Installing 120 volt AC line-voltage smoke detectors.

Electrical Finish Work:

Installing AC-Powered Smoke Detectors

In This Article:

The outer jacket on the 3-conductor electrical cable is cut away. The ground wires are connected together and attached to the metal J-box. The black, white and red wires are stripped and connected to the wiring pigtail provided with the new smoke detectors. The base plate is installed and the smoke detector is mounted to the ceiling.

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Skill Level: 2-3 (Basic to Intermediate) Time Taken: About 30 Minutes Each

By , Editor


This article describes how line voltage (AC-powered) smoke detectors are installed when the wiring "rough-in" has been completed and the drywall has been finished and painted. While the electrical rough-in procedures are not complicated, it is beyond the scope of this basic article. At the end of this page I'll provide some links to other information to help explain the rough-in process.

Smoke detectors can be purchased in "contractor packs" like this 6-pack.

Each smoke detector comes with a base plate and a 3-wire pigtail that is wired into the house wiring.

AC powered residential smoke detector.


14-3G wires sticking out of electrical junction box in ceiling. My starting point for this project: After the electrical "rough-in" has been completed and the wallboard has been finished and painted.


I used a sharp knife to carefully slit the outer jacket on each cable.

This metal ceiling junction box has three 14-3G cables entering it. When multiple AC smoke detectors are wired together, 3-conductor wire must be used.

The third wire is for interconnecting with other smoke detectors so all detectors will beep in the event one detector senses smoke.

Slitting outer coating on wire to expose conductors inside.


Wiring for smoke detectors. After I peeled away the outer jacket, I separated the individual conductors.


Metal junction boxes must be grounded.

There usually is a threaded hole (red arrow) in the back of a metal J-box that will accept a ground screw.

Location of ground screw hole in metal junction box.


Connecting ground wire to ground screw in back of metal J-box. I screwed a green ground screw (red arrow) part-way into the hole, then I wrapped one of the bare ground wires around the screw, and let the end of the wire dangle so it could be connected with the other ground wires.

I tightened the screw and made sure the wire was snug. Note that a good half-circle of wire needs to be beneath the screw head.


Then I clipped the wires to the same length.

The wires need to stick out of the junction box by at least 6 inches. I avoid leaving too much wire sticking out, because then it's difficult to pack all those wires into the J-box.

Bunching together ground wires so they can be connected.


Twist-on wire connector (wire nut) applied to ground wires. I twisted a "wire nut" over the ends of the wires.

With three number 14 wires, I find that the wires become twisted when I apply the wire nut. With heavier gauge wires, or more wires, I find it necessary to pre-twist the wires with a pair of lineman's pliers.


Then I folded the ground wires neatly and pushed them into the junction box, leaving room for the remaining wires. Tucking ground wires into back of electrical box.


Stripping insulation from end of wire. Using a pair of wire strippers, I stripped about 1/2" to 5/8" of insulation from the end of each insulated wire.


I also stripped some more insulation from the pigtail wires that came with the smoke detector.

This is stranded wire, and I always strip stranded wire slightly longer than the solid wires it's being connected to.

Next I twisted each stranded wire so the individual strands would stay together and not fray.

Stripping additional insulation from stranded wire for smoke detector.


Twisting ends of wire so wire nut can be install. I bundled together the three solid wires and the stranded pigtail wire, then I twisted the ends with a pair of lineman's pliers.


The smoke detector pigtail after being connected to the house wiring.

Note that the yellow pigtail wire gets connected to the red solid wire.

Smoke detector "pigtail" connected to household wiring.


Wiring folded into junction box. Then I folded up all the wires and carefully tucked them into the junction box.


I attached the base plate to the junction box, using the screws that came with the J-box.

Sometimes the J-box is too deep behind the drywall, and I need to use longer screws. These are #8-32 machine screws, which can be found at any hardware store.

Fastening smoke detector base plate to junction box.


Connecting power to back of smoke detector. The pigtail is connected to the back of the smoke detector.

Note: I am not aware of any standard size/style of smoke detector connector. Different brands of smoke detectors use a different pigtail connector, so if a detector needs to be replaced, and it's a different brand, the pigtail will likely also need to be replaced.


I plugged the pigtail into the back of the smoke detector. This just snapped into place.

The red arrows point to the holes that engage the tabs in the base plate.

USI Electric 120 Volt AC smoke detector and  mounting holes that engage baseplate.


Mounting tabs on USI Electric brand of smoke detector base plate. The tabs (red arrows) need to be aligned with the corresponding holes in the back of the smoke detector.

Easier said than done.


When I finally got the tabs to line up, I twisted the smoke detector into place. Turning smoke detector to install on baseplate.


