How to Install a Hardwired Smoke Alarm – Part 3

A new smoke alarm is mounted and wired in the suspended drywall ceiling for the basement living room using an old work electrical box. NM-B 14/3 cable is pulled from an existing smoke alarm to power and interconnect the new smoke detector. This project is continued from How to Install a Hardwired Smoke Alarm – Part 2.

Smoke Alarm Installation

A second Kidde/Firex Dual Sensor (Ionization and Photoelectric) AC powered hardwired smoke alarm will now be installed in the finished basement living room. The smoke alarm, NM-B 14/3 cable roll and old work electrical box are shown here:

Kidde/FIREX Smoke Alarm Model # PI2010 and NM-B 14/3 Wire

Kidde/FIREX Smoke Alarm Model # PI2010 and NM-B 14/3 Wire

Smoke Alarm Wiring Diagram

The NM-B 14/3 branch circuit for the new smoke alarm was connected to the octagonal ceiling box in Part 2. The NM-B 14/3 wire will be strung along the floor joists to the basement living room. The wiring diagram for this work is:

Smoke Alarm Drywall Ceiling Wiring Diagram

Smoke Alarm Drywall Ceiling Wiring Diagram

Install the Old Work Electrical Box in the Drywall Ceiling

After pulling NM-B 14/3 cable along the floor joists and fastening it with insulated cable staples every 4 or 5 feet, I wedged myself into the crawlspace above the finished basement and reached out as far as I could to position the electrical box on the suspended drywall ceiling above the basement living room. The smoke alarm must be at least 4 inches away from the walls and I’m at least 2 feet away here.

I positioned the old work electrical box parallel to the metal stringers and traced the outline of the box on the drywall ceiling with an pen. The box outline is highlighted in red for better viewing. Note: Do not trace around the tabs on the ends of the old work  box because the tabs must seat against the face of the drywall to so the box doesn’t pull through the drywall mounting hole.

Old Work Electrical Box and NM-B 14/3 Wire Drop for Smoke Alarm

Old Work Electrical Box and NM-B 14/3 Wire Drop for Smoke Alarm

I cut along the box outline traced on the ceiling with the Stanley FatMax Jabsaw. The FaxMax made quick work of cutting the hole. When starting a cut, I find it best to work the pointed nose into the drywall to puncture through rather than pounding the saw in – you’ll have a cleaner starting cut with less tearing of the finished drywall face. My helper was on a ladder inside the room holding a plastic tub against the ceiling to catch the dust:

Install a Smoke Alarm: Saw the Drywall Ceiling for the Old Work Electrical Box

Install a Smoke Alarm: Saw the Drywall Ceiling for the Old Work Electrical Box

I slipped the old work electrical box in the drywall ceiling to ensure it would fit when I installed the bop from the finished side of the ceiling. It fit perfectly the first time. Note how the four plastic tabs prevent the old work box from falling through the mounting hole:

Smoke Alarm Installation: Fitting the Old Work Electrical Box in Drywall Ceiling

Smoke Alarm Installation: Fitting the Old Work Electrical Box in Drywall Ceiling

I removed the old work electrical box and fed the NM-B 14/3 cable through the drywall ceiling to wire the smoke alarm:

Install a Smoke Alarm: NM-B 14/3 Cable Drop in Drywall Ceiling

Install a Smoke Alarm: NM-B 14/3 Cable Drop in Drywall Ceiling

View of the ceiling cutout and cable drop for the new hardwired smoke alarm from the basement living room. You can get by with about 12 inches of cable hanging from the ceiling:

Install a Smoke Alarm: Drywall Ceiling and NM-B 14/3 Cable Drop

Install a Smoke Alarm: Drywall Ceiling and NM-B 14/3 Cable Drop

Mount the Old Work Electrical Box in the Drywall Ceiling

The NM-B 14/3 cable is fed through the old work electrical box then the box is inserted in the drywall ceiling. Swing clamps that grab the back of the drywall when the two corner screws are tightened. Be careful not to over tighten the mounting screws because the swing clamps can crush the drywall. Tighten the screws until the box is snug. Notice the cordless drill/driver torque is set to a low torque value of 4:

Install a Smoke Alarm: Mount the Old Work Electrical Box in the Drywall Ceiling

Install a Smoke Alarm: Mount the Old Work Electrical Box in the Drywall Ceiling

Smoke Alarm Ceiling Box Wiring

The NM-B 14/3 wires are cut so at least 6 inches extends into the box and at least 3 inch extends past the box. Only 1/2 inch of insulation is stripped from the wires ends to match the exposed wire of the Kidde PI2010 AC Quick Connect Harness. Notice the copper ground wire is wrapped out of the way since it’s not used and there’s nothing for it to connect to on this terminating circuit. Plastic electrical boxes do not require grounding. (Metal junction boxes must be grounded with a green ground screen and ground wire pigtail.)

