Step-by-step: wiring smoke and CO alarms

Smoke and CO alarms are life-safety devices. The wiring is simple, but the code, the interconnect logic, and the placement rules trip up even experienced installers. This is the field-ready sequence for getting them installed correctly on a residential rough-in or retrofit.

Know what code applies before you pull wire

Smoke alarms fall under NFPA 72 and the local building code (usually IRC R314 for dwellings). The NEC governs the branch circuit feeding them. CO alarms are covered under IRC R315 in most jurisdictions. Combination smoke/CO units satisfy both when listed for the purpose.

Power requirements: hardwired with battery backup, on a dedicated or shared circuit that is not controlled by a switch, GFCI, or AFCI in a way that creates nuisance trips. NEC 210.12(A) requires AFCI protection in dwelling unit bedrooms, hallways, and similar areas, which is exactly where alarms live. Use a combination AFCI breaker and confirm the alarm manufacturer lists compatibility.

• NFPA 72: alarm spacing, interconnect, and sounder requirements
• IRC R314 / R315: locations required in new and altered dwellings
• NEC 210.12: AFCI protection for the supplying circuit
• NEC 300.3(C)(1): interconnect conductors must be rated for the circuit voltage

Locations and spacing

Every sleeping room, outside each sleeping area within 21 feet of any bedroom door, and one on every story including basements and habitable attics. CO alarms go outside each separate sleeping area and on every level with fuel-fired appliances or an attached garage.

Keep smoke alarms at least 3 feet from bathroom doors, supply registers, and ceiling fan blade tips. On a flat ceiling, keep the unit at least 4 inches off the wall. On a wall mount, the top of the alarm sits between 4 and 12 inches from the ceiling. On a vaulted ceiling, mount within 3 feet of the peak measured horizontally, but not in the apex dead-air pocket.

Field tip: frame the ceiling box before drywall. Retrofitting a smoke alarm into a finished ceiling with no backing is an hour of work you will not bill for.

Circuit and cable

Pull 14/3 or 12/3 NM-B between alarms. Two conductors carry line and neutral, the third carries the interconnect signal. Ground bonds through the cable as usual. Most residential jobs run 14/3 on a 15 A AFCI circuit shared with bedroom lighting, which satisfies the "not easily disabled" intent while giving you a circuit that gets used and noticed if it trips.

Do not land the alarms on a dedicated circuit that no one ever looks at. A dead breaker on a dedicated smoke circuit can go unnoticed for years. Sharing with bedroom or hallway lighting is common practice and code-compliant as long as the circuit is not switched at the alarm.

1. Home run from the panel to the first alarm box (or through a junction)
2. 14/3 daisy chain from alarm to alarm, black to black, white to white, red to red
3. Ground pigtailed at every box
4. Label the circuit "Smoke/CO Alarms" at the panel per NEC 408.4(A)

Interconnect: wired vs wireless

Traditional interconnect uses the red conductor. When any alarm in the chain detects smoke or CO, it energizes the red wire and every other alarm on that loop sounds. The limit is typically 12 alarms and 18 total devices (including relays) per NFPA 72 and most manufacturer listings. Mixing brands on a wired interconnect is a no-go: the signaling voltage and pulse pattern are not standardized.

Wireless interconnect modules pair alarms over RF so you do not need the traveler conductor between every device. Useful on retrofits where fishing 14/3 through a finished home is impractical. Still hardwire each alarm for primary power; the wireless module only handles the interconnect signal.

Field tip: on a retrofit, pull 14/2 to each new alarm location and use wireless-linked units. You get hardwire power and code-compliant interconnect without opening up ceilings.

Testing and commissioning

After energizing, press the test button on one alarm. Every interconnected unit in the dwelling should sound within 10 seconds. If one does not sound, check the red conductor continuity and confirm all units are the same brand and compatible model family.

Verify battery backup by pulling the harness on a live unit: the alarm should continue to function and chirp the low-battery warning when tested later. Document model numbers, install date, and circuit number on a sticker inside the panel door. The homeowner will not remember in ten years, but the next electrician will thank you.

• Test button on each unit, confirm full-chain response
• Trip the AFCI breaker, confirm alarms switch to battery and chirp
• Label circuit at panel and note alarm model in the job file
• Walk the homeowner through the hush button and expected end-of-life chirp

Common callbacks to avoid

Nuisance tripping from AFCI incompatibility is the number one callback. If the alarm manufacturer lists specific breaker brands, use them. Mixing photoelectric and ionization units in the same interconnect loop is fine functionally, but some listings restrict it, so check the instruction sheet before mixing.

Alarms installed too close to kitchens, bathrooms, or HVAC registers generate false alarms and get disabled by the occupant. That disabled alarm is now a liability. Measure, mount correctly the first time, and note any deviations on the final inspection walkthrough.