to call a licensed electrician and ask him to trace the wires.

      Let me know what you find.


  4. chris May 19, 2013 at 5:34 pm - Reply

    thanks for the speedy reply. All circuits are in working order. All light switches and receptacles are working. The GFCI’s are not tripped either. I wish I could follow the wires in the attic but I have plywood covering the joists. I guess I have to call an electrician.
    I will let you know

    • BobJackson May 19, 2013 at 7:42 pm - Reply

      > I wish I could follow the wires in the attic but I have plywood covering the joists.
      It shouldn’t be difficult to pull up only those sections of plywood necessary to follow the wires with a hammer and flat nail bar. It’ll make the electrician’s job easier (i.e. less expensive) too. Use wood screws to reattach the plywood so future access will be easy.

      Good luck!

  5. chris May 21, 2013 at 1:09 pm - Reply

    well here is the deal. I traced all the wires coming out of the hallway box. I had to raise a few pieces of attic flooring to do so but it helped. As I said earlier all the smoke detectors had 14/3 in them. Hallway (box #1) had 3 lines (ABC), 2 bedrooms (boxes # 2&3) had 2 lines and one bedroom (box #4) had 1 line. So lines (ABC) out of the hallway, I discovered that line( A) went to bedroom( box #3) which had a line out to (Box #4) bedroom. So back to the hallway, line (B) went to bedroom (Box #2). So where did line (C) go…???? It went to the (Box #2) as well.
    Basically the hallway box had two lines of 14/3 going to one bedroom, one line going to another bedroom and then that bedroom tied over to the last bedroom with a single line. I disconnected and pulled out one of the two lines to the #2 box. I then ran some 14/2 from a hot source and now I have power to all the smoke detectors.

    • BobJackson May 21, 2013 at 8:42 pm - Reply

      I’m happy you found the wiring problem by pulling up the plywood attic floor and tracing the smoke alarm wiring.

      To summarize, what you found was a closed system with no power feed:

      [Hallway Box #1]—NM-B 14/3—[Bedroom Box #3]—NM-B 14/3—[Bedroom Box #4]
      || Two runs of NM-B 14/3 cable to Box #2
      [Bedroom Box #2]

      And fixed the problem by removing the redundant cable strand and wiring a new circuit for power:

      [Hallway Box #1]—NM-B 14/3—[Bedroom Box #3]—NM-B 14/3—[Bedroom Box #4]
      | Remove one redundant run of NM-B 14/3 cable to Box #2
      [Bedroom Box #2]
      | Install a new run of NM-B 14/2 cable to power the smoke alarms
      [Existing junction box with power]

      Nice work!

  6. chris May 22, 2013 at 8:28 am - Reply

    Thanks Bob,
    Glad you were able to understand my explanation of what I did.
    So now I do have just one more question. Being that Bedrooms 2 and 4 only have 1 line each going to them, there is the unshielded ground wire in each box that is not connected to anything. What to do with the loose ends?
    On the other end of the lines, the grounds are connected in Hallway 1 and Bedroom 3. The new 14/2 power line is properly grounded as well.
    I do appreciate you help

    • BobJackson May 22, 2013 at 7:02 pm - Reply

      If the smoke alarm junction boxes are metal, the boxes must be grounded by connecting the NM-B ground wire to a #10-32 green ground screw fastened to the box. Use a ground wire pigtail from the ground screw and nut it to the NM-B ground wire as shown here.

      If your smoke alarm ceiling boxes are plastic, just fold the ground wire for an “end node” into the junction box with no connection as shown in this photo. Some plastic junction boxes may have a metal ground wire lug as a convenience for connecting several ground wires, for example the NM-B and ceiling fan ground wires.

      The reason metal junction boxes must be grounded is for shock protection in case a hot or neutral wire comes loose or is damaged and contacts the metal box. This would make an ungrounded metal junction box “hot” (energized with electricity) and a shock hazard if touched (your body becomes the electricity ground pathway!), whereas a properly grounded metal box will short the current to the ground wire and trip the circuit breaker to let you know something is wrong.

      Plastic junction boxes are non-conductive and therefore not grounded.

  7. Jonathan May 24, 2013 at 8:46 pm - Reply

    Bob – first of all, kudos again on this page. The wiring diagrams were especially helpful.

    That written, I have one question for you. In Box 2 (Smoke Alarm in Unfinished Area) you spliced the wires, with one run ending in the basement and another run ending in the upper floors. From what I have read, you are “supposed” to wire these alarms in series, going from box to box, with only one end run – the last box on the line.

    Electrically speaking, if the alarm trips, current will flow through the traveler (red) wire to all of the boxes in your diagram. Therein lies my confusion – is there a reason why these installation guides only mention series runs and seem to discourage parallel runs?


    • BobJackson May 25, 2013 at 9:57 am - Reply

      Hi Jonathan,
      The “daisy chain” wiring diagram illustrated in the Kidde/Firex smoke alarm datasheet and user manual are representative for ease of understanding. The wiring requirements stated by the manufacturer are:
      * A maximum of 24 Kidde devices can be interconnected in a multiple station arrangement.
      * The interconnect system must not exceed the NFPA (National Fire Protection Association)
      limit of 18 initiation devices, of which 12 can be smoke alarms.
      * Make certain all alarms are wired to a single, continuous (non-switched) power line,
      which is not protected by a ground fault interrupter.
      * A maximum of 1000 ft. of wire can be used in the interconnect system.
      * Use standard UL listed household wire (18 gauge or larger as required by local codes).

      The U.S. Consumer Safety Product Commission ( Directorate FOR Engineering Sciences published CONSIDERATIONS FOR INSTALLATION OF SMOKE ALARMS ON RESIDENTIAL BRANCH CIRCUITS which illustrates several wiring configurations. In particular, Figure 16 on page 19 in section 4.1 Dedicated Circuits and Smoke Alarms illustrates a splitting the smoke alarm wiring in a junction box T configuration at the 2nd floor.

      The junction box wiring configuration as I’ve used meets the wiring requirements, including:
      1. Interconnected alarms wired in parallel (pigtail connection to the alarms). The failure of a smoke alarm unit will not interfere with the remaining alarms.
      2. Single, continuous (non-switched) power line connection.

      The critical result is when I press the Test button on any of the 7 interconnected smoke alarms in my home (I tested every device); all the alarms sound in unison, but slightly out of phase like an echo as the other alarms respond to the trigger signal. What a racket! The dogs bark, wife and kids yell “Hey! What are you doing?!!”. I smile knowing that the system works correctly.

  8. Jonathan May 25, 2013 at 7:28 pm - Reply

    Thanks, Bob, for the detailed explanation and the links. I am sure my wife and kids will have the same reaction when I test our system.