I’ve set two flat head 8/32 thread by 3/4″ long screws in the box ends to secure the smoke alarm trim plate. The old work electrical box doesn’t include these screws so you’ll have to provide your own:

Wiring a Smoke Alarm in the Drywall Ceiling

Wiring a Smoke Alarm in the Drywall Ceiling

The Kidde PI2010 AC Quick Connect Harness is fastened to the NM-B 14/3 wires with wire nuts:

Kidde PI2010 Smoke Alarm AC Quick Connect Wiring Harness

Kidde PI2010 Smoke Alarm AC Quick Connect Wiring Harness

Almost done! The trim plate is fastened to the ceiling electrical box with the two screws. The wiring harness is plugged into the smoke and the smoke alarm ratcheted onto the trim plate. Remember to pull the plastic yellow ribbon from smoke alarm to activate the battery:

Kidde PI2010 Smoke Alarm Ceiling Installation

Kidde PI2010 Smoke Alarm Ceiling Installation

Kidde PI2010 Smoke Alarm Testing

I turned on the electricity at circuit breaker to restore the AC power to the smoke alarm circuit then walked through the house to verify the green AC power LED is illuminated on all smoke alarms. Yes! Steady green lights on the units. I also watched each smoke alarm to see the red LED flash about once a minute indicating the smoke alarm is operating correctly in standby mode.

With my helper on the 2nd floor, I visited every smoke alarm in the house and pressed the “Test” button on each unit for at least 5 seconds. The alarm sounded followed by all the other alarms ringing as it should for an interconnected system. Very loud and annoying, but working as advertised!

The green LED is illuminated on the smoke alarm in the finished basement (click on the image for full size view):

Kidde PI2010 Smoke Alarm on AC Power: Green LED Illuminated

Kidde PI2010 Smoke Alarm on AC Power: Green LED Illuminated

Install a Bedroom Smoke Alarm

I later finished the basement bedroom and a smoke alarm is required in each sleeping area per the Building Code. That job is explained in How to Install a Hardwired Smoke Alarm – Part 4.

Stay safe!

Bob Jackson

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7 Responses to How to Install a Hardwired Smoke Alarm – Part 3

  1. Phil Esra July 6, 2015 at 2:22 pm #

    Great article–thanks.

    I’m confused about plastic vs. metal boxes. It’s not OK to use a plastic box in an unfinished space, but it’s OK to have the back side in an unfinished space? (vs. contained on both sides in a stud bay)

    Also, what are the green bushings in one of the pics? Some kind of drywall spacer?

    THANKS!

    • Bob Jackson July 6, 2015 at 7:18 pm #

      Old work electrical boxes are available in both plastic and metal. I prefer the plastic old work boxes with the swing style clips. For new construction I use metal electrical boxes for the benefit of grounding the box, better cable clamps and strength.

      > It’s not OK to use a plastic box in an unfinished space, but it’s OK to
      > have the back side in an unfinished space? (vs. contained on both
      > sides in a stud bay)
      Not quite sure what you mean here. You can use a new work plastic or metal box in an unfinished area. “Unfinished space” implies there no way to mount an old work metal or plastic box with swing style, screw clips or plaster ears.

      For a ceiling installation the back side of the old work box naturally will extend into the area above the ceiling.

      > Also, what are the green bushings in one of the pics? Some kind of drywall spacer?
      The “green bushings” on the right side of this photo are cap nails and typically used for roofing. The electrician used them to fasten the 1×2 inch strips to the floor joist to protect the NM-B 14/3 cable. They’re also great for fastening a tarp over a leaky roof after storm damage! The plastic cap may be green, blue, orange, etc. and is unimportant.

  2. Phil Esra July 9, 2015 at 12:44 am #

    Ah, got it, I was confused–I thought plastic boxes were only allowed when inserted in walls (whether new work or old work). It’s very helpful to be corrected about that! Now that you mention it, the fact that plastic ceiling boxes exist SHOULD have made me question myself. And thanks for the info about the nails, too.

  3. Yohannes July 17, 2015 at 9:40 pm #

    My Kidde box comes with Black, White and Red Harness wires. From the ceiling I have Yellow, White and Red wires for me to connect the Fire and Carbon Monoxide Alarm detector. I connect Black to Yellow, White to White and Red to Red, when I turn on the breaker it beeps continuously . When I disconnect the Battrey it is ok, and No Green LED comes on. What did I do wrong? Please advice

    • Bob Jackson July 19, 2015 at 2:34 pm #

      Do you have other smoke detectors in the home? If they’re not the same manufacturer and/or older models they may not be interconnect compatible with your new smoke/CO alarm. You can check by calling Kidde Product Support: 1-800-880-6788 M-F 8am – 5pm EST. I think this is the problem. The solution is to replace all alarms with the same model.

      The smoke & carbon monoxide detector has three wires:
      * black – 120VAC hot wire for power
      * white – 120VAC neutral wire for power
      * red – low voltage interconnected alarm signaling wire.

      When the detector is triggered it sends an ~9VDC signal over the red wire to trigger all other interconnected alarms throughout the house. If you have different alarms the signaling format may not be compatible.

      Let me know if Kidde says it’s a compatibility problem.

  4. Daniel October 22, 2015 at 8:25 am #

    I went to replace all of our hardwired smoke detectors. I noticed that no current is present in the wires. I checked the circuit box and no switches were tripped. What could be wrong? They all worked before. Thanks

    • Bob Jackson October 23, 2015 at 12:41 pm #

      Unfortunately you’ll need to trace the electrical wiring to figure out if the smoke detectors are improperly connected to a switch controlled circuit, a wire is loose/damaged or maybe the branch circuit was never connected to the circuit breaker panel. See my advice to other people having this problem here and here.

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