How to Install a Hardwired Smoke Alarm – Part 7

This tutorial explains how to install a hardwired smoke alarm in a bedroom drywall ceiling. The branch circuit for the new smoke alarm is connected to the main junction box for power and signal. This project is continued from How to Install a Hardwired Smoke Alarm – Part 6.

Smoke Alarm Junction Box Wiring Diagram

Now I’ll make the final wiring connections to the main junction box in the unfinished area of the basement for the new smoke alarm circuit per the following wiring diagram. At present the new smoke detector NM-B 14/3 black, white and red wires are not connected to the junction box in the center of the diagram:

Smoke Alarm Wiring Diagram: Branch Circuit Junction Box Wire Splice

Smoke Alarm Wiring Diagram: Branch Circuit Junction Box Wire Splice

Electrical Safety

I double checked the circuit breaker serving the smoke alarm junction box was still Off, then verified my non-contact voltage tester was working by checking a live NM-B 14/2 cable for the basement lights – which is on a different breaker to ensure it’s not broken the batteries are dead. You can also test the voltage detector with a wall outlet. The voltage detector lighted up and beeped repeatedly indicating a live wire:

Non-Contact Voltage Detector

Non-Contact Voltage Detector

Next I checked the junction box wires for voltage with the non-contact voltage detector; no light, no beeps means the electricity is Off and the wiring is safe to work on:

Smoke Alarm Junction Box Wiring: No Voltage / Electricity is Shutoff

Smoke Alarm Junction Box Wiring: No Voltage / Electricity is Shutoff

Smoke Alarm Branch Circuit Wire Spice

The new bedroom smoke alarm circuit is wired in parallel at the main junction box as follows:

  • Match and wire like colors together: red (alarm signal), white (neutral) and black (hot).
  • Remove the wire nut from the existing wires.
  • Strip 3/4″ of insulation from the wire.
Smoke Alarm NM-B 14/3 Junction Box Wiring: Red Communications Wire

Smoke Alarm NM-B 14/3 Junction Box Wiring: Red Communications Wire

  • Twist the new NM-B 14 gauge wire to the other wires of the same color with linesman pliers.
    Linesman pliers have extra wide flat jaws especially suited for gripping and twisting wires.
  • Trim the end of the new NM-B 14 gauge wire even with the existing wires if necessary.
Smoke Alarm Junction Box Wiring: Connect the Red Signaling Wires

Smoke Alarm Junction Box Wiring: Connect the Red Signaling Wires

  • After twisting the NM-B 14 gauge copper wires together, hold the smoke alarm AC Quick Connector red wire even with the wires and twist on an appropriated sized nut. I’m using Ideal 76B Red Wire Nuts.
    • The AC Quick Connector smoke alarm wire is soft stranded wire; twist this around the NM-B 14 gauge wires with your fingers, no need for linesman pliers.
  • Strip 3/4 inch of insulation from the white (green box in photo) and black (light blue box in photo) wires.
Smoke Alarm Junction Box Wiring: Wire Nut Connection

Smoke Alarm Junction Box Wiring: Wire Nut Connection

  • Connect the new NM-B 14 gauge white wire and AC Quick Connector white to the white junction box wires using the same method.
  • In the next photo, the white (neutral) wires are connected.
    Now I’m making the NM-B 14 gauge black (hot) wire connections.
Smoke Alarm Junction Box Wiring: Black (Hot) Wire Connections

Smoke Alarm Junction Box Wiring: Black (Hot) Wire Connections

Fold the wires in the octagon junction box after finishing the wiring connections. Ensure the AC Quick Connector is hanging out of the box:

Smoke Alarm Junction Box Splice Wiring: Fold the Wires in the Box

Smoke Alarm Junction Box Splice Wiring: Fold the Wires in the Box

Remount the smoke alarm trim plate and tighten the two screws:

Install the Smoke Alarm Trim Plate

Install the Smoke Alarm Trim Plate

Plug the AC Quick Connector into the smoke alarm unit and mount it to the trim plate with a twist until locked. Turn on the circuit breaker and observe the green light is illuminated on all smoke alarms.

Kidde/Firex Hardwired Smoke Alarm Installation

Kidde/Firex Hardwired Smoke Alarm Installation

Smoke Alarm Testing

Visit every smoke alarm in the home and press the Test button. ALL alarms should sound. I have seven interconnected smoke alarms in my home (basement, main floor and 2nd floor). The other alarms sounded slightly out of phase, sort of like a (very loud) echo. Everyone will be annoyed and fuss. Nice! Due to the distance between floors and quantity of smoke alarms, I asked my son to be on the other floors and called him on the cell phone to coordinate the tests and confirm each alarm was triggered.

Be safe,

Bob Jackson

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