We discovered a non-functional smoke detector that's hard-wired into the electrical system of the house we recently bought. I removed the smoke detector and am replacing it with a battery operated one. But there is one set of 4 small wires hanging out of the ceiling. They all run through one single wrap. The colors are black, red, green and yellow. The hole in the ceiling is small. I won't be able to cap each wire and push it back through. Not sure if they can all be capped together into one wire nut or what....

  • 4
    Why not get a new hardwired smoke detector? In many locations, they are required for new construction.
    – BMitch
    Nov 25, 2012 at 22:28
  • I'm finding the wiring is really wonky throughout the house. 100amp instead of 200 amp, and lots of dead outlets and blowing fuses. We had an electrician come in and fix a bunch of stuff amd verify general safety, but we need an overhaul, I think. I trust battery powered smoke detectors more than the wiring in this house....
    – Sarah
    Nov 25, 2012 at 22:38
  • 2
    Every wired smoke detector that I've ever seen has a battery backup. There's no need to have blind faith in the wiring.
    – BMitch
    Nov 26, 2012 at 2:04
  • 3
    @Sarah 100amp is not "wonky". houses built in the 60's or 70's had a lot lower then that usually! Just depends how much power you need!
    – Steven
    Nov 26, 2012 at 3:22

3 Answers 3


Those wire colors (black, red, green and yellow) are commonly found in the wiring for security systems; smoke detectors that hardwire into security systems typically run on 12VDC or 24VDC, unlike the 120VAC detectors that you generally find at home improvement stores. I'm guessing the wires are stranded instead of solid core (like mains wiring) and of a smaller gauge wire. Another clue is that there's a small hole in the ceiling; if it was a 120V detector, there should be a ceiling box, and the wires would be colored black and white, possibly with a third, red, insulated wire, and probably a bare copper wire for ground.

Since this smoke detector isn't working, the security system may have been disconnected or removed. Do you have other smoke detectors in the house, and do they work? What about other security system components like motion detectors, glassbreaks, the control panel and/or keyboard?

Use a multimeter to check that there's no voltage between any of the wires; the supply for the detector was probably on the red and black, but check all the combinations to be sure. I would seal the hole with some expanding foam to prevent air leaks into the ceiling, then wrap each exposed wire individually with electrical tape and just leave them behind the base plate for the new detector.


As someone who has had smoke detectors save his house twice please rethink your plan. The big advantage to the electric detectors are they are wired together, one goes off,they all go off. The cost of having the electrician come back is minimal to the added benifit. There is also the chance that modifications to the wiring may make them all inoperative.

  • Thanks, you two. There used to be a central alarm system. But there are no other smoke detectors wired like this. All the others are battery powered. There are plenty! I added one in every bedroom in addition to the others throughout the house. I'll test the hanging wires as you suggest. But I can't cap them and leave them in place because the new detector has a flush backing. Maybe I ought to just cap them and leave them hanging until I have an electrician come in.
    – Sarah
    Nov 27, 2012 at 0:23

If the old security system is gone, you should be able to just push the wires, uncapped, into the wall. If you want to be really smart about it, wrap them in electrical tape before you do and leave a tiny bit protruding out the hold and taped flat to the wall so you can retrieve them at a later date if you choose.

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