Code-compliant approach to wiring smoke and CO alarms

Where the code actually lives

Smoke and CO alarms sit at the intersection of NEC Article 760 (fire alarm systems), NFPA 72 (National Fire Alarm Code), NFPA 720 (now folded into NFPA 72 for CO), and the local building code adopted by the AHJ. For single and two-family dwellings, you are almost always working with single or multiple-station alarms powered by the branch circuit, not a listed fire alarm system, so 760 does not apply the way it would in commercial work.

Check the adopted edition before you rough in. IRC R314 and R315 drive placement and interconnection in most residential jurisdictions, and NFPA 72 Chapter 29 governs household installations. Commercial occupancies flip to NFPA 72 Chapters 10, 17, 18, 23, 24, plus NEC 760 for the circuits themselves.

Branch circuit selection for residential alarms

The hardwired 120V supply to interconnected smokes and combo units is a general-purpose branch circuit. Most jurisdictions and manufacturer instructions require a dedicated or shared lighting circuit that is not protected by a GFCI or AFCI in a way that compromises alarm function. Current NEC 210.12 requires AFCI on most dwelling branch circuits, and alarms listed for use on AFCI circuits are now the norm. Verify the product listing before you pull wire.

Do not land alarms on a circuit fed from a switched outlet, a garage door opener, or anything a homeowner can kill at a switch. The circuit must be always-hot, and the disconnect must require a tool.

• 120V, 15A or 20A general lighting circuit, always energized
• AFCI per 210.12(A) unless the listing prohibits it
• No GFCI protection unless the alarm is listed for it
• No switched legs, no load control, no smart relays upstream
• Neutral and equipment ground landed per 250.148 at every box

Interconnect wiring and conductor count

Traditional interconnect uses 14/3 or 12/3 with ground between alarms: hot, neutral, and a dedicated traveler (usually red) for the interconnect signal. Mixing manufacturers on the same traveler is a listing violation. Most units cap the loop at 12 alarms or 18 devices total including relays and remote sounders, but the spec sheet is the final word.

Wireless interconnect modules have changed rough-in. You still pull 14/2 to every location for 120V power, and the RF module handles the signal. That lets you retrofit without fishing a third conductor through finished ceilings.

Field tip: label the red traveler at the panel and at every J-box. On a service call two years later, the next electrician will thank you when they are not chasing a red wire that could be a three-way traveler, a switch leg, or an interconnect.

Placement that will actually pass

NFPA 72 29.8 and IRC R314.3 require a smoke alarm in every sleeping room, outside each sleeping area within 21 feet of the bedroom door, and on every level including basements. CO alarms per IRC R315.3 go outside each sleeping area and on every level with fuel-burning appliances or an attached garage.

Keep smokes at least 3 feet from HVAC supply registers, 3 feet from bathroom doors containing a tub or shower, and 10 feet from cooking appliances, or use a photoelectric listed for closer placement. Ceiling-mounted alarms sit 4 inches minimum from any wall. Wall-mounted units go 4 to 12 inches below the ceiling.

1. Every bedroom, centered when practical, away from ceiling fans
2. Hallway outside sleeping areas, within 21 feet of every bedroom door
3. Every story including finished basements, near the stair head
4. CO within 10 feet of every sleeping area entry
5. Not in kitchens, bathrooms, unconditioned attics, or dead air spaces

Box fill, support, and finish

Most alarm bases land on a 4-inch round or octagon box. Run box fill under 314.16: count each ungrounded and grounded conductor, add one for all grounds combined, one for the device yoke if applicable, and one for each internal cable clamp. A 4-inch round 1-1/2 inch deep box gives you 21.5 cubic inches, which is usually enough for two 14/3 cables plus the alarm pigtail, but do the math on three-ways.

Support the box per 314.23. If the alarm is ceiling-mounted and over 6 ounces, which most combo units exceed once you add the battery, the box must be listed for the weight. A bar hanger between joists is the clean answer. Do not rely on old-work brackets in drywall for combo units.

Battery backup, commissioning, and handoff

Every hardwired alarm in a dwelling needs a secondary power source per NFPA 72 29.6.3. Ten-year sealed lithium is now the default and is required in several states regardless of NFPA edition. When you replace a single alarm in an interconnected system, replace all of them if they are past 10 years from the manufacture date printed on the back.

Commissioning is not optional. Energize the circuit, wait for the green power LED on every unit, then trigger one alarm and confirm every other alarm in the loop sounds within 10 seconds. Document the test, the date codes, and the circuit number on the panel directory.

Field tip: photograph the back of each alarm showing the manufacture date before you install it. That one photo settles warranty disputes and proves code compliance if an inspector or insurance adjuster ever asks.

Permits, inspections, and the paper trail

Alarm work is almost always part of a permitted electrical or building scope. For replacements in kind, many jurisdictions allow it under a minor work permit or no permit at all, but new circuits, new locations, or whole-house upgrades trigger a full inspection. Check with the AHJ before you quote the job.

Leave the manufacturer instructions in the panel with the load calc and the permit card. Inspectors read them. So does the next electrician.