Quick reference for wiring smoke and CO alarms

Power Source and Branch Circuit

Hardwired smoke and CO alarms run on a 120V branch circuit per NEC 210.12 and the dwelling unit requirements in 760 and the manufacturer listing. The circuit must be dedicated where the manufacturer specifies, but most residential listings allow the alarms to share a circuit with general lighting in the same area. Avoid putting them on a circuit that gets switched, controlled by a GFCI/AFCI nuisance trip path, or fed from a kitchen or bathroom small appliance branch.

AFCI protection is required for the alarm circuit when it serves a dwelling unit bedroom, living room, hallway, or similar area under NEC 210.12(A). GFCI is not required unless the outlet location falls under 210.8. Use a combination AFCI breaker for new work, or an OBC AFCI receptacle only if the manufacturer permits.

Label the breaker clearly. Homeowners and the next electrician need to find it fast.

Interconnection Wiring

Code and most listings require all alarms in a dwelling to be interconnected so one trigger sounds them all (IRC R314.4, NFPA 72). For hardwired interconnect, run 14/3 or 12/3 NM between devices: black is hot, white is neutral, red is the interconnect signal. Match conductor size to the breaker, 14 AWG on a 15A or 12 AWG on a 20A circuit.

Wireless interconnect is permitted where the listed alarms support it, which saves time on retrofits where fishing a third conductor is impractical. Mixing brands on a wired interconnect bus is not allowed. The signaling voltage and protocol vary by manufacturer and a mismatch will either fail to trigger or damage the units.

Field tip: on a remodel, if you can only fish 14/2, use wireless interconnect modules from the same manufacturer rather than running an unsafe makeshift signal path through the ground.

Box Selection and Mounting

Use a 4 inch round or octagon ceiling box rated for the device weight. Most alarm bases mount with 3.28 inch on-center screw spacing, so confirm the box has the right ears or use an adapter plate supplied with the alarm. Plastic boxes are fine on wood framing; metal boxes need bonding per NEC 250.148 when the EGC passes through.

Box fill calculations under NEC 314.16 still apply. A typical alarm whip counts as one device with two or three current-carrying conductors plus the ground. Add the interconnect conductor when present.

• Ceiling: minimum 4 inches from any wall (NFPA 72)
• Wall-mount: 4 to 12 inches below the ceiling
• Sloped ceiling: within 3 feet of the peak, measured horizontally
• Keep at least 3 feet from HVAC supply registers and bath doors

Location Requirements per Dwelling

Smoke alarms are required in every sleeping room, outside each separate sleeping area within 21 feet of the bedroom door, and on every level including basements (IRC R314.3). CO alarms are required outside each separate sleeping area in the immediate vicinity of bedrooms, and on every level containing a fuel-burning appliance or attached garage (IRC R315.3).

Combo smoke/CO units satisfy both requirements at one location and reduce ceiling clutter, but verify the listing covers both functions for the install height. Some combo units are wall-only because the CO sensor needs a specific air column.

For new construction, the alarms must be hardwired with battery backup. Battery-only is allowed only on existing dwellings where no construction is occurring that would expose wiring (IRC R314.6 exceptions).

Battery Backup and Listings

Every hardwired alarm needs a backup power source per NFPA 72 29.6.3. Modern alarms use sealed 10-year lithium batteries that cannot be replaced; the whole unit gets swapped at end of life. Older 9V backup units are still legal where installed, but recommend a replacement when you see them on a service call.

Check the date stamp on the back. Smoke alarms expire 10 years from manufacture, CO alarms 7 to 10 years depending on model. An expired alarm is a code violation under NFPA 72 even if it still chirps and tests.

Field tip: write the install date on the alarm housing with a permanent marker the day you mount it. The date stamp on the back is the manufacture date, not the install date, and saves a callback when the homeowner asks why it is beeping in eight years.

Testing and Closeout

After power up, every alarm should chirp once and the green LED should hold steady. Press the test button on one alarm and confirm every interconnected unit sounds within 10 seconds. If only the tested unit sounds, the interconnect conductor is open, miswired, or the wireless pairing failed.

Document the install: alarm model, manufacture date, install date, and circuit number. Hand the test procedure to the homeowner and walk them through the monthly test and the hush button.

1. Verify 120V at each base before mounting the alarm head
2. Mount and twist-lock each alarm head onto its base
3. Restore power and confirm steady green LED on every unit
4. Test interconnect from at least two different alarms
5. Log model, dates, and circuit on the panel directory