All my smoke detectors are on one circuit, but one needs to be moved about 4 feet from where it is. At its present location is where I would like to have a light fixture which will be on a different circuit. Is it ok to use the round ceiling box for both the light and as a junction for relocating the smoke detector?

The issue is that from there, the bulk of the rest of the smoke detectors for the house are wired through the first floor ceiling joists, up through walls and to 2nd floor ceiling joists all of which are inaccessible without creating a lot of extra work (there are electrical staples about every 2 feet). Also that location no longer makes as much sense for the detector, but 4 feet away is the landing for the stairs going up.

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    As far as I know it is okay to have more than one circuit in a junction box. Think the only limit is the number of wires, but that is more for small outlet/switch boxes.
    – crip659
    Commented May 6, 2022 at 20:08
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    In case you haven't already, please read the applicable code for your area about smoke detector placement. If located incorrectly, their ability to detect smoke can be greatly diminished. .
    – RetiredATC
    Commented May 6, 2022 at 22:44
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    Mark the cables carefully so it is apparent it is for smoke detector wiring. If you want to mark wires differently, an "alternate" color for neutral is gray (the neutrals MUST NOT be tied together!) and alternate colors for hot are anything except white gray and green. I'm fond of purple! Commented May 7, 2022 at 0:37
  • Thank you. That's part of the reason for the move. I'd like to move the detector to the base of the stairway. It had been at the end of a hall heading to the stairs, but that hallway no longer exists with the new floor plan. I'll make a point of tagging the smoke detector wires as such.
    – Trevor
    Commented May 7, 2022 at 15:40

1 Answer 1


If you have room in your junction box without overloading fill rules, there shouldn't be any problem with that, assuming that there is not a low voltage signaling component to your detectors which needs to be isolated in the junction box from high voltage wiring.

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