Hardwired smoke alarms in U.S. homes run on 120V AC power; the low-voltage wire between them is only an interconnect signal, not a power source.
The two numbers people mix up live in the same junction box. One pair of wires carries 120 volts from your breaker panel and powers the alarm. The other carries a small signaling current that tells every alarm in the house to sound at once. First Alert’s SMI100-AC manual states: “This unit must be powered by a 24-hour, 120V AC pure sine wave 60 Hz circuit.” Wire the interconnect as if it were power and you will destroy the alarm or trip the breaker.
What Voltage Do Hardwired Smoke Detectors Actually Use?
Standard residential hardwired smoke alarms in the U.S. are 120V AC, 60 Hz devices with battery backup, not low-voltage devices. Kidde’s i12010S documentation describes a continuous 120V AC, 60 Hz household circuit, and the BRK 9120B product page labels that alarm 120V AC/DC. The battery backup keeps the alarm alive during an outage.
Brands diverge on wire color, so read your manual. First Alert uses black for hot, white for neutral, and orange for interconnect only. Kidde uses black for hot, white for neutral, and red for interconnect. In a standalone installation, First Alert instructs you to tuck the unused interconnect wire inside the junction box.
Why People Call The Interconnect Wire “Low Voltage”
The interconnect conductor is a Class 2 low-voltage signaling circuit, different from the branch-circuit wiring feeding the alarm. A code summary source notes that where hardwired interconnection is used, that signal wire must be kept separate from branch-circuit wiring under IRC requirements. It triggers the other alarms; it does not power them.
Confusion also comes from commercial equipment. The Honeywell D4120W duct smoke detector manual lists separate input options — 24VDC/24VAC at 50–60Hz and 120VAC at 50–60Hz — because those commercial detector families genuinely come in both voltage classes. A residential hardwired smoke alarm does not. Do not assume every “smoke detector” runs on the same voltage.
| Wire | First Alert Function | Kidde Function |
|---|---|---|
| Black | Hot (120V AC) | Hot (120V AC) |
| White | Neutral | Neutral |
| Orange | Interconnect only | Not used |
| Red | Not used | Interconnect only |
How Do You Wire A Hardwired Smoke Alarm Safely?
Turn the power off at the breaker first. Then:
- Mount the bracket to the junction box.
- Connect black to hot and white to neutral using wire nuts or the supplied connector.
- Connect the interconnect conductor (orange on First Alert, red on Kidde) only if linking alarms; otherwise cap or tuck it as your manual instructs.
- Plug the power connector into the alarm, then twist the alarm onto the bracket.
- Activate the battery backup by pulling the tab, if present.
- Restore power and test interconnected alarms so one unit sets off the others.
When power returns, the alarm chirps once and its indicator light settles into a steady glow.
A few gates matter before you buy. These alarms are built for unswitched 120V household circuits, not switched circuits or dimmers, and not GFCI-protected circuits when the manual prohibits them. They are designed for U.S. 120V systems; 240V service is not the intended supply. Never connect hot or neutral to the interconnect wire, and never cross hot and neutral between alarms — First Alert warns against both. When replacing an old unit, matching the brand and model family keeps the interconnect wiring consistent. A current roundup of tested 120 volt smoke detector picks can help you match voltage and wire color before you buy.
Do You Need A 120V Smoke Detector Or A Battery-Only Model?
Hardwired 120V alarms with battery backup are standard where a home already has interconnect wiring, since one alarm sounds them all. Battery-only models skip the wiring but do not interconnect unless they use a wireless link. For most U.S. homes with existing ceiling wiring, staying with a 120V hardwired model is simpler.
Confirm the model number on the back of your old alarm against the manual before ordering a replacement. Wire color and interconnect behavior are brand-specific, and a five-minute check saves a second trip up the ladder.
FAQs
Can I replace a 120V smoke alarm with a battery-only one?
Yes, though you lose the interconnect feature that sounds every alarm at once when one detects smoke. If your home has existing hardwired interconnect wiring, keeping a 120V model with battery backup preserves the protection you already paid for.
Is the orange or red wire carrying 120 volts?
No. On First Alert alarms the orange wire and on Kidde alarms the red wire are interconnect conductors only — low-voltage signaling paths, not power. Connecting a hot or neutral conductor to that wire can damage the alarm and creates a shock and fire risk.
Will a 120V smoke alarm work on a 240V circuit?
No. These residential alarms are designed for U.S. 120V systems, and 240V service is not their intended supply. If your home uses a different voltage class, you need a device rated for that system rather than an adapter or workaround.
References & Sources
- BRK Electronics. “9120B BRK 120V AC/DC Smoke Alarm with Alkaline Battery.” Confirms the 9120B is a 120V AC/DC hardwired alarm.
- First Alert. “SMI100-AC Hardwire Smoke Alarm Manual.” Specifies the 24-hour, 120V AC pure sine wave 60 Hz power requirement and orange interconnect wiring.
- Honeywell. “D4120W Duct Smoke Detector Manual.” Shows commercial detectors offered in both 24V and 120VAC input classes.