I have a new CAFI breaker in a Square D Sub-Panel (older panel - T5 Series, 24 slots, but new installation of breakers and wires due to renovation work, whole cottage re-wired). I did the rough and finish myself (as a homeowner, allowed in Canada). The CAFI breaker is supplying 8 plugs (outlets) and one ceiling smoke detector. The ceiling smoke detector is not connected now, I just have wire connectors/yellow marrettes on the neural and hot wires to cap them.

Here's the wiring: From the panel, I bring a 14/2 into a first plug. That plug has two outs: 1 out that feeds four more plugs; in series. The second out goes to a sixth plug with also two outs (one out that feeds two more plugs in series; the second out feeds the smoke detector).

That CAFI breaker keeps on tripping randomly. Sometimes it trips within a few minutes, sometimes a few hours, completely random. Nothing is plugged into any of the eight outlets and the smoke detector is not installed.

I have another CAFI breaker (for 9 other plugs) that does not trip. So I switched the circuits to make sure it was not the breaker and it's not. When I switched the circuits, it's the other breakers that started tripping and the other one not, so clearly not a breaker problem, since both breakers trip when connected to the 8 plugs+smoke detector circuit.

I connected the circuit on a regular 15 amp breaker and it holds power (no trips).

I tripled checked all my outlets and smoke detector metal boxes. No wires are exposed and all connected properly. I also did not back-stab the outlets, I used the side screws and pigtails when I had two outs. All outlets work, I have a small tester and neutral, hot, and ground are connected properly.

Possibilities I'm exploring:

  • The drywallers finished last week, maybe they hit a wire inside the wall with a screw (although wouldn't that make the breaker trip right away and not after a few hours?).

  • Maybe a faulty outlet? Sometimes (not always) when the breaker trips I hear a "pouff" coming around the outlet that feeds the smoke detector. Other times, I don't hear the noise, the breaker just trips (hear the click of the trip, that's it).

  • Maybe I stapled a wire and hit the insulation (although again, wouldn't that make the breaker trip right away?)

  • I check the info sheet that comes with the breaker and it says to test it after it trips (hold the test button and turn it back on). If it trips right away, they say it's a ground fault, within 2 seconds, an ARC fault, within 5 seconds, no faults, thermal overload, or short circuit. Mine tripped right away when I tested it, which would suggest these random trips are due to a ground fault.

Other details: This is a subpanel in a separate dwelling, 100 amp, square D, Series T5. The ground and neutral are not bonded (at least I think). I noticed I have two grounds: one coming from the 3/3 service wire and one going to a rod (I thought that might be the problem, but I read that separate buildings need their own ground rod on a subpanel). Both these grounds are connected on the same small strip (in the portion of the panel where the main breaker is).

I'm really not sure what is going on. It's so intermittent. Anyone has some thoughts on this situation?



  • 3
    Would turn off breaker and disconnect the wires/cable from the last outlet to last device(sounds like smoke detector). Turn on breaker and see if it still trips. If still trips repeat with next last outlet and repeat. This should let you find device or section of cable giving problem.
    – crip659
    Oct 4, 2021 at 10:25
  • 2
    @user263983 OP did mention he switched breakers around to check for a faulty breaker.
    – crip659
    Oct 4, 2021 at 11:23
  • 1
    Are you 100% certain that it's code compliant to have the smoke detector on the same circuit that's serving general-use receptacles? I'm not an electrician, but I thought I'd read that the detectors have to be on a dedicated circuit. Again, just a recollection, so I could be wrong, but something for you to double check.
    – FreeMan
    Oct 4, 2021 at 12:36
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    @FreeMan Kind have thought people would not want a smoking outlet to cut power to smoke detector.
    – crip659
    Oct 4, 2021 at 13:01
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    @OlivierSt-Cyr The wire/cable could be the problem area, not the load. Isolating cables/devices in sections is easiest, but slow. It sounds like the outlet that goes pouff and the smoke alarm cable are at the end of the cable run and might find the problem there.
    – crip659
    Oct 4, 2021 at 14:35

1 Answer 1


If you have audible arcing near the last outlet or two in the circuit, then disconnect the cable supplying those two and try the circuit again. You can methodically disconnect some parts of the circuit in this way to figure out roughly where the problem is.

It sounds like you're already confident that the breaker is installed correctly. Just remember to follow the instructions and that these breakers need more than one wire attached.

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