The smoke detectors are currently wired to each other (2 wire) on a dedicated circuit. We want to add something that gets used often, so that we'll notice if the circuit has tripped.

We would like this something to be a light that is one a three-way switch. I am guessing this is not possible if the light is closest to the source, because when the light's off the smokes won't get power. But what if the light is at the very end of the circuit? Would this work ok?

If a light on a three-way switch is not possible, can we do a light on a one-way switch, and just ignore the red wire when we go to wire that bit, ie run the red through but don't connect it to anything. Thx

  • I am an electrician but have no idear what you are talking about. Picture and sketches would help a lot.
    – Ride Sun
    Oct 19, 2019 at 22:56
  • Where do you want to put the switches and the light relative to where you want the smokes to go, and is replacing existing wiring an option here? Oct 19, 2019 at 23:51
  • It used to be somewhat common to put a fridge on the same circuit as the smoke detectors -- that way you won't be tempted to turn them off at the breaker.
    – gbronner
    Oct 21, 2019 at 4:28

2 Answers 2


Adding anything is against code in most jurisdictions. They want wired smoke detectors to be certain of working.

Edit: 3PhaseEel points out that having smoke detectors on an unswitched branch circuit is allowed. See his comment below.

Good wired detectors still have a battery and they chirp annoyingly when they are without power. And will do so for weeks.

  • NFPA 72 specifically permits the use of an unswitched branch from a lighting circuit in 29.6.3 point 4: "(4) AC primary (main) power shall be supplied either from a dedicated branch circuit or the unswitched portion of a branch circuit also used for power and lighting." Oct 19, 2019 at 23:50
  • Thanks @ThreePhaseEel, that answers the question of whether it is legal. As Sherwood Botsford points out, the chirping is enough (and we have noticed it when batteries die, usually at 1 a.m., I just forgot about it). So we are abandoning the idea. ^pI'll also point out that this is part of the wiring for a house extension that is all permitted, including an electrical permit (we live in OR, where owner can do wiring). We would never do this kind of work without a permit, it's just not worth the risk.
    – Catherine
    Oct 21, 2019 at 0:43
  • Thank you for the correction. I will edit my answer. Oct 21, 2019 at 0:55

You can branch out and make your lighting circuit from any smoke fixture or any place along the wire run all the way up to the breaker box.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.