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How to Install a Hardwired Smoke Alarm

This project explains how to install Kidde/Firex hardwired (AC powered) smoke alarms in the finished- and unfinished sides of the basement. A new 120 volt AC branch circuit is wired to power the alarms and an alarm is installed in the suspended drywall ceiling.


Project Update

If you have functioning hardwired smoke alarms on 120VAC power and only need to install new smoke alarm(s), skip to How to Install a Hardwired Smoke Alarm – Part 4 for a streamlined installation because the first part of this tutorial concerns fixing two problems:

  • Relocating an abandoned smoke alarm and junction box that was improperly concealed when the basement was finished.
  • Wiring the smoke alarm circuit to 120VAC full time power.

After correcting these problems the new smoke alarm is installed in the finished basement.

Smoke Alarm Building Code Violation

The prior homeowners hired a contractor to finish the basement. The work was done without a building permit and required inspections in an attempt to avoid a property tax increase. When I purchased the house, the home inspector identified several ungrounded AC electrical outlets (which were fixed) but he didn’t notice that the smoke alarms were either disabled or missing – a clear violation of the building code and a safety hazard. Aside: When I filed for a permit to finish the basement bathroom the building inspector noted I had a finished basement that was not on record and the county increased my property tax assessment accordingly. I thought the tax increase was very reasonable.

This next photo shows the problem; the single smoke alarm is disabled and in an inaccessible location above a finished room in the basement. Smoke alarms should not be installed in a crawl space such as this and it will be relocated to the center of the utility room:

Disabled and Improperly Concealed Smoke Alarm in Finished Basement

Disabled and Improperly Concealed Smoke Alarm in Finished Basement

The smoke alarm is a “hardwired” type that runs on the 120VAC house electricity with a 9 volt battery backup. The smoke detector battery door is open and the green LED is not illuminated which indicates there is no 120VAC power. The 9 volt battery stamp said it was manufactured in 1999! No additional smoke detectors were installed when the basement was finished to reflect the new floor plan. Were there a fire in the basement I probably wouldn’t know until it was too late!

Disabled Smoke Detector with 10 Year Old Battery

Disabled Smoke Detector with 10 Year Old Battery

I removed the smoke alarm by twisting the alarm slightly to release it from the mount ring, disconnected the AC power plug and loosened the screws to release the mount ring. This Firex smoke alarm model # 120-1182 was manufactured on Feb. 8, 2000 as stamped on the back label. Smoke alarms should be replaced every 8 to 10 years according to the National Fire Protection Association, U.S. Fire Administration and the manufacturer Firex. This smoke alarm will be replaced with a new unit:

Remove the old Firex Smoke Alarm Model # 120-1182

Remove the old Firex Smoke Alarm Model # 120-1182

The smoke detector LED will illuminate when 120VAC power is present. I checked for voltage with the non-contact voltage detector to confirm the electricity was Off. Now that I think of it, the green smoke alarm LED was Off on all of the smoke alarms in my house.

No AC Power to Hardwired Smoke Detector

No AC Power to Hardwired Smoke Detector

I’m very cautious when it comes to electricity and debugging wiring problems, so I grabbed my digital multimeter to verify the voltages on the NM-B 14/3 alarm wiring. There was zero volts across all wire combinations. There was 0.048 volts (or 48 hundredths of a volt) across the black (hot) & white (neutral) wires and 9 thousandths of voltage across the hot and ground wires is practically 0.0 volts and reflects an insignificant induced voltage from other AC wires adjacent to the smoke alarm wiring:

Hardwired Smoke Alarm - NM 14/3 Wire Voltage Check

Hardwired Smoke Alarm – NM 14/3 Wire Voltage Check

Troubleshooting Smoke Alarm Wiring

I needed to figure out why there was no 120VAC power to any of the hardwired smoke detectors in the house as the green LED indicating AC power is not illuminated on any of the units. Was an AC power circuit disconnected? Was a circuit breaker thrown? Were the smoke detectors improperly wired to a light switch? Did a rodent chew through an electrical cable?

The smoke detectors are interconnected with NM-B 14/3 (non-metallic, 14 gauge, 3 conductor plus ground) electrical wire so that if one detector goes off, the other smoke alarms will go off too. This is so you’ll be alerted to a fire anywhere in the house.

I followed the smoke alarm wires in the basement and attic, but didn’t find any disconnected or damaged wires. It required a fair amount of crawling around the attic and brushing away insulation to trace all the wires and ceiling boxes:

Troubleshoot Smoke Detector Wiring: Attic Ceiling Box

Troubleshoot Smoke Detector Wiring: Attic Ceiling Box

I flipped on all the light switches in the house and that made no difference.

At the main electrical panel, there were only a couple of NM 14/3 wires and I traced these to other purposes.

Next I removed each smoke alarm in the house to inspect the ceiling box wiring. The good news is the alarms all all interconnected (red and yellow wires), but there was no AC power present at any of the smoke detectors:

Hardwired Smoke Detector Troubleshooting: Hallway Unit Wiring

Hardwired Smoke Detector Troubleshooting: Hallway Unit Wiring

While I couldn’t access the smoke alarm wiring inside the walls I was reasonably confident the smoke alarms were never wired into a non-switch controller AC power circuit. An example of switch-controlled circuit is a light switch and smoke detectors must never be wired to an On/Off switch.

This project is continued in How to Install a Hardwired Smoke Alarm – Part 2.

Take care,

Bob Jackson

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