Project Guide

How to Install Smoke Alarms

Cut Holes for Remodel Boxes
Someone cutting a hole in a ceiling for a smoke alarm.
  • Identify where the smoke alarm will be installed. 
  • Use a stud finder to determine the location of wall studs around each alarm location. Mark a spot for each alarm in the open space between studs on the wall 12-inches down from the ceiling. 
  • Use a marking tool to trace the outline of the remodel box on the wall. Use a level to make sure the box outline is straight. 
  • Drill a 1/2-inch starter hole at the corner of your outline. 
  • Starting at the hole, cut along the outline with your drywall saw. 
  • Test the fit remodel box. It should fit into the hole snuggly, but not so tightly that you have to force it. Do not set the remodel box yet. 
  • Repeat this process at each location where an alarm will be installed.
Run the Cable to the First Alarm
A man running a cable from the ceiling to a smoke alarm.
  • See our Running Electrical Wire Behind Walls project guide for step-by-step instructions on how to run wires behind walls. 
  • Run Romex 14-2 NM-B cable from the breaker panel to the location of the smoke alarm closest to the breaker panel. Do not connect the cable to the breaker panel yet. You’ll do this at the end of the project. 
  • When the cable reaches the alarm location, pull about 10-inches of the cable through the knockout of the first remodel box.
Install the Next Runs of Wiring
A man wiring a smoke alarm in a ceiling.
  • Run Romex 14-3 NM-B cable from the first alarm to the second alarm, leaving 10 inches of cable hanging out of the work box. Unscrew the faceplate from the starter outlet. 
  • Run Romex 14-3 NMB-B wire from the second box to the third, from the third to the fourth and so on until you reach the last alarm.
Secure the Cable and Set the Box
A person securing the cable box of a smoke alarm.
  • At the last alarm, pull roughly 10-inches of the cable through the knockout of the remodel box. 
  • Secure the cable in the box by tightening the wire clamp in the box. 
  • Set the box in place. 
  • See our Installing Remodeling Boxes project guide for detailed instructions.
Strip the Wires
A man using wire cutters to strip a wire.
  • Still working at the last remodel box, use wire strippers to cut the NM-B cable’s outside sheath and any paper insulator to expose the black, white, red and bare copper wires within. 
  • Attach the copper wire to the box with a ground screw. 
  • Use wire strippers to strip 3/4-inch of insulation from the ends of the wires.
Wire the Alarm
Wires for a smoke alarm hanging from a hole in the ceiling.
  • The alarm has two parts: the alarm itself and the harness with wires coming out of it that plug into the alarm after the wires have been connected. Use the harness with black, white and red wire for the following steps. Do not attach the plug to the alarm yet. 
  • Connect the black wire coming out of the box to the black wire on the harness and secure the wires together with a wire connector. 
  • Connect the white wire to the white wire on the harness in the same way. 
  • Connect the two bare copper ground wires together and connect them to the remodeling box. 
  • Connect the red wire from the harness to the red wire coming out of the box. 
  • Secure all wiring with wire connectors. 
  • Wrap electrical tape around the base of each wire connector.
Attach the Mounting Plate
A person attaching the mounting plate on a smoke alarm.
  • Place the alarm’s mounting plate on the remodel box and align the screw holes. Make sure the wire connection runs through the mounting plate. 
  • Carefully put the connected wires and plug through the mounting plate. 
  • Connect the wired harness to the back of the smoke alarm. These are usually configured to ensure they can only fit one way. 
  • Use appropriate sized screws to connect the mounting plate to the remodel box.
Attach the Alarm to the Base
A person attaching a smoke alarm to the ceiling.
  • Line up the notches on the back of the alarm with those on the base. 
  • Twist the alarm until it locks into place. 
  • Check the connection to ensure it is secure on all sides.
Connect and Mount the Remaining Alarms
A person wiring a smoke alarm.
  • Continuing to work back toward the alarm closest to the breaker panel, repeat the actions in steps 4 and 5. 
  • At each location, connect the wires according to color. Connect the black wire on the harness to the black wire coming out of the box. Connect the white wire on the harness to the white wire coming out of the box. Connect the red wire on the harness to the red wire coming from the box.
  • Secure all wiring with wire connectors. 
  • Wrap electrical tape around the base of each wire connector. 
  • Connect the harness to the smoke alarm. 
  • When you reach the alarm closest to the breaker panel, connect the black wires, the white wires and the red wires. 
  • Mount each alarm as detailed in steps 7-9.
Connect at the Breaker Panel
A man adjusting wires at a breaker panel.

Connect the 14-2 NM-B cable to the beaker panel following the step-by-step instructions in our Adding a New Circuit project guide.

Test the Alarms
A person pressing the button on a smoke alarm.
  • Turn on the power to the circuit. 
  • Confirm each alarm’s power light is on. 
  • Press the test button on each alarm. You should be able to hear all the alarms each test. 
  • If the alarms don’t sound, turn off the power at the breaker panel and double-check your wiring connections. If the connections are correct but a unit’s power light won’t come on, replace the unit.
Install Backup Batteries
A person installing the battery for a smoke alarm.
  • When you have confirmed all alarms are operating correctly, open the battery compartment cover on each alarm. 
  • Install a battery, being careful to position it so positive contact meets positive contact and negative contact meets negative. 
  • Close the battery compartment.