  9. Jonathan May 28, 2013 at 8:14 pm - Reply

    Hey, Bob, quick update on what has happened. I wired my house in a similar manner to your wiring diagram: service entrance to a basement detector to first floor detector and basement to 2nd floor detectors. Thus, there was a parallel run, which was spliced in the basement, near the service entrance. The county inspector came, looked around, and gave me my close-in permit. Obviously, code enforcement is different everywhere, but this work passed. I fall under a modified NEC 2008 – thus an AFCI breaker for the run – and a modified NFPA 72. Thanks again for the advice. Jonathan.

    • BobJackson May 28, 2013 at 8:52 pm - Reply

      > Thus, there was a parallel run, which was spliced in the basement, near the service entrance.
      The key requirement being the smoke detectors are all on the same circuit (via the basement junction box splice) for alarm signal continuity.

      > I fall under a modified NEC 2008 – thus an AFCI breaker for the run – and a modified NFPA 72.
      My home was built in 2002 prior to the updated NEC 2008 requirement for AFCI breakers for bedrooms, etc. and I’ve been replacing the older breakers in my panel with AFCIs. If you type “AFCI” in the search box at the top right page, you’ll find that I’m a believer in AFCIs.

  10. Wayne August 31, 2013 at 2:15 pm - Reply

    Bob I have a very similar scenario as you depicted above. My home is a 1952 home in Washington DC that has no smoke alarms. I purchased 5 of the FireX PI2010 and will be installing them on 3 floors running them in line directly to a 15amp dedicated circuit/breaker.

    I am under the impression that the 18 gauge wire is preferable for installation of fire alarms. Have you heard mention of this and could you give some insight as to the rational behind the use of 18 gauge wire for smoke alarm installation.

    Any information you have would be of value. Thanks in advance.


    • BobJackson September 3, 2013 at 6:10 pm - Reply

      18 gauge wire is too small for use on a 15 AMP 120VAC circuit and would be:
      * A fire hazard!
      * Violate the National Electrical Code (NEC) and your local Building Code.

      The Kidde/Firex® PI2010 User Manual states:
      “The Make certain alarms are wired to a continuous (non-switched)
      power line. NOTE: Use standard UL listed household wire (as
      required by local codes)
      available at all electrical supply stores
      and most hardware stores.”

      The problem with 18 gauge wire is the ampacity, meaning “The current in amperes a conductor can carry continuously under the conditions of use without exceeding its temperature rating” is not rated for a 15 AMP circuit. Imagine the 18 gauge wire heating until it glows like toaster coils while melting and setting the house on fire.

      You should use a minimum of NM-B 14/2 (14 gauge) or optionally NM-B 12/2 (12 gauge) wire to comply with the NEC/Building Code requirements for a 15 AMP 120VAC circuit.

      While the PI2010 smoke alarm only draws 80mA (equal to 0.08 Amps) and the maximum of 24 interconnected alarms would only draw 1.92 Amps, so it would appear to work on 18 gauge wire, that not the point. The point is it violates the National Electrical Code (NEC)/Building Code because:
      * 18 gauge wire is too small for a 15 Amp circuit (ampacity rating).
      * the overcurrent protection provided by the 15 Amp circuit breaker would not protect the 18 gauge wire from overloading.

      250 feet of NM-B 14/2 wire costs about $50 at Home Depot as of this writing, which is a small price to pay for living safely.

      Also, if you were to sell the home, the home inspector would likely notice the odd looking 18 gauge wires connected to the circuit breaker panel and flag it as a “must correct” by a licensed electrician.

      • KArime March 16, 2018 at 11:34 am - Reply

        I’m very confused by this. the wiring harnesses for my alarms (BRK) are 18 gauge, and the installation sheet states that the wires must be 18 gauge or larger, so how would using 14 or 12 between alarms do anything to prevent said glowing “like toaster coils” at the wiring harnesses? I understand code says that 14 gauge must be used on a 15 amp circuit. Is there some kind of a reducer that could be installed at the first alarm? Why don’t the alarms state that it’s needed, as I understand code doesn’t even allow for a breaker smaller than 15 amps? Maybe there’s some point here I completely don’t understand, but I would love to know what I’m missing.

        • Bob Jackson March 16, 2018 at 7:04 pm - Reply

          The reasons the 18 gauge smoke alarm wires don’t glow like toaster coils are:

          1 – It must be wired in parallel or at the end-of-run with the NM-B 14/3 or 12/3 house wiring. See Figure 4 “Interconnect Wiring Diagram” in the User’s Guide. When wired in parallel it’s siphoning AC power from the house wiring instead of passing the full load through the device.

          For example, compare the electrical outlet parallel wiring diagram with the series diagram.

          2 – The alarm draws very little AC current (amperes or AMPs). The current is low enough that it doesn’t exceed the ampacity of the 18 gauge wire.

  11. Steve Hnath January 13, 2014 at 11:34 am - Reply

    I replaced an old hardwired smoke detectors since the clips broke off to hold in in place. But when the electric was turned back on, all the other smoke detectors on the line are going off. I replaced the old one and everything works.

    • Bob Jackson January 13, 2014 at 6:09 pm - Reply

      I’m not quite following your description. The smoke detector plastic mounting clips broke and you replaced it. When you turned on the electricity, all smoke detectors started beeping. Was the replacement unit defective?

  12. cw April 15, 2014 at 7:32 pm - Reply

    If a smoke detector is still working but the mounting clips are broken, does it need to be replaced or is there some other way it can be clipped togeher?

    • Bob Jackson April 15, 2014 at 8:20 pm - Reply

      You could try reattaching the smoke alarm to the trim ring with three small 1/4 inch squares of double-sided sticky tape. It’s a tradeoff between enough tape to hold the smoke alarm in place but not so much tape that it can’t be pulled off to replace the batteries or install a new unit some years down the road.

  13. Todd April 23, 2014 at 8:25 pm - Reply

    Bob…I have a similar problem as CW. I have attempted to replace two of several smoke detectors in my house with Kidde Smoke/CO detectors. In my house, it is a hardwired, interconnected, non-switched system with NM-B 14/3 wires. When I reattached the new ones to power, all detectors in the house go into alarm state; not chirping. I have tried the units separately, but to no avail. I have returned them to the store and got two new ones. Same problem. If the two units are left out, the other detectors seem to work, but I would like to get these two locations back into service ASAP.

    I cannot find any information of a similar problem. Has anyone heard of such an issue?

    • Bob Jackson April 24, 2014 at 3:07 pm - Reply

      Hi Todd,
      The new Kidde Smoke/CO detectors are probably incompatible with the existing smoke detectors in your home.

