How to Install a Smoke Detector (Step by Step)

Before you open a single box, know what your local code wants. Most US jurisdictions require detectors that are hardwired with battery backup, interconnected on a three-wire daisy chain so they all sound when any one trips, and placed in every bedroom, every hallway connected to a bedroom, and on every level of the house. Hallways usually need a combination smoke + carbon monoxide unit.

Plan your runs first. The first detector in the chain only needs two-wire power coming in. Every detector after that runs on three-wire so the interconnect signal can pass through. Mark each ceiling location with a pencil before you start cutting drywall or fishing cable.

Pop the box and you should find four things: the round mounting flange (bracket) that screws to the ceiling, the wiring harness pigtail that connects the house wires to the detector, a plastic dust cover for use during construction, and the detector itself.

Wire nuts are not usually included. Grab a small bag of orange and red wire nuts before you start - you will need at least two sizes (smaller for the red interconnect pair, larger for the white and black pairs).

At the breaker, kill power to the detector circuit and verify dead with a non-contact voltage tester. Climb the ladder, gently work the cables out of the ceiling outlet box, and let them hang down where you can reach them. For the first detector in the chain you will have one two-wire cable (power in). For middle detectors you will have two three-wire cables (one from the previous detector, one heading to the next).

Address the ground wires first. The First Alert BRK detectors used in this video are not grounded, so tie all the bare copper grounds together with a wire nut and tuck them up out of the way.

Strip the outer sheathing off the wiring harness so you have three loose conductors: white (neutral), black (hot), and red (interconnect). The harness comes pre-slit so you can just peel the jacket back. The red wire on the harness has a factory tip you need to strip with wire strippers before you twist.

Hold the matching colors together - all the whites in one bundle, all the blacks in another, all the reds in another - and twist them with a pair of lineman's pliers until you have a clean spiral about an inch long. Pre-twisting makes a much stronger connection than relying on the wire nut alone.

Wire nuts are sized by the number and gauge of conductors they grip. Use the larger orange or yellow nut for the white pair and the black pair. Use the smaller red nut for the red interconnect pair (it has fewer wires to grab so the nut has to be smaller to bite).

Twist the nut on clockwise until it is firmly snug. Do not gorilla it - the nuts crack if you go too hard. Once it stops turning easily, give each wire a tug. If anything pulls out, redo the connection. A loose wire nut is the most common cause of an intermittent detector and a failed inspection.

Tuck all the wired connections back up into the outlet box, leaving just the harness pigtail hanging down. Start two drywall screws by hand into the threaded holes on the outlet box and run them in with a drill until they stick out about a quarter inch.

Position the mounting bracket over the two screw heads with the keyhole slots lined up, then twist the bracket clockwise. The slots tighten down over the screws and lock the bracket in place. Snug the screws the rest of the way with a screwdriver - just enough to clamp the bracket flat against the ceiling. The flange is plastic and it will crack if you torque it too hard.

Snap the wiring harness into the matching socket on the back of the detector. Confirm the 9V backup battery is seated in its compartment - some detectors ship with a pull-tab you have to remove first. Lift the detector up to the bracket, line up the keyhole arrows, and twist clockwise until it clicks home.

Flip the breaker back on. The green power LED should come on. Push the test button firmly for two seconds and the detector should let out a loud chirp + alarm pattern. If the units are interconnected, every other detector in the house should sound at the same time. Set a recurring reminder to press the test button once a month and to swap the backup battery once a year per NFPA recommendations.