How to Install Hardwired Smoke Detectors

Smoke detectors are required by the residential electrical code nearly everywhere, and for good reason: Research demonstrates that smoke d...

Smoke detectors are required by the residential electrical code nearly everywhere, and for good reason: Research demonstrates that smoke detectors save lives and property by alerting occupants to early, smoldering fires.1 House fires often begin so quietly that occupants have no idea that the home is on fire until it is too late. Smoke may kill occupants long before flames are visible. Smoke detectors remain awake all day and all night, continually sensing even the faintest evidence of smoke and fire.

Smoke detectors work correctly only when they have a continuous electrical charge, which can be provided in one of two ways. Smoke detectors come in two styles: battery-only smoke detectors that are powered by replaceable batteries, and hardwired smoke detectors that are powered by household circuits, with internal backup batteries that step in should the power source fail.

Consult your local permitting agency for codes and regulations regarding hardwired smoke detectors. In some communities, hardwired smoke detectors are increasingly required in new construction and home remodeling work. Even if the building code does not require you to install hardwired smoke detectors in your home, you may wish to consider doing so.

What Is a Hardwired Smoke Detector?

The term “hardwired” refers to any electrical fixture or appliance in which a circuit cable runs directly into an electrical connection box on the device rather than with a cord that plugs into an electrical outlet. From the outside, hardwired smoke detectors look much like battery-powered smoke detectors and are located in the same areas within the home. The difference is that hardwired smoke detectors include an electrical cable that runs unseen behind the ceiling or wall directly into the back of the smoke detector. The electrical cable provides power to the smoke detector all the time, except in the event of a power failure. Should the power fail, an onboard battery takes over and continues to power the smoke detector.

Before You Begin

You can install a hardwired smoke detector yourself, but only if you have a good familiarity with home electrical work and an understanding of electrical circuits before attempting this project. If you don’t have this experience and understanding, it’s best left to a professional electrician to install and replace hardwired smoke detectors.

The electrical code does not require that hardwired smoke detectors need to be connected to a dedicated circuit, though there is nothing wrong with installing a new circuit for this purpose. More often, though, hardwired smoke detectors are installed by splicing into a general lighting circuit or outlet circuit. Either a 15-amp circuit (wired with 14-gauge wire) or a 20-amp circuit (wired with 12-gauge cable) is acceptable for powering hardwired smoke detectors.

Wiring the smoke detectors is fairly straightforward for an experienced DIYer or a professional electrician. First, old-work electrical boxes are installed at appropriate spots where the smoke detectors will be installed. Then, a 2-wire cable is run from the power source to the first smoke detector. This power source can be a circuit breaker panel, an existing wall outlet, a wall switch, or a light fixture that has pass-through wires. Next, 3-wire cables are installed to link the smoke detectors in sequence. Then, the various wire connections are made and the devices are installed.

All standard precautions should be followed when working with electrical circuits. Power must be shut off and tested for voltage before making any feed wire connections.

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Equipment / Tools

6-foot step ladder
Tape measure
Pencil
Stud finder
Drywall saw
Cordless drill
Fish tape
Cable ripper
Wire stripper
Voltage tester

Materials

Hardwired smoke detectors
Old-work electrical boxes
UL-approved wire connectors
2-wire NM cable
3-wire NM cable