      You didn’t indicate the model # of the new Kidde detectors, but the Kidde PI2010 User Guide states:

      Kidde alarms and accessoriesCAN ONLY BE interconnected with other Kidde alarms and accessories
      as well as specified brands and models of interconnect compatible
      alarms. Connection of Kidde products to a non-specified manufacturer’s
      interconnect system, or connection with non-specified equipment
      from another manufacturer into an existing Kidde system could result
      in nuisance alarming, failure to alarm, or damage to one or all of the
      devices in the interconnect system. Refer to the User’s Guide supplied
      with each Kidde product for interconnect compatible models, brands,
      and devices.


  14. Joan McDermott August 15, 2014 at 4:50 am - Reply

    Less than 40 days ago we had 8 brand new Kiddie smoke alarms installed by a professional. This was done because the alarms went off after they were serviced by the same professional, cleaning, new batteries, wiring checked, etc.
    Tonight, the alarms again went off, not all eight at first but one in the MBR, A very short while later they all went off.

    Do we have a major problem or what? Thank you,

    • Bob Jackson August 15, 2014 at 3:49 pm - Reply

      If the alarms are correctly installed and wired, then it may be a environmental factor triggering the false alarms.

      Check the items in the User Manual for external causes. See Section 2. LOCATIONS TO AVOID and 5. NUISANCE ALARMS.

      > not all eight at first but one in the MBR
      Is the alarm in the master bedroom within 3 feet of the bathroom door (steam/humid air), too close to a heating/AC vent or a fluorescent light (electrical noise)?

  15. Andy Vitch August 21, 2014 at 8:16 am - Reply

    I too have had issues with nuisance alarms in the kidde or garrison systems. I have suspected in the past that spiders or dust has triggered the alarm from locations in crawl spaces. I have a difficult one today. A series of 7 fired last night. I suspect, not 100% confident, the primary alarm going off is in a cathedral ceiling hallway. I am considering moving to a location out of the hallway where potentially there will be more air flow. Does this make sense to you? Do you have any other ideas? There was no shower/bath moisture issue as it was 3am but it was raining slightly outside and windows were open in the room.

    • Bob Jackson August 24, 2014 at 10:03 am - Reply

      > There was no shower/bath moisture issue as it was 3am
      > but it was raining slightly outside and windows were open in the room.
      That could have been the problem due to condensation. The Kidde Knowledgebase states:

      “Your smoke alarm may sound when its very cold outside, or if a door adjacent to a heated area is opened, like in an entryway. This is due to condensation (water vapor) in the detection chamber. The sensor is a particle sensing device, when water condenses in the sensor the unit will go into alarm.”

      The FireX Alarm FAQ Answer for “What causes my FireX alarm to false alarm (alarm sounds when no smoke is present)?” lists several possible cause for nuisance or false alarms:
      * Age if the unit is over 8 to 10 years old
      * Location – near steamy areas (bathrooms/kitchens), in direct airflow near heating/cooling vents, dusty areas
      * Dust or insects in the sensing chamber
      * Cleaning solvents – especially ammonia

      Electrical interference from fluorescent lights is another cause.

      Also see: Preventing False Alarms by the City of Surrey.


  16. Wire Nut September 30, 2014 at 7:00 pm - Reply

    A smoke alarm system should NEVER share a circuit with receptacles or lights. What if another yuck with no knowledge electrical safety disconnects the circuit for a reno or whatever? No protection. And pulling up plywood in the attic? What, did you get a deal on electrical work by the carpenter? Junction boxes are never allowed to be concealed, they have to be accessible. Most states the electrician has to be licensed and insured. We take code updates to renew licenses. And we have a responsibility to protect life and property. But if you want to put your safety in the hands of Joe Blow the carpenter or yourself who has to ask how to wire smoke alarms go for it.

    • Bob Jackson September 30, 2014 at 8:34 pm - Reply

      Hi Wire Nut,
      I don’t understand the reason for your rant.

      Are you replying to the thread about Chris’ problem where he pulled up the attic floor plywood trace the wiring and locate the missing connection?

      > A smoke alarm system should NEVER share a circuit with receptacles or lights.
      Yes, I stated in the project:

      “In How to Install a Hardwired Smoke Alarm – Part 1 I described how the smoke alarm in the finished basement was disabled and improperly concealed in a ceiling crawl space in violation of the Building Codes.

      120VAC power for the smoke alarms will provided by a new branch circuit from an existing junction box on a continuous (i.e. non-switched) circuit (emphasis added in this reply). The new work is indicated by the green box in this wiring diagram

      Nor are any of the junction boxes concealed as shown in the project.

      Wire Nut wrote:
      > A smoke alarm system should NEVER share a circuit with receptacles or
      > lights. What if another yuck with no knowledge electrical safety disconnects
      > the circuit for a reno or whatever? No protection.
      You’re talking about several different issues. A smoke alarm must be connected to a non-switched circuit. Lights are controlled by a switch and agree with you. However, the statement about “receptacles” is just plain wrong.

      The NFPA 72®: National Fire Alarm and Signaling Code Handbook, 2013 Edition states in article 29.6.3(4) AC Primary Power Source:

      “AC primary (main) power shall be supplied either from a dedicated branch circuit or the unswitched portion of a branch circuit also used for power and lighting.”

      Take care not to confuse Fire Alarm requirements as commonly found in commercial buildings with interconnected smoke alarms in single family residential dwellings.

      Recall the National Electrical Code (NEC) is part of the National Fire Codes published by the NFPA.

      There is the possibility the local Authority Having Jurisdiction (AHJ) may have amendments requiring a residential smoke alarm be connected to a dedicated circuit. If you know of any, please cite references.

      The major change for residential smoke alarm wiring requirements is the 2008 NEC which requires it to be protected by an AFCI circuit breaker if it’s in a bedroom.


  17. Cole October 30, 2014 at 9:03 pm - Reply

    This might sound like a silly question: is the fuse switch associated with connected smoke alarms the same one for ceiling lights? Thank you.

  18. brian October 31, 2014 at 11:29 am - Reply


    Your explanation to Wire Nut regarding the power source for Residential fire alarm smoke detectors seems to make a lot of sense.

    “AC primary (main) power shall be supplied either from a dedicated branch circuit or the unswitched portion of a branch circuit also used for power and lighting.” Although I prefer the latter of your statement

    In a nutshell if I have my smoke detectors connected to one of my bedroom circuits. I would know that power to my fire detection system is on (energized). Namely because the lighting and receptacles in that bedroom are working (energized).

    • Bob Jackson October 31, 2014 at 11:50 am - Reply

      Hi Brian,
      I think Wire Nut (pseudonym) is an electrician by trade, which worries me because he’s clearly not up to speed the National Electrical Code (NEC).

      That quote you cited straight out of the NEC 2013 Handbook and it’s good advice.


