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How to Add More Smoke Detectors

Summary: Adding smoke detectors to your home existing smoke detector system will help bring your home up to current codes while providing additional protection to your home and family and adding value to your home as well.

Adding 3-Wire Smoke Detectors to a 2-Wire System

The 2-Wire System - Older Homes Many homes still only have one 2-wire smoke detector usually found in the hallway leading to the bedrooms or a stairwell.
Adding additional smoke detectors in other required areas such as individual bedrooms while converting the original smoke detector is relatively easy, as shown in the installation photos shown below.

A typical construction project will require a smoke detector in each bedroom and on the ceiling or wall at a point centrally located in the area giving access to bedrooms such as hallways.

Mount smoke detectors at least 4 inches from wall or on the wall with the top of the detector within 4 inches and 12 inches of the highest point of the ceiling. In multi-story units there shall be a detector at each level and shall be placed at the center of the ceiling 24 inches or more higher than the hall are required to have a smoke detector on the hall ceiling and in the bedroom. In new construction when more than one smoke detector is required to be installed within an individual dwelling, the detectors shall be interconnected in such a manner that the actuation of one alarm will activate all of the alarms in the dwelling.
Below you will see some photos of recent projects that I have selected to help you with your project.

Smoke Detector Plan

check local code reqs

Take a look at a typical Smoke Detector installation

After the circuit is identified and shut off the wires are capped off with wire connectors for an extra measure of safety.
A 3-wire romex is installed to each location linking the detectors together. The wire size must match the existing smoke detector. Even though most detectors do not require a ground, always install and bond the ground conductors together.

The new 3- wire romex is installed from the existing junction box to the new smoke detector location. Carefully install the romex along existing rafters or other supports stapling the wire firmly every 3 feet or less.

Our new location has the cut out with the new wire ready for the cut-in box. Installing the wire and aligning the box into place. The new box is fastened into place and the new wire is ready for attaching to the new smoke detector

The wires are stripped and ready to make up. Notice that the ground wire is attached to the ground screw provided in the junction box. After the mounting adapter is screwed to the cut in box the wire plug which came with the smoke detector is attached using wire nuts. With the smoke detector plugged in the smoke detector is attached to the mounting adapter.

Back at the existing smoke detector junction box the new wires are ready to be attached to the existing wires, blacks to blacks, whites to whites and the grounds bonded to each other and to the metal junction box. The new red wire of the 3-wire romex is attached to the smoke detector's yellow wire and is used to send the alarm signal between all the smoke detectors that will be installed. The adapter plate and plug are ready fro the new smoke detector which takes the place of the old smoke detector which did not have the third wire capability to interconnect all the smoke detectors together

The power is turned back on and the green power indicator LED lights up. To smoke detector is tested by pressing the test button until the alarm sounds. The alarm usually sounds once then sends the signal to the other smoke detectors causing them to all go into alarm for a short test period.

* Not all smoke detectors are exactly alike, so be sure to carefully read the instruction sheet that came with your brand of smoke detectors.


Questions about Adding Smoke Detectors

Questions about Wiring Smoke Detectors

Question from Mike, an Electrician from Patchogue, New York:

Can the carbon monoxide alarm be connected to the smoke detectors in series and be the last on the line? I haven’t been on the job for a few years and I just want to make sure that is how it is done again. I know the smoke detectors are in series, but I forget if the carbon monoxide can be connected to it as well.

Dave’s Answer:

Many local areas require permanently installed carbon monoxide detectors with battery backup which are used to detect odorless gas which could be fatal. Check with your local building jurisdiction for the specific requirements, and if combining the circuit wiring of carbon monoxide detectors and smoke detectors is permitted.
Citation Reference: NEC NFPA 72 Article Section 29.5

Question from Mike, a Homeowner from Twin Falls, Idaho:

I want to install smoke alarms. Can I use 14/3 wire or do I have to use 12/3?

Dave’s Answer:

Smoke alarms may be wired from a circuit using 14/3 wire.


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