The Spruce / Hilary Allison

Mark Locations for the Smoke Detector Boxes
Find the best locations for the smoke detectors. Ceilings are generally the best place for smoke detectors since smoke rises. If installing the detector on a wall, install it within 12 inches of the ceiling. The manufacturer’s instructions, as well as your local building code, will have recommendations for where the alarms should be installed.
First, use a stud finder to locate the ceiling joists or wall studs. Hold the electrical box backward and use the perimeter of its face as a template to draw a cutout around the perimeter. With old-work boxes, be sure to locate the boxes between joists or studs, not over them.
Make Cutouts
With a drywall saw, cut openings in the drywall to hold the electrical boxes for the smoke detectors. You may need to drill a pilot hole to establish a starting point for the saw.
Run NM Cable to the First Box
From the power source, run a 2-wire cable (with ground) to the first box location. The power source can be any one of several locations:
- An existing wall outlet
- A wall switch
- A ceiling light fixture with a pass-through circuit cable that is not controlled by a switch
- The circuit breaker panel
Fishing cable through walls is often the most difficult part of the installation. It can take some ingenuity to figure out the best route for the cables to run. Some professional electricians prefer to fish cables up into attic spaces, along ceiling joists, then down into the smoke detector boxes. This method may require more cable, but it can be much easier to fish the cables in this way.
As you route the cables, leave a good amount of excess cable extending through the hole in the drywall. You will cut it down to the proper length during the installation of the boxes.
Run Cables to Other Boxes
Beginning at the first smoke detector box location, run 3-wire cable (with ground) to each subsequent smoke detector. The extra wire in these cables allows the smoke detectors to “talk” to one another, so that all detectors will sound the alarm when any of them detect smoke or fire.
Again, leave plenty of excess cable extending through the holes in the drywall.
Insert Cables into Electrical Boxes
At each box location, first cut away the excess cable so that about 8 inches of wire will extend into each electrical box. Then, strip away outer sheathing from the cables using a cable ripper, and insert them into the electrical boxes so that about 1/2 inch of cable insulation extends past the clamps and into the box.
The way in which the cables are clamped into the boxes will vary, depending on the type of box you are using. With some metal boxes, you may need to install a cable clamp in knockout openings on the box, while other types have internal clamps that hold the cables.
Mount the Boxes
With cables secured in each box, install each old-work box into the opening in the drywall. Tighten the screws in the box, which will draw the retaining tabs up tight against the back of the drywall, securing it in place.
Install Mounting Plates
At each smoke detector location, feed the circuit wires through the detector’s mounting plate. Then, align the screw holes on the mounting plate with the holes in the electrical box, and use the included screws to secure the plate to the box.
Connect the Smoke Detectors
In the first box, use wire nuts or other UL-approved wire connectors to connect the wire leads on the smoke detector:
- Join the black circuit wire(s) to the black wire lead on the smoke detector.
- Connect the white circuit wire(s) with the white wire lead on the smoke detector.
- Connect all bare copper ground wires together. If you are using a metal box also attach a bare copper or green insulated pigtail wire to the box.
- Connect the red circuit wire(s) to the interconnect (traveler) wire from the smoke detector, usually marked as yellow.
Carefully tuck the wires through the mounting plate, then secure the smoke detector to the mounting plate, following the manufacturer’s directions. For most detectors, you will need to slide notches on the detector into slots on the base, then twist to secure the body of the smoke detector to the mounting plate.
Tip
Connection methods for smoke detectors vary from model to model, so make sure to follow the manufacturer’s directions. On some models, the wire connections are made to a mounting plate that has a socket into which the smoke detector is plugged. In other models, the wire connections are made directly to the smoke detector.
Install the Backup Smoke Detector Battery
Insert the battery into the smoke detector, aligning positive and negative terminals in the correct configuration. Replace the cover.
Connect to the Power Source
The trickiest part of the installation is connecting the feed cable that brings power to the first smoke detector to its power source. This is where some DIYers may choose to have a professional electrician make the hookup.
If you are feeding the smoke detectors off an existing outlet or light fixture circuit, this connection is made by turning off the circuit and testing for power, then opening an outlet, light switch, or ceiling fixture box and pigtailing into the circuit wires to join the smoke detector cable to the circuit.
It’s important, though, that you splice into circuit wires that are not controlled by a wall switch. This means that if you are splicing in at a light fixture box, it must be a box where there are pass-through wires that are not controlled by a wall switch. If splicing in at a wall switch, you must connect to the black feed wire that feeds the switch, not the outgoing wire that brings power to the light fixture.
If you are installing a new circuit, the feed connection involves installing a new AFCI-protected circuit breaker to feed the smoke detectors.

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