  19. Jonathan November 17, 2014 at 9:39 pm - Reply

    In your images above you have an unfinished basement ceiling. If i were to install pine on my ceiling, can i cover the metal box and drill a hole through the pine to access the wires, then place the plastic plate over the wood?

  20. Waseem December 9, 2014 at 2:48 pm - Reply

    Hi Bob,
    I am installing a brand new run of interconnected smoke detectors and wanted to know if it is required to have the smoke detectors powered by the same line a light would be powered by, before the light switch? reason for this is if the circuit ever stopped working, you would detect it faster because your light would not be working, so it would work as an indicator. does this make sense?


    • Bob Jackson December 9, 2014 at 5:27 pm - Reply

      The NFPA 72®: National Fire Alarm and Signaling Code Handbook, 2013 Edition states in article 29.6.3(4) AC Primary Power Source:

      “AC primary (main) power shall be supplied either from a dedicated branch circuit or the unswitched portion of a branch circuit also used for power and lighting.”

      Your reasoning the nonworking lights would be an indicator the smoke alarm AC power circuit is also dead is discussed in the NFPA 72 handbook article at the above link.


  21. Dora Jones January 25, 2015 at 9:36 am - Reply

    Hi Bob,

    My house was built in 2004. I moved out 4 years ago and have had one tenant. I’m getting it ready for the next tenant. Problem: the smoke detectors beep intermittently, the lights are green, but one of them I could not get it to work. Also the house security alarm keeps chirping. Will all of these chirps not stop until I replace the smoke alarm that’s not working. This happened several years ago and one day when I returned home everything was alarming!!! I cannot remember how I stopped it. My smoke alarms are by USI and my alarm system is by Spectra. Can you help me?

    Thank you.

    • Bob Jackson January 25, 2015 at 10:46 am - Reply

      For a house built in 2004 and it’s now 2015, the smoke detectors are at least 10 years old, out of warranty and should be replaced. You can find troubleshooting information at the Universal Security Instruments website.

      It’s possible the smoke detectors are interconnected with the home alarm system, which is a best practice. You’ll need to consult with a home security alarm maintenance technician to know for certain and verify the replacement smoke detector signaling is compatible.


  22. Amanda June 27, 2015 at 5:22 pm - Reply

    I have firex brand smoke alarms and the new ones have red, black and white. My old ones have yellow, black and white. Does the red and yellow replace each other?

    • Bob Jackson June 28, 2015 at 10:01 am - Reply

      That’s correct. The quick connect wiring harness may have a red or yellow wire for inter-alarm signaling and are interchangeable.

      • Jerry April 11, 2016 at 1:17 pm - Reply

        thanks — that saved me some anxiety!

  23. Christine July 7, 2015 at 7:48 am - Reply

    It’s 4am, and all of my alarms just went off. Wide awake after getting my 5 year old back to sleep. This is probably the 3rd time in the past year this has happened—and why I’ve been googling this. Great site.

    I’m going crazy wondering why this always happens in the MIDDLE OF THE NIGHT??? but after reading this I’m wondering if it’s not a bug or something crawling into one of these?

    My original thought was that there was some bad wiring that was causing it to trigger when a battery was low–and the alarm would go off instead of chirp. Is that possible?

    The bug is making more sense to me. The poor thing was probably terrified when the alarm went off. Just like the rest of us.

    The best thing I learned here is that in the future I can go right to the fuse box and turn them all off with one switch. thank you!

    • Bob Jackson July 7, 2015 at 7:44 pm - Reply

      How old are your smoke alarms? The Firex Replace and Upgrade your Smoke Alarms Every 10 Years FAQ states:

      “Aging smoke alarms do not operate efficiently and often are the source for nuisance alarms. Older smoke alarms are estimated to have a 30% probability of failure within the first 10 years. Newer smoke alarms do better, but should be replaced after 10 years.”

      The smoke alarm manufacture date should be printed on the back of the unit.

      > The best thing I learned here is that in the future I can
      > go right to the fuse box and turn them all off with one switch.
      False alarms are annoying but only pull the fuse after checking every location with a smoke alarm for fire/smoke. Next time it could be the real thing!

  24. Chris August 27, 2015 at 8:43 am - Reply

    Just purchased Kiddie Alarm, my old old smoke detector has white, black, yellow, and green wires coming out of it. There is also a red wire inside the metal electrical box. Black to black, white to white, and then what?

    It is interconnected with other smoke detector on 3rd level.

    • Bob Jackson August 27, 2015 at 8:45 pm - Reply

      The electrical box red wire should be the smoke alarm interconnect signaling wire. If one alarm is triggered, it sends a 9 volt DC signal to sound all other interconnected alarms.

      First verify there is no voltage on the electrical box red wire with a voltage tester. If the voltage tester indicates 110/120 volts AC then the electrical box is wired incorrectly and do not connect the red wire to the smoke detector because it’ll damage the smoke detector. Just cap the red wire with a wire nut for a no connection.

      If AC voltage is not present on red wire then you can assume the red wire is the alarm interconnect signaling wire. Connect the red wire to the yellow smoke alarm wire.

      Take care that mixing different brands of smoke alarms can cause erratic behavior/false alarms if interconnected. See this comment about smoke alarm compatibility.

      Lastly, the green wire is the ground wire. Connect it to the other ground wires if present as shown in the project.

  25. Anna September 1, 2015 at 4:21 pm - Reply

    My smoke detector is a Kidde, and i recently had it go off in the middle of the night (who’s surprised there…) and I took it down to change the batteries.

    However, in my late-night angst i accidentally detached a red wire from the ceiling, and it’s hanging down now. It has a little cap on the end, like seen in the picture at the top (the part labeled AC Quick Connect Harness).

    Is this red wire supposed to reattach in the ceiling? Or does it not plug into anything?

    What should my course of action be?


    • Bob Jackson September 1, 2015 at 5:10 pm - Reply

      The capped red wire of the AC Quick Connect Harness is signal wire for interconnecting with other smoke alarms. Since it’s still capped (was not connected) you can just push it back into the ceiling.

  26. Cody September 18, 2015 at 3:34 pm - Reply

    hoping someone can help me here. im helping remodel a 1991 built “modular” and someone else unhooked the only two smoke detectors in the house. im not familiar with the wire type in this modular. my problem is this…. the old wires and receptacles are the kind that the wire, 14-2 im assuming, doesn’t get cut and wired nutted together or pigtailed or anything. instead it’s like a crimp version of an receptacle. meaning you just crimp the receptacle to the continuous wire thats feeding it without cutting them. my problem is i cant tell which wire is which. Could someone please humor me and help me out with this please? Thanks in advance anyone!!

