Safety guide for installing a smoke detector circuit

Circuit selection and load planning

Smoke detectors in dwellings fall under NEC 210.12 and must be on an AFCI-protected circuit when installed in bedrooms, hallways serving bedrooms, and most other habitable spaces. Hardwired 120V smokes are almost always fed from a general-purpose 15A or 20A lighting circuit, never a dedicated GFCI receptacle circuit or a circuit that can be killed at a wall switch.

Do not share the smoke circuit with anything a homeowner is likely to kill at the panel. Shared lighting is fine because a dark room gets noticed fast. A dedicated circuit is not required by the NEC, but many AHJs in California, Massachusetts, and parts of the Northeast push for one on new construction. Check locally before you pull wire.

Sizing is simple. A typical interconnected string of ten 120V photoelectric or dual-sensor smokes draws well under 1A total, so conductor ampacity is never the constraint. The constraint is the AFCI breaker and the interconnect conductor count.

Conductors, boxes, and interconnect

Run 14/3 with ground for interconnected smokes on a 15A circuit, or 12/3 on a 20A circuit. The red conductor is the interconnect (IC) signal line. All smokes in the string must share the same neutral and the same IC wire, and they must all be fed from the same branch circuit per NEC 300.3(B) and the manufacturer's listing.

Box fill matters. A single-gang 4 inch round or octagon box with a smoke mounted to it is tight once you bring in a 14/3 in and a 14/3 out. Use a deep box, 21 cubic inches minimum for two 14/3 cables plus the device. NEC 314.16(B) gives you the math: count each conductor, add two for the device yoke, add one for all grounds combined.

• 14/3 in: 3 current-carrying plus ground
• 14/3 out: 3 current-carrying plus ground
• Device: 2 conductor volumes
• Grounds: 1 conductor volume total
• Minimum fill: 9 x 2.0 cu in = 18.0 cu in

Placement per NFPA 72 and local amendments

The NEC points you to NFPA 72 for placement. You need a smoke in every sleeping room, outside each separate sleeping area within 21 feet of the bedroom door, and on every level including basements. On vaulted or tray ceilings, mount within 3 feet of the highest point measured horizontally, not on the peak itself where a dead air pocket forms.

Stay out of the dead air space where walls meet ceilings. Wall-mount units go 4 to 12 inches from the ceiling. Ceiling mounts go at least 4 inches off the wall. Keep smokes at least 3 feet from bathroom doors, supply registers, and ceiling fan blade tips to cut down on nuisance alarms.

Field tip: in kitchens and laundry rooms, spec photoelectric only or a combo unit with a 10 foot minimum standoff from the cooktop and dryer. Ionization heads within 20 feet of a range will nuisance-trip every Thanksgiving.

Power, battery backup, and AFCI

NFPA 72 and most state building codes require primary power from the building wiring with a secondary battery backup. Sealed 10-year lithium backups are now standard on new installs and are mandatory in California (SB 745) and a growing list of states. If you are swapping heads in an existing string, match the manufacturer and model family, or replace the entire string. Mixing brands on a common IC line is a listing violation and often will not communicate.

AFCI nuisance tripping on smoke circuits is almost always a shared-neutral issue or a staple that nicked the cable jacket. Meg the circuit before you energize. If the AFCI trips on first energization with nothing connected, you have a wiring fault, not a bad breaker.

1. De-energize and lock out the branch circuit at the panel
2. Disconnect the neutral at the breaker and megger line-to-ground and line-to-neutral, 500V DC minimum
3. Look for less than 1 megohm, which indicates a pinched or stapled cable
4. Reconnect, energize, and test each head with canned smoke, not the test button alone

Testing, commissioning, and sign-off

The test button on a smoke only verifies the horn and the electronics, not the sensing chamber. Use listed aerosol smoke for photoelectric heads and listed aerosol CO for combo units. Trigger one head and confirm every other head on the IC line alarms within 10 seconds. If a head does not join the chorus, you have a broken red wire, a reversed polarity, or a head from a different family.

Document the install: circuit number, AFCI model, head manufacturer and date code, and the date you pressed the aerosol. Many jurisdictions now want this on the rough and final inspection card. California Title 24 inspectors will ask for the date code to verify the 10-year clock.

Field tip: write the install date on the back of each head with a silver Sharpie before you mount it. Saves you a ladder trip in year nine when the chirp starts.

Common callbacks and how to avoid them

The three calls that come back are chirping at 3 a.m., random full-house alarms, and AFCI trips after a storm. Chirping is almost always end-of-life or a loose battery door on older units. Random alarms trace back to dust, spider webs, or a unit mounted too close to an HVAC supply. Storm-related AFCI trips point to a shared neutral on a multiwire branch circuit that was never properly handled under NEC 210.4(B).

Train the homeowner before you leave. Show them the hush button, the test button, and the location of the breaker. Leave a laminated card with the head model, install date, and replacement date. A five minute walkthrough kills 80 percent of the callbacks on this work.