How to Wire Smoke Alarms in Parallel

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Wire Smoke Alarms in Parallel
Wire Smoke Alarms in Parallel (Image: tendringdc.gov.uk)

According to the National Fire Protection Association, smoke alarms should be installed in every bedroom, outside of each sleeping area and on every level in the home. In addition, if you have a home with more than one floor, it's important for the smoke alarms to be wired in parallel so if one goes off in the basement, every alarm will sound throughout the home. Why is this important? Because it is estimated that approximately 890 lives can be saved each year if all homes have working smoke alarms installed. Here is how to wire your smoke alarms for added protection.

Things You'll Need

  • Smoke alarms
  • Old-work boxes
  • 12-2 NM cable
  • 12-3 NM cable
  • Screwdrivers
  • Electrical wire stripper
  • Wire strippers
  • Wire connectors
  • Drywall saw
  • Non-metallic fish tape
  • Electrical tape

Determine how many smoke alarms your home will need. For the wire, you will need enough 12-2 NM cable to reach from the electrical panel to the first smoke alarm. From the first smoke alarm on, you will need 12-3 NM cable, so purchase only what you need of each.

Additionally, you want to have a dedicated circuit for your hardwired smoke alarms. There are some helpful guides in the Resources section below if you need help with running the new circuit or installing a new breaker.

At each smoke alarm location, you will have to cut away the drywall to install the old-work box. Hold the box where you want it installed, trace around it with a pencil and cut out the drywall. Do not set the box yet.

Fish the 12-2 NM cable from the electrical panel to the first smoke alarm. Do not make any connections in the electrical panel yet; that is the last step.

Fish the 12-3 NM cable from the first smoke alarm to the second alarm. Then fish another length from the second to the third, and so on and so on until you reach the last smoke alarm.

Starting at the last smoke alarm, pull the wire through the knockout on the old-work box until about 10 inches of wire is coming out of the box. Secure it by tightening down the wire clamp inside the box. Set the box in the ceiling and secure it in place by tightening the wing screws.

Use the ROMEX® stripper to slice the outer sheath of the NM cable, revealing the individual wires. Cut away the outer sheath and the strands of paper insulation. This should leave you with a black, white, red and bare wire. Secure the bare wire to the box using a ground screw.

Use your wire strippers to remove about 3/4 inches of insulation from the ends of each wire. Take the wiring harness that came with your smoke alarm and connect the black wire on the harness to the black wire coming out of the box. Connect the white wires the same way. Then take the yellow harness wire and connect it to the red wire coming out of the box. Make sure that all of your wire connectors are on tight and wrap a strip of electrical tape around the base of each.

Install the smoke alarm's mounting bracket to the old-work box and pull the wiring harness through the center access hole. Snap the wiring harness plug into the back of the smoke alarm and twist the alarm onto the mounting bracket until it snaps into place.

Again, we are moving backward from the last alarm to the first, so go to the next alarm in the sequence. Follow the same procedure for installing the box and prepping the wires. Here you will want to make the following connections:

The black wire on the wiring harness with the two black wires coming out of the box

The white wire on the wiring harness with the two white wires coming out of the box

The yellow wire on the wiring harness with the two red wires coming out of the box

Connect the two ground wires together.

Follow the same procedure for mounting the smoke alarm that you used for the previous one.

Follow this procedure until you reach the last smoke alarm (first from the panel). Here you will connect black to black, white to white, ground to ground and yellow to red.

With all of the smoke alarms now wired, make the connection at the electric panel. Again, the guide for that job is listed below.

With the circuit complete, turn the breaker on and check your alarms to see if each one's "ready" light is lit. If they are, test your smoke alarms by pressing the "test" button on one of them. Each alarm should sound. Press it again to cancel the test, insert the backup battery in each smoke alarm and you're finished.

Tips & Warnings

  • Batteries, even backup batteries, need to be replaced every year in smoke alarms.
  • Smoke alarms should be replaced every 10 years, even if they are still working.

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