    • Bob Jackson September 18, 2015 at 5:39 pm - Reply

      Manufactured and mobile homes often have “self-contained wiring devices” – including receptacles and wall switches. The wires are pressed/clamped-down on the self-contained receptacle contacts (blades) which pierces the wire insulation to make the electrical connections; the wires themselves are not cut and pass through the receptacle. This is a very different wiring system compared to conventional home wiring.
      > my problem is i cant tell which wire is which
      The black wire should be hot, white neutral and bare copper is ground. You may need to peek under the NM-B cable outer insulation jacket to see the color of the insulation on the individual wires.

      You can also use a voltage tester to identify the hot and neutral wires:
      * Refer this receptacle diagram.
      * With the receptacle in the wall and ready for use…
      * Stick one test probe in the “hot” plug slot per the diagram at the above link, and the other test probe in the ground slot. The tester should indicate 110 volts.
      * Now move the test probe from the hot to neutral plug slot while keeping the 2nd probe in the ground slot. The tester should read 0 volts.

      If both tests display 110 volts then the receptacle is wired incorrectly and it’s best to call an electrician!

      You now know which side of the receptacle is hot and neutral. The corresponding hot and neutral wires will be on the same sides on back of the receptacle.

  27. steven September 27, 2015 at 1:32 pm - Reply

    I bought a KIDDE FIREX model i4618A as a replacement for the smoke detector in my hallway. Its connector doens’t match the existing mating connector on the wall. Also, the make and model of the smoke detectors in the other bedrooms that are all interconnected are not KIDDE FIREX.

    Will the KIDDE FIREX smoke detector work ok with the other (bedroom) smoke detector?

    • Bob Jackson September 28, 2015 at 9:52 pm - Reply

      Most likely the new Kidde smoke detector won’t be compatible with the existing non-Kidde units in the home. See this comment for more information.

      Your choices are to buy the same smoke detector as the existing units – or – change out all units to the Kidde model. Check the manufacture date on the back of the other units in the home, it’s best to replace them if they’re over 10 years old.

  28. Gabe October 9, 2015 at 11:49 pm - Reply

    I have a new construction home in WA (may 2011), and the Smoke Detectors have some how lost power causing them to run on Back-up battery only. There is no voltage between Black and White wires, Black and Red, White and Red. I have tested all of the Detectors to make sure they work, and during tests one will send off all 4. Any ideas?

    • Bob Jackson October 10, 2015 at 11:22 am - Reply

      At least you know the NM-B 14/3 red wire for the alarm interconnect signaling is wired correctly between all four units because if one unit is triggered, the other three units are activated too. Since the red wire is part of the same NM-B 14/3 cable containing the black, white and ground wires then these are most likely properly wired between the smoke alarms, too. Therefore the problem is finding the “line side” section of NM-B cable that feeds back to the circuit breaker panel that should be supplying 110/120 VAC to power the circuit.

      Troubleshooting steps:
      * Check the circuit breaker panel for a tripped or Off breaker.
      * Turn On each light switch and check the alarms for AC power. The alarms may have been incorrectly wired on a switch-controlled circuit.
      * Follow the wires in the attic checking for damage, junction boxes with loose or no connections, etc. You may discover the alarm branch circuit is a dead-end that was never connected to the house power.

      If you still can’t find the problem, a continuity tester/tracer is really helpful for identifying wires. Shutoff the electricity at the circuit breaker panel before using the tester:

  29. Duane January 4, 2016 at 4:31 pm - Reply

    I am looking for a ceiling light (normal light) with a built in smoke detector. Not having any luck, can anyone help? Can only find them with strobe lights.

    • Bob Jackson January 4, 2016 at 8:00 pm - Reply

      You won’t find a ceiling light with built-in smoke alarm because the alarm must be wired to a non-switch circuit, otherwise when you turned off the light it would also cut power to the alarm.

      As stated in the project:

      120VAC Branch Circuit Smoke Alarm Wiring

      A new run of NM-B 14/2 cable is installed between the octagonal ceiling box and a convenient junction box on the wall about 10 feet away. The junction box is on a circuit that is connected at the main electrical panel. It’s extremely important the smoke alarms are on a continuous non-switched circuit and not to a light switch.

      Also see my reply to Wire Nut dated Sept. 30, 2014 for detailed electrical code requirements.

  30. Bill Bock March 19, 2016 at 3:51 pm - Reply

    Hi Bob,

    I’ve been in my present home for going on 9 years so I figured it was almost time to replace my smoke detectors so I went out and bought new ones. As was changing them I noticed some were installed with 14/3 romex and some with 14/2 romex. Upon further inspection I noticed the one’s connected with 14/2 used the bare copper ground wire as the alarm interconnect circuit. This appears to me to be a code violation. What’s your take on this?

    Thanks, Bill

    • Bob Jackson March 20, 2016 at 1:54 pm - Reply

      Using the NM-B 14/2 ground wire for the alarm signaling interconnection is an unsafe and incorrect installation. IRC R314.3 Location states:

      “When more than one smoke alarm is required to be installed within an individual dwelling unit the alarm devices shall be interconnected in such a manner that the actuation of one alarm will activate all of the alarms in the individual unit.”

      The alarm interconnection signaling wire must be a separate insulated wire, this is most often the red wire in NM-B 14/3 cable. The problems with using the uninsulated ground wire for signaling are:
      * It violates the smoke alarm manufacturer’s installation requirements.
      * It may be grounded/shorted disabling the 9VDC alarm interconnect signal violating IRC R314.3.
      * Someone may have connected the ground wire to the branch circuit ground. If 110/120VAC is ever passed it will damage the smoke detectors.
      * Metallic boxes must be grounded per the NEC. If any of your smoke alarms are mounted to a metal ceiling box the 14/2 ground wire must connected to the box.
      * Using the ground wire for signaling probably violates NEC 300.3 “Mixing. Control, signal, and communications wiring must be separated from power and lighting circuits so the
      higher-voltage conductors do not accidentally energize them.” See the bottom of page 3 in the “Wiring Methods Part One” presentation. The issue being the ground wire is repurposed for low-voltage signaling.

      I’d abandon the NM-B 14/2 smoke alarm wiring and pull new 14/3 cable. Cap the 14/2 wires with a wire nut and label all exposed locations as “Abandoned – disconnected. Do not use”.

  31. Josh March 30, 2016 at 12:18 am - Reply

    Great page you have here. About 5 weeks ago our alarms started sounding randomly (5 units interconnected built in 1998) I figured they were old and bought new ones. I installed the new ones and I’ve been getting the same results. I’ve sprayed for bugs, swapped breakers in the breaker box, double checked all of my new pig tailed connections and nothing is making a difference. We are at our wits ends with this. Any suggestions on what to do next would be greatly appreciated.

