Installing Hard-Wired Smoke Detectors

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  • 20 hours
  • Intermediate
  • 500
What You'll Need
Smoke detectors (wired)
Terminal (junction) boxes
14/3 Cables
14/2 Cable
Cable connectors
Wire nut connectors
5/8" (16mm) Drill bit
Utility knife
Wire stripper

Smoke alarms are a critical part of home safety, especially when most house fires occur at night or early morning when everyone is asleep. The way a house is set up and protected should be a major concern for everyone since it concerns the well-being and safety of the whole family and considering a system upgrade might not be such a bad idea, especially since detectors only have a 10 years lifespan.

Hard Wired or Battery Operated

Battery-powered detectors are easy to install and use since they don’t require terminal boxes or wiring, and they keep working during a power outage. They are a cheaper option for an older home where hard-wiring inside finished walls and ceiling can quickly become a costly project. They do have the set-back, however, to need immediate attendance when battery replacement is required, which should be once or twice a year unless it has a10-years lithium-powered battery (Figure 1a & 1b).

smoke detectors with wiring

Some battery-powered detectors are equipped with the wireless capability to communicate with other units through radio frequency transmission, making it a much more effective alternative than its solitary counterpart, by activating an alarm throughout every unit installed in the house, alerting everyone at the premises during a fire. Since they also rely solely on batteries, however, they should be purchased with the lithium batteries.

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The Hard-wired alarms are probably the best option while a new building is under construction. They are approved by construction code and are much easier to install in a new construction project while the wood framing is still exposed, but the terminal boxes and wiring required throughout the house make it more costly for the installation, but much less so than after the walls and ceilings are finished (Figures 2-4). Hard-wired alarms inter-connected together throughout the house will all sound at once when one unit is triggered by fire or smoke, but is much more reliable than the wireless system, since electrical interferences around the house or battery failure is never an issue, especially when equipped with the backup 10 years lithium batteries.

smoke detector wiring design

Planning Before Committing

Smoke alarms should be placed:

In hallways leading to a bedroom

In or near living areas such as den, living room, family room

In the kitchen but 10’ (3 M) away from any appliances

At the bottom of a staircase leading to an upper floor

Keeping in mind that dust, steam, and exhaust can trigger a nuisance alarm, the units should not be installed near kitchen appliances, in garages, or in bathrooms. Areas near AC units, doors, windows, ceiling fans, or register where drafts could affect their sensitivity should be avoided. Units should also be at least 3 ft. (1 M) from a furnace.

A Step by Step Installation for a System Upgrade

A Hard-Wired smoke alarm installation involves wiring the closest smoke detector to a 120 VAC source by tapping it from a distribution terminal box nearby (Figure 5), or to a dedicated circuit breaker at the main panel (Figure 6).

smoke detector outlet wiring

Preparing Each Location for Wiring

For each alarm:

1. The location of the alarm and the outline of the terminal box is traced on the drywall alongside a stud or joist.

2. A keyhole saw or any drywall saw is used to cut out the outlined section of the drywall.

3. A 5/8” (16 mm) hole is drilled where the wire will be fished through the wallplates.

4. The stud finder can also detect if there’s any Fire Block inside the wall. If so, a small piece of the drywall can be cut out large enough to be able to fish on the top and bottom of the Fire Block. A recess can then be bored and chiseled out of the exposed section of the Fire Block to let the cable across.

Cables Preparation

5. Starting with the electrical box that will receive the 120-Volts feed from an outlet or junction box, a 14/2 cable is to be fished through to that box with an extra 10” (250 mm) at both ends of each cable.

6. The rest of the wiring will be done using 14/3 Romex to connect all the alarms together. 14/3 cables are to be passed through the previously drilled holes to go to each one of the alarm locations, going from one location to the next and always with the extra 10” (250mm) at each cable end.

7. The cables’ jackets can then be stripped off of an 8” (200 mm) length from each end, and 1/2” (12mm) of insulation removed from each insulated wire.

Wiring the Alarms

smoke detector outlet wiring

8. Removing a knockout for each cable at every location, the wires can be passed through to the inside of the box. Using the knockouts at the back of the box will make it easier to set the box back into its final position through the drywall opening.

9. Each cable can be firmly secured in place using cable clamps.

10.The box can then be set in the wall flush with the surface.

11. The box is then screwed to the stud or joist.

12. To connect the alarm, all the black wires are gathered together and secured using a wire nut. This will be the line voltage feed.

13. All the white Neutral wires can now be gathered together and secured with another wire nut.

14. The green wire from the alarm is then hooked up to the bare copper wires from every cable in the box with a wire nut while securing to the ground screw in the box.

15. This leaves one red or yellow wire from the alarm and one loose wire from each cable except from the first 14/2 cable supplying the voltage. The extra wires (usually red) must be gathered together and secured into a wire nut enabling the system to transmit and receive the signal when one of the alarm is triggered by smoke or fire.

16. With that step completed and after placing all the wiring inside the terminal box, the alarm is secured to the box.

17. Steps # 9 to 17 must be followed for each one of the remaining alarms in the circuit.

Connecting the System

Once all the alarms are in place and wired, the circuit can finally be connected to its 120 Volts source at the junction box.

18. The circuit breaker to the terminal box to tap from is to be turned Off, to avoid personal injury.

19. The cover and the receptacle, if its the case, must be removed next, exposing the wiring. The wires can be pulled out to see inside the box.

20. One of the knockouts can be removed from the box and a cable clamp secured in its place.

21. The 14/2 cable can be put through it and secured in place with the cable clamp.

22. The 120-Volts line feed can be located and the black wire from the alarm connected to the black from the feed, the white from the alarm to the white from the feed, and the bare copper wire to the ground terminal screw. All connections are to be secured again with the wire nuts.

23. The box can finally be closed back up.

24. The circuit breaker can finally be switched back on, and the alarm system tried out.

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