Two codes, one system
It's easy to think of smoke alarms as purely a "building code" item, but the wiring that feeds them falls squarely under the Ontario Electrical Code. The Ontario Building Code (OBC 9.10.19) sets out how many alarms you need and where — one on every storey, and one outside each sleeping area, interconnected so that if one sounds, they all sound. The OEC (Section 32) governs how that interconnected circuit is actually wired: hardwired alarms must be connected to a lighting circuit — not a receptacle circuit — and each alarm must have battery backup in case of a power outage.
Why hardwired alarms connect to a lighting circuit
This is one of the most misunderstood rules in residential wiring. Tying smoke alarms into a lighting circuit — rather than a general-purpose receptacle circuit — means the light itself acts as a visual indicator of the alarm circuit's status. If that breaker trips or the circuit loses power, a light going dark gives an obvious visual cue that the smoke alarms on that circuit have lost their primary power source (they'll still run on battery backup, but the light serves as an early warning that something needs attention).
This applies everywhere in the home, not just kitchens and bathrooms: hardwired smoke and fire alarms must be connected to a lighting circuit, not a receptacle circuit, so the light continues to act as a power/alarm-status indicator no matter which room the circuit serves.
Carbon monoxide (CO) alarms: when they're required
- Any home with a fuel-burning appliance (gas furnace, gas water heater, gas stove, wood/gas fireplace)
- Any home with an attached garage
- Required outside each sleeping area, on every storey containing a bedroom
- Combination smoke/CO units are common, but confirm the unit is certified for both functions — not all smoke alarms include CO detection
Recommended: interconnected smoke/CO combo alarm
Hardwired, battery-backup, interconnectable units simplify meeting both the smoke and CO requirements with one device per location.
Placement requirements at a glance
- Every storey of the home, including basements
- Outside each sleeping area — typically the hallway serving bedrooms
- Inside each bedroom is increasingly required or recommended, especially for new construction and renovations — check your local amendment
- Interconnected so that every alarm sounds together, whether hardwired interconnect or wireless interconnect
Step-by-step: replacing a hardwired alarm (like-for-like)
- Kill the power at the panel for the lighting circuit feeding the alarm — confirm with your voltage tester at the alarm's junction box.
- Remove the old alarm from its mounting bracket, then disconnect the wiring harness.
- Note the wire configuration. Interconnected alarms typically have three wires: black (hot), white (neutral), and a third colour (often red or yellow) for the interconnect signal between alarms.
- Connect the new alarm's harness to the matching wire colours, cap with wire nuts, and tuck neatly into the junction box.
- Mount the new alarm on the bracket and insert the battery backup.
- Restore power and test every interconnected alarm in the home — pressing the test button on one should sound all of them.
⚠️ Never disconnect or "silence" an alarm by removing power
Pulling the battery or capping the wires to stop nuisance alarms (like cooking smoke) leaves the home unprotected until someone remembers to reconnect it — a common cause of preventable injuries and deaths. Relocate the alarm, add a hush/silence button model, or improve kitchen ventilation instead of disabling it.
When this needs a permit and a pro
A like-for-like alarm swap on existing wiring is typically not permit-triggering. However, adding new interconnected wiring — running a new circuit, adding alarms in a renovation, or converting battery-only alarms to hardwired — generally requires an ESA permit, since you're altering the branch circuit wiring itself, not just replacing a device.