    • Bob Jackson March 30, 2016 at 7:54 am - Reply

      Is there a pattern to the false alarms? Only happens in the morning, rainy days, when the window is open, after a shower, etc? Wind, dust, humidity can trigger false alarms.

      Troubleshooting steps:

      Step 1 – Try disconnecting one unit at a time to troubleshoot. Wait for however long the false alarm usually happens (day, week).

      Step 2 – If a false alarm happens you know it’s caused by one of the other units. Reconnect this unit and redo from Step 1.

      Step 3 – If no false alarm occurs then the disconnected unit is the culprit. Go to Step 4.

      Step 4 – Review the manufacturer’s installation instructions for proper placement and false alarm issues. It’s probably something with where it’s located causing false alarms.


      • Josh March 30, 2016 at 10:35 pm - Reply

        No pattern to the alarms. I did start disconnecting each alarm that caused them to go off until I was down to one. Then it went off. We’ve lived in the house for 13 years and have never had this problem so I don’t think it’s a placement issue. One thing was last night when I reset and reconnected all of them, they started sounding before after I got the last one plugged in but before I had made it to the garage to turn the power back on.

        • Bob Jackson March 31, 2016 at 6:33 pm - Reply

          I’m wondering if it’s an electrical problem due to interference, loose connection or voltage drop. Have you recently installed a new freezer in the garage or fluorescent light?

          Try measuring the AC voltage level at each detector with a multi-meter? This manufacturer’s website discusses nuisance alarms caused by “A Loose Electrical Connection on AC or AC/DC Smoke Alarms” and “Are there any appliances on the same circuit as the alarms?” which may cause a “voltage drop of the wiring to the load will be imposed on the interconnect wire thus causing the alarm to sound”.

          I don’t know exactly what voltage drop would be a problem but if the multi-meter shows 120VAC at 4 detectors and say 100VAC at the 5th unit then it may be suspect. Measure the voltage when the lights and appliances are On.

          RF interference from fluorescent lights may cause false alarms. The Kidde PI2010 User Guide states:

          “2. LOCATIONS TO AVOID

          * Near fluorescent lights. Electronic “noise” may cause nuisance alarms.”

          The NFPA Journal article titled “Proximity Question: Smoke detectors and fluorescent lighting fixtures” suggests 3 feet separation.


  32. Barbara June 14, 2016 at 8:27 am - Reply

    Hi Bob,

    I’m a single mom and I had chirping on one unit in the middle of the night. When I replaced the battery, I saw an uncapped red wire. Not sure what to do. I checked the other units and it looks like one has the red wire inserted into the clip along with the black and white, and the other unit has it dangling but capped.

    I’m thinking a bare red wire crammed back into the box is a hazard and I should have an electrician come cap it?

    This site is so informative! Thanks

    • Bob Jackson June 15, 2016 at 11:17 am - Reply

      The red wire in the NMB-14/3 electrical cable is the alarm interconnect signaling wire. That way if one smoke alarm is trigger all other interconnected smoke alarms will sound, too. The red interconnect wire should only carry 9 volts DC.

      > I’m thinking a bare red wire crammed back into the
      > box is a hazard and I should have an electrician come cap it?
      It’s sloppy to leave the red wire uncapped with a wire nut if unused because someone might mistakenly connect it to the black (hot) wire which carries 120 VAC.

      Call an electrician and request the following:
      * Inspect the smoke alarms and verify they support interconnect signaling.
      Check the manufacture date on the back of the alarm. Smoke alarms should be replaced every 10 years and if yours are over 5 yrs old or don’t support alarm interconnect wiring have them replaced.
      * Verify the red interconnect wire isn’t carrying 120 volts AC and wire the alarms correctly.


  33. Doug August 7, 2016 at 8:03 pm - Reply

    This is an awesome web site. Your comments are very helpful and (forgive me, I was raised by teachers) well written! Just had a false alarm in the middle of the night, vacuumed out all sensors, and am crossing my fingers. I have nine detectors. Eight are just hard-wired, made by Kidde, no batteries. The final one, in the upstairs master, is hard-wired with a battery. I replaced the battery. But it is not a Kidde — it is a USI-5204 by UST Electric. House built in 2004, so all are at 12 years. If I start replacing, should I make them all by the same manufacturer? And if so, do you have a preference?

    • Bob Jackson August 8, 2016 at 6:51 am - Reply

      I installed Kidde and it’s done well for me. The Kidde PI2010 User Guide states: “Kidde alarms and accessories CAN ONLY BE interconnected with other Kidde alarms and accessories as well as specified brands and models of interconnect compatible alarms.”

      Also see What products do Kidde alarms interconnect with?

  34. Sandra Snider December 23, 2016 at 4:19 pm - Reply

    Bob, we live in OKLA, built a house in 2014…smoke alarm has gone off about 6 times(we called the electrician each time, had things checked out)…Still happens….Cant this smoke system be on it’s OWN circuit breaker switch , so we can switch it off without cutting off other things in the house ? He said the system HAD to be installed with another circuit…..could not be alone. These malfunctions are driving us crazy…Even the builder is of no help…And I cant find a “code” in our Norman city info. Any ideas ? thx, Sandra

    • Bob Jackson December 25, 2016 at 9:07 am - Reply

      Smoke alarms can be the only devices on a dedicated branch circuit and circuit breaker.

      The City of Norman Oklahoma Building Codes website lists State and Local amendments.

      Amendments page 18 states (bold emphasis added):

      “SMOKE ALARMS R314.4 Power source. Smoke alarms shall receive their
      primary power from the building wiring when such wiring is
      served from a commercial source, and when primary power is
      interrupted, shall receive power from a battery. Wiring shall be
      permanent and without a disconnecting switch other than those
      required for overcurrent protection
      . Smoke alarms shall be

      A circuit breaker is an “Overcurrent protection” device.

      The above amendment is consistent with the NFPA 72®: National Fire Alarm and Signaling Code Handbook, 2013 Edition states in article 29.6.3(4) AC Primary Power Source:

      “AC primary (main) power shall be supplied either from a dedicated branch circuit or the unswitched portion of a branch circuit also used for power and lighting.”

      In summary:
      * Smoke alarms can be the only devices on a branch circuit.
      * Smoke alarms must not be on a switched portion of a branch circuit. e.g. Power shutoff by a light switch.
      * It is good practice, but not required, to have a light on the switched portion of the branch circuit. That way if the circuit breaker trips the light won’t work letting you know the power is Off to investigate the problem. Otherwise you might not notice the smoke alarms are not on AC power.

      As for the frequent nuisance alarms, it could be caused by electrical noise from other devices on the circuit. Florescent lights are suspect.