Some Additional Tips:

Plastic "remodel box" in ceiling, with 3 wires sticking out or smoke detector. This is a "remodel box" or "old work junction box", which is sometimes used in remodeling projects.

First a 4-inch hole is cut into the wallboard and the electrical wire is fished through the space behind the drywall. Then this junction box simply clamps to the drywall.

Since this J-box is plastic, the ground wire does not need to be attached to the box. Since smoke detectors do not use a ground connection, the ground wire in this 14-3G cable is not needed.


I just folded up the ground wire (red arrow) and tucked it into the back of the plastic junction box.

It's never a good idea to cut off unused ground wire... somebody else might change this box someday and need the ground wire.

Unused ground wire folded up in back of plastic junction box.


Slight kink in solid wire to help stranded wire stay connected. When connecting stranded wire to a single copper wire, I find it helpful to make a slight "S"-shaped kink in the stripped end of each wire.

When the solid wire is perfectly straight and connected to a stranded wire, I find that the wire nut often falls off because the stranded wire gets all bunched up in the wrong spot. Making this kink in the solid wire seems to help.


To fasten the smoke detector base plate to this plastic remodel box, I could not use the mounting holes (B) that I used with the metal J-boxes.

The mounting screws (A) had to be used with the slots in the base plate.

Different mounting options on smoke detector base plate.


Supplying Power To The Group Of Smoke Detectors:

The last smoke detector I installed was in the basement, where the power was supplied to the smoke detector portion of the circuit. I turned off the circuit breaker to this circuit before doing any work. (All the earlier work was done on a dead part of the circuit.)

Electrical wiring for smoke detectors. This ceiling box in the basement had two electrical cables entering it:
A 14-2G cable on the left (marked "LINE IN") and a 14-3G cable on the right (marked "OUT").

The LINE IN cable begins at a junction box that is part of the basement lighting circuit. Note that this wire is NOT SWITCHED, and it is NOT PROTECTED BY A GFI (GROUND-FAULT INTERRUPTER).


Some Thoughts On The "Rough Wiring" Procedures:

In my case, I added new wiring to AC-powered smoke detectors while I remodeled the second floor of my 1 story house, so I was able to get above the first-floor ceiling and cut holes in the ceiling drywall for round metal junction boxes, and drill holes in floor joists to run the new 14-3G wire to each junction box. I also was able to fish some wire through the sloped ceilings on the second floor and install "remodel boxes" in the flat section of the ceiling, without cutting away any drywall. Running electrical cable in an existing house can be easy or difficult... it all depends on the circumstances.

These older articles may help explain some of the procedures used when adding new wiring to an existing house:

However, if you intend to do your own wiring, I must advise you to seek more information than what is published on There are many good books available on the subject of house wiring. I like the book "Wiring A House" by Rex Cauldwell, published by Taunton Press. This book is good for the serious do-it-yourselfer, and may be available at Home Depot and/or Lowe's. There are many lighter-duty books available, which might be better to start with.

When adding any new wiring, you should obtain an electrical permit and inspections.

Before you hurt yourself, read our Disclaimer.



Choice Of Circuit For Smoke Detectors:

This is my understanding of the rules and/or recommendations for smoke detector circuits:

- Smoke detectors should not be on the same circuit as receptacles (outlets). Why? Because some heavy-current-drawing appliance could be plugged into the outlet and trip the circuit breaker, rendering the smoke detectors powerless.

- Smoke detectors should not be used on a circuit that is dedicated to just smoke detectors, with nothing else on the circuit. Why? Because if the circuit breaker gets tripped, there will be no unworking lights or outlets to alert you of any problem.

- Smoke detectors should be connected to a lighting-only circuit.

- Some electricians prefer to connect smoke detectors to whatever lighting circuit is used for any 3-way lights in the house (such as stairway or hallway lights). Since 3-way switch wiring typically involves 14-3 wire, and smoke detectors need 14-3 wire, it makes sense to group these together. If the circuit breaker ever gets tripped, you will know something is wrong because one or more lights won't work.

- In theory all this concern over tripped circuit breakers shouldn't matter because today's electrical code requires AC-powered smoke detectors with battery backup. I guess that the National Electrical Code people don't want to rely on battery power. Or... these code rules were written quite a few years ago, when AC smoke detectors were common but battery-backup AC-powered detectors were not common.


More Info:

Tools Used:
  • Utility Knife
  • Diagonal Wire Cutters
  • Wire Strippers
  • Needle-Nose Pliers
  • Lineman's Pliers
  • Phillips Screwdriver
  • Nutdriver, 5/16"
Materials Used:
  • Smoke Detector, AC Powered With Battery Backup
  • Twist-On Wire Connectors (Wire Nuts)
  • Ground Screws
  • 8-32 Machine Screws, Assorted Lengths
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Written February 15, 2010