Smoke + CO Alarm Code Compliance Reference

Why this matters

Smoke and carbon-monoxide alarm requirements come up in remodels, basement finishes, panel upgrades, addition permits, and pre-sale inspections. Failing to bring alarms to current code is the single most common reason a remodel permit fails final inspection in jurisdictions that adopt the IRC fire-protection chapter wholesale. The electrician who walks the home before quoting and includes the right alarm scope in the original estimate avoids the "wait, I have to do what?" conversation at final.

Where smoke alarms are required (residential)

Adopted in most jurisdictions through IRC R314 and NFPA 72:

• Inside each sleeping room
• Immediately outside each separate sleeping area (the hallway serving the bedrooms)
• On every floor including basements (habitable attics and basements with sleeping areas count as floors)
• Within 21 feet of any sleeping-room door, horizontally measured
• A new alarm is required when an existing one is replaced, when a remodel exposes wiring, or when an addition adds sleeping space

Where CO alarms are required

Adopted through IRC R315 and NFPA 720 in most state codes:

• Outside each separate sleeping area in the immediate vicinity of the bedrooms
• On every level of the home (some jurisdictions require this; some only the sleeping-area level)
• In any dwelling unit with a fuel-burning appliance, an attached garage, or a fuel-fired fireplace

The CPSC reports approximately 400 unintentional non-fire CO deaths per year in the United States from consumer products, and CO alarm code adoption has consistently driven that number down where enforcement is tight.

Photoelectric vs. ionization

NFPA 72 (2019 and later) requires every smoke alarm installed in or near a kitchen to be photoelectric or to use a documented kitchen-nuisance algorithm - pure ionization within 20 feet of a cooking appliance creates nuisance trips that customers solve by removing the battery, which kills the protection entirely. UL 217 was revised in 2020 to require new alarms to distinguish cooking smoke from real fire smoke; alarms manufactured before that revision do not meet the new standard but remain legal until they reach their 10-year replacement date.

Required wiring topology

• Hardwired with battery backup - required in all new construction and in most remodels that expose wiring (IRC R314.4)
• Interconnected - when one alarm sounds, all alarms sound. Required in new construction. Wired or RF-interconnected both acceptable per the AHJ
• 120 V AC primary with 10-year sealed lithium battery backup - current best practice. Some jurisdictions specifically require 10-year sealed; verify locally
• Battery-only alarms - permitted only in existing dwellings where running new wiring is impractical and the AHJ accepts; new dwellings cannot use battery-only

Replacement and retirement

UL 217 and NFPA 72 require smoke alarm replacement at 10 years from the date of manufacture. CO alarms have varying manufacturer-stated end-of-life, typically 5-10 years. The date is printed on the back of every alarm. Replace, do not just swap the battery, when an alarm reaches end of life - the sensor degrades.

When replacing one alarm in an interconnected system, replace them all together - mixing 8-year-old and brand-new units in the same RF cluster causes intermittent failures the homeowner will blame on the new device.

Wiring practice

• Use a dedicated branch circuit for smoke alarms when AHJ requires (some jurisdictions do; many do not)
• AFCI protection is required on smoke-alarm circuits in dwelling units under NEC 210.12 since the 2014 cycle - verify the breaker is AFCI before signing off
• GFCI protection is not required on smoke-alarm circuits and should be avoided - a GFCI trip removes life-safety devices
• 14/3 NM-B cable carries the interconnect signal between hardwired alarms (red conductor = interconnect)
• Junction boxes serving alarms must be accessible - no buried boxes behind drywall

Mounting rules

• Ceiling-mount preferred; minimum 4 inches from wall
• Wall-mount permitted if ceiling impractical; mount within 12 inches of ceiling, never closer than 4 inches
• Not within 3 feet of an HVAC supply register (drafts cause nuisance trips and dilute smoke)
• Not within 3 feet of a bathroom door (steam)
• Not in dead-air spaces (apex of cathedral ceilings - relocate down the slope)
• CO alarms can mount at any height per NFPA 720; manufacturer instructions take precedence

Combination alarms

Combination smoke/CO alarms are listed under UL 217 + UL 2034 and satisfy both code requirements with a single device. Acceptable in most jurisdictions. Verify the AHJ has not specifically excluded combination units before specifying. Some inspectors prefer separate devices for redundancy in case one type of sensor fails.

When inspection requires retroactive upgrade

Pre-sale inspections and some insurance underwritings demand current-code alarm compliance regardless of when the home was built. This is the most common retroactive electrical scope. Quote it as:

1. Walk-through alarm count by location
2. Existing alarms removed and replaced with 10-year sealed hardwired interconnected alarms
3. New rough wiring where existing locations are missing
4. AFCI verification at the panel
5. Functional test of every alarm with smoke and CO simulant after install
6. Documentation packet for the closing or the insurer

Common pitfalls

• Skipping the bedroom-interior alarm - IRC requires one INSIDE every sleeping room, not just the hallway
• Mixing manufacturers in an interconnected system - interconnect signals are proprietary and 24 V across brands but message frames differ
• GFCI protection on the alarm circuit - nuisance trips disable life-safety
• Installing combination devices near the kitchen - combination smoke/CO often triggers nuisance smoke alarms from cooking; specify photoelectric kitchen-zone units
• Not testing post-install - every alarm must sound the others within 10 seconds of trigger; verify with canned smoke (UL 268 test gas)

References

• IRC R314 - Smoke Alarms
• IRC R315 - Carbon Monoxide Alarms
• NEC 210.12 - Arc-Fault Circuit-Interrupter Protection
• NFPA 72 - National Fire Alarm and Signaling Code
• NFPA 720 - Standard for the Installation of Carbon Monoxide Detection and Warning Equipment
• UL 217 - Standard for Smoke Alarms (revised 2020 for kitchen-nuisance discrimination)
• UL 2034 - Standard for Single and Multiple Station Carbon Monoxide Alarms
• CPSC Non-Fire Carbon Monoxide Deaths Associated with the Use of Consumer Products (annual report)
• Manuall internal: AFCI + GFCI + Dual-Function Protection Deep Reference, Annual Electrical Inspection SOP