  35. Rog January 22, 2017 at 1:01 pm - Reply

    I just wanted to thank you for this comprehensive explanation and pictures you posted. I was just replacing an old smoke detector with a Nest Protect, and the wiring behind the old device did not conform to the Nest youtube tutorial. Your procedure (and pictures :) ) helped me get a much clearer understanding of the wiring, and my son and I were able to complete this without an issue.
    Thanks again! I appreciate the time and effort you put into this, and to helping us novices :)

  36. AJ Larsen February 9, 2017 at 3:05 pm - Reply

    I finished two bedrooms in my basement and I used 14-3 wire to hook the smoke detectors up to the already hardwired smoke detector located at the base of the stairs outside of one the finished rooms. I took a wire from one bedroom to the other, and then from that room to the hardwired smoke detector. There is a switch box next to the existing smoke detector that a wire runs through, but it has an outlet attached to it as well.

    The batteries are out of the smoke detectors and the light is green in all three, but when I push the existing smoke alarm all the alarms go off upstairs, but the two new smoke alarms do not go off. Any ideas what is wrong or additional trouble shooting that I can do. Should I connect all the wires in the box and just have one piece of wire come out from the box and connect to the existing smoke detector and get rid of the outlet plugs?



    • Bob Jackson February 9, 2017 at 5:27 pm - Reply

      Are the new smoke detectors the same brand as the existing smoke detectors? See What other products will Kidde alarms interconnect with? for details.

      > Should I connect all the wires in the box and just have one piece of wire come out from
      > the box and connect to the existing smoke detector and get rid of the outlet plugs?
      By “get rid of the outlet plugs” if you mean the AC Quick Connect harness don’t do it. Wire the AC Quick Connect per the manufacturer’s instructions. Your new detector wiring should be very similar to the units in the dotted green box in this wiring diagram.

      Also recheck the red signaling wire nut connections in case there’s a bad or loose connection.

  37. Mike K February 28, 2017 at 4:03 pm - Reply

    This question may be similar to one I saw last year…. house built 1986, there are 3 wired detectors, one on each level. They used to work but They have not worked for 15 years or so. ( yes we have battery operated ones for now). 2nd floor has 1 input wire 14/2 it appears I can’t find a red and this has power. 1st floor has 14/3 no power 1 input wire. Basement has 14/3 no power 2 input wires ( or 1 in 1 out). What would be best way to trouble shoot this?

    • Bob Jackson March 1, 2017 at 9:25 am - Reply

      The 1st floor and basement alarms served by 14/3 cable are likely on the same branch circuit. Try visually tracing the basement 14/3 cable back to the circuit breaker panel. Most of the cables will be 14/2 with only a few 14/3’s. Are there any abandoned cables, breakers in the Off position or unlabeled breakers? You might have to *shutoff power at the service entrance to the entire house* and remove the panel cover to see if the wires are disconnected from a breaker. Most people should hire an electrician if any work is needed on the circuit breaker panel.

      A Fluke Networks Pro3000 Tone Generator & Probe Kit is very useful for tracing wires. It must only be used on non-energized AC circuits. Verify the electricity is Off with a voltage detector before using the probe.

      Because the 2nd floor detector is served by 14/2 cable you’ll have to pull a new 14/3 to connect it to the existing 14/3 serving the 1st floor and basement units so all are on the same circuit with inter-alarm communication.

      It may help to systematically check every circuit breaker and correct/update the breaker labels. I recently went through my panel, turning Off breakers one-by-one to see what outlets, lights and appliances were actually served by that breaker. Found a couple of mislabeled breakers, oddly routed circuits, an ungrounded outlet installed by the former homeowner (he cut off the ground wires!) and a breaker with no connection. I rewired the ungrounded outlet, removed the disconnected breaker, installed a blank cover plate and printed new breaker labels.

  38. tedcohen March 2, 2017 at 4:53 pm - Reply

    Hi Bob,

    Two of my five detectors – all five are hard-wired with battery backup – failed to go back to AC power after a smoke-triggering event.

    All five are interconnected and all five showed constant AC power green light before the event.

    Why would two of the five now fail to reconnect to the AC?

    I shut off the breaker at the box controlling the detectors and turned it back on thinking they might reset but it didn’t work.

    Thank you in advance for your help.

    • tedcohen March 2, 2017 at 5:35 pm - Reply

      Bob -you inspired me. To wit, I did a push-button test on each of the affected units and – bingo – they came back to AC power.

      You made it happen.

      Thank you.

  39. Sam March 2, 2017 at 5:03 pm - Reply

    Hi Bob,
    I need help in this situation. We moved in a townhouse (Toronto) few years now and recently just noticed the AC smoke detector not in order because there is no electricity to the detector (we do have a separated battery drive detector in place). I have tried to find out where its power come from and knowing it MUST be somehow it has been disconnected accidentally without notice.
    Typical Townhouse (built in 1977 as per sticker on power panel).
    – does this typical kind of townhouse building code requires AC smoke detector connect to the Master bedroom/washroom ? or
    – must connect to stair hallway ? or
    – those two smaller bedrooms ?
    Sure, sure it was disconnected by the previous owner but I just unable to figure it out where. The building code must has these information unfortunately I am unable to find it from anywhere.
    Hope you can shed some lights…just wanted know by building code, which circuit it suppose to be connected to.

    • Bob Jackson March 2, 2017 at 10:05 pm - Reply

      How old are the hard-wired alarms given the was built in 1977? Alarms should be replaced at least every 10 years. Use the same make/model alarm for compatibility.

      Smoke alarms must be on permanent circuit that cannot be disconnected by a light switch. Beyond that IRC R314.4 doesn’t specify how the alarm circuit is connected to the circuit breaker. The breaker could supply power to other branch circuits and devices.

      Some things to check are:
      * Turn on all the light switches in case it’s incorrectly wired to a switch-controlled circuit.
      * Look for tripped circuit breakers. Sometimes a breaker will trip but the switch toggle only moves a little and could be mistaken for being in the On position. Flip the suspect breakers Off then On.
      * Remove each alarm to see if the house wiring is disconnected.
      * Are there any dead outlets? Check each with a receptacle tester. The alarms might be downstream of the bad outlet.
      * Are there any GFCI outlets? If so, press the Reset button. If power is restored to the smoke alarms then it’s incorrectly wired to the GFCI outlet.

      Also see my advice to Mike K dated February 28, 2017 for a similar no power / wire tracing problem.

  40. Rick Peterson March 18, 2017 at 11:27 pm - Reply

    I am replacing the original smoke detectors and noticed that in one bedroom the black and white wires are both connected to the white wire. The red is connected to the yellow wire as expected. The black wire is not connected to the smoke detector. Why would that be done and how could it work?

    • Bob Jackson March 19, 2017 at 12:28 pm - Reply

      Just so I understand, the NM-B 14/3 black (hot) wire is not connected to anything; and the smoke alarm AC Quick Connect Harness black & white wires are both connected to the NM-B 14/3 white (neutral) wire. Correct?

      If so, the smoke detector is miswired and has no power because the NM-B 14/3 black (hot) wire isn’t connected to anything. Hopefully whoever miswired it capped the 14/3 hot wire with a wire nut to prevent a short circuit and/or shock.

      To correct the wiring, shut off the electricity at the circuit breaker, verify the power is Off at the smoke detector with a voltage meter and disconnect the wiring. Rewire the smoke detector correctly. Turn on the electricity and verify the smoke detector is operating on AC power and working.

      • Rick Peterson March 19, 2017 at 10:08 pm - Reply

        The black wires are capped. Just wanted to understand if there was a valid reason to do this. I’ll fix it up. Thanks!

  41. Bradley E Rose March 30, 2017 at 10:32 am - Reply

    one detector is beeping all green light are on but the basement decor is still beeping what did i do rong

  42. Mark November 6, 2017 at 1:15 am - Reply

    Great article. I searched around the internet for some information on smoke detectors as I was totally surprised at what I just found in my home. It was time to replace my smoke detectors, as I hired someone to replace them when I bought the house ~10 years ago. The brand is Kiddie — I have 8 total (3 down, 5 up). Doing some research, along with pulling down one detector (ionization type), I decided on getting two dual photo/ion type (one upstairs and one down stairs) and the rest photoelectric. I bought all hardware style AC/interconnect as that is what I have.

    By everything I need at one of the big box stores (Kidde). I go to replace the first three (two downstairs, one is really high and I will do last), and first upstairs in hallway. All good. I move onto the next, pull down, remove mount plate, and pull out the cable connector (as I am going to replace all of them — yes, turned power off). What I noticed is only AC connection (White and Black). There is NO interconnect cable. I proceeded to look at all remaining 5 — same thing. AC powered, but no interconnect.

    Home was built in 1996. I was very surprised to find this as AC was run to each location. 3 have interconnect cable + AC, remaining 5 just AC.

    Think this was a mistake that got overlooked years ago, perhaps the rules changed around ~1996 (or later).

    Anyway, I decided, for now, to put all new smoke detectors up just like old configuration, i.e. 5 are on “islands”, but changed from ionization sensors to photoelectric. 3 (2 down, 1 up) are hard wire interconnected (2 are duel sensor, 1 is photoelectric). Set me back ~$180 in materials, ~4-6 hours of my time.

    I think I will rework this whole solution with First Alert sensors, using a wired bridge for one of the 3 I replace, all 5 on “islands” I will get a wireless/battery only style I guess. I am trying to figure out if the Wired/Wireless style (such as First Alert SA521CN-3ST) can all work wireless while still using the AC power. None of the manuals or marketing literature promote this configuration.

  43. Brian November 13, 2017 at 6:53 am - Reply

    One of my Kidd hardwire smoke detectors was acting up so I decided I would just replace all 4 of the smoke detectors in my house, which was built in 2008. They were all 9 years old so I just figured they were getting close to their life span. All are Kidd hardwired detectors and are only smoke detectors (no CO2) and run off a separate circuit breaker. All went fine but the one that I was initially having problems with doesn’t seem to be getting power. Before I started, I checked it to see if there was power, but no power was getting to that one detector but the other 3 and they all had power.

    I replaced the detectors and the green light appears on the 3 that had power, but the problematic one still doesn’t have power, which I knew it wouldn’t because a new detector wasn’t going to bring power when it didn’t exist before. I recently moved and I honestly can’t remember if power was ever getting to the one detector.

    Any thoughts on how I am figure out why power is not getting to that one? I know I should go up in the attic and follow the wire for damage, but are there any other ideas I can do to see why the one detector is not getting power? I tried turning light switches on and off just in case it was wired to a light switch and it doesn’t change anything. I also checked all receptacles in that area and they all work.

    Any help would be great. Thanks.


    • Bob Jackson November 13, 2017 at 6:46 pm - Reply

      Yeah, you’ll have to trace the wires in the attic. Other readers with no power to a smoke detector have found a dead-end cable run, no connection at a junction box or squirrel/rodent damaged wires.

  44. Bill V January 7, 2018 at 11:21 pm - Reply

    I have eleven (11) FireX FADC ionization SD’s six (6) up and five (5) down.
    I was taking inventory for replacement when I noticed that there is no green power indication on all the 5 down SD’s.
    I have one breaker at the electric panel labeled “Smoke Detector” so I assume they are all on the one circuit.
    How can I find out what might be the issue with the down 5? The up six all have power.
    I want to replace them all but I would like to resolve the power problem first.
    My house was built to code for first occupancy in 2003 calling for the SD’s to be with wired power, interconnection and battery backup.
    I’m not sure when the power to the down 5 was interrupted.
    I have checked the power with all lights turned on and verified that all GFCI circuits are powered.
    All wiring is enclosed behind sheetrock so exposing the wiring would be a significant effort.
    Any suggestions you can offer would be appreciated.

    • Bob Jackson January 9, 2018 at 6:49 pm - Reply

      Steps for tracing the smoke alarming wiring. Have a non-contact voltage detector ready:

      1 – Remove the each of the five downstairs smoke alarms one at a time to expose the junction box wiring.
      2 – What type of wire is in the box? NM-B 14/3 or 14/2? NM-B 14/3 with the red wire is required for interconnected alarms.
      3 – Check for voltage with the non-contact voltage detector.
      4 – How many NM-B cables enter the box? Each box should have two NM-B cables if it’s in the middle of the run except for an alarm at the end of the run.
      5 – Is the NM-B cable the same type, color and brand at each of the smoke alarms? If not the wiring may have been modified and worthy of closer investigation.
      6 – Are any wires loose (not properly nutted) or disconnected?

      Also see my comment dated October 10, 2015 at 11:22 am.

  45. Jamie February 12, 2018 at 9:55 am - Reply

    Hi Bob. Found your website and article fairly informative. I’m also wondering about bringing in power to a light first rather than a hard wired detector. What’s the best approach to a situation like that? Since power needs to be unswitched for the detector is it the neutral at the light junction box that is spliced for both the light and the detector or is it the hot wire that’s spliced for both the wall switch and the detector. I know that the red wire is basically a communication wire for other smoke detectors, so that wouldn’t be spliced at the light box. Would like to learn more about this.

    • Bob Jackson February 16, 2018 at 4:21 pm - Reply

      It took me a while to respond because I needed to create new wiring diagrams. See the new section at the end of the article titled “Smoke Alarm on Unswitched Branch Circuit also Used for Power and Lighting“.

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