Seemingly randomly today two breakers in my panel tripped at the same time. Someone was vacuuming, so I initially assumed it was because of too much load, but A) the two circuits in question are lighting and not tied to the outlet used for vacuuming (the vacuum kept running) and B) flipping the breakers back on immediately trips them even after unplugging the vacuum.

Here's what I've done/observed thus far:

  • The breakers only trip with BOTH are on (A on with B off is fine, B on with A off is fine)
  • Swapping both breakers with new breakers results in the same behavior (doesn't seem to be faulty breaker)
  • The breakers are on different legs (makes me think there is a hot-hot short somewhere)
  • The breakers do not trip when repositioning them such that they are on the same leg (further suggesting a hot-hot short)
  • Wire/cable insulation appears undamaged in the panel, cables take different paths once exiting the panel
  • These circuits have been working fine for 6 months
  • Multimeter shows no continuity between the two hot legs of each circuit (I would expect to see continuity if there was a short)

What can I do to further troubleshoot this?

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Red boxes are around the two circuits in question (currently sharing a leg and working), and the empty spot where placing either of them results in a trip for both (different leg).

  • What type of breakers? simple or do they have a test button? Does one breaker have a red wire and maybe part of of MWBC.?
    – crip659
    Commented Dec 18, 2023 at 21:25
  • @crip659 One breaker is a combination arc-fault, but I swapped it for a standard single-pole 15 amp and the same behavior occurs. There are no MWBCs in the house (full rewire completed 6 months ago).
    – Travis
    Commented Dec 18, 2023 at 21:28
  • And now after returning everything to its initial state, the issue is no longer happening... I have turned on all lights that are on the circuits to ensure all of the wire paths are closed/connected.
    – Travis
    Commented Dec 18, 2023 at 21:31
  • Playing/fooling around in the panel only and a difference happens, might have narrow down the problem to the panel/breakers/wires near the panel, or might need the extra power from a vacuum or similar device(partial insulation breakdown).
    – crip659
    Commented Dec 18, 2023 at 21:43
  • @crip659 What do you mean by partial insulation breakdown? The vacuum was running on a different circuit located in a subpanel.
    – Travis
    Commented Dec 18, 2023 at 23:30

2 Answers 2


TLDR - a 4-way switch group was wired to two hots on each end instead of a hot on one and a fixture on the other. The hots were from different circuits, and when the corresponding breakers were on different legs, flipping any of the switches in the group would cause a short between each leg.

I finally had some time to dig into this today and looked at the wiring for the switch plate closest to the outlet where the vacuum was being used. There is a 3-gang box with a non-functioning switch I assumed had no wiring (I asked for an extra gang in a couple of places for future expansion of a Lutron system thinking they would install blank fillers in the extra spots, but they used switches instead; I assumed it was because the electrician had them on hand). I turned the lighting breaker off at the panel, pulled the bogus switch and was surprised to find A) it wired at all B) it wired as a 3-way and C) one traveler of the 3-way was still hot with my multimeter. Then I flipped the switch and all of the lights on the circuit I turned off at the breaker panel came back on.

So I went to the other location that had what I thought was a dummy switch and it was the other end of the 3-way. Its hot wire was still live as it came from a different circuit.

  • Backfeeding into a circuit that's turned off at the breaker panel is really dangerous. This situation is completely unacceptable and ought to cost someone their license.
    – Ben Voigt
    Commented Jan 12 at 22:11

Seemingly randomly

that's how it always seems

circuits have been working fine for 6 months

they always work fine for some amount of time before the problem happens

What can I do to further troubleshoot this?

you could call an electrician. Based on what you have described, I would start with inspecting the outlet (outlets?) you had the vacuum plugged in. And systematically figure out all the outlets and switches is on each breaker. In addition to knowing the make/model of breaker in question which you did not specify.

  • All of this should be comments, not an answer.
    – FreeMan
    Commented Dec 19, 2023 at 14:39
  • maybe, but since the guy included a pic since I responded, and he did not mention the breaker type previously, his pic shows AFCI breakers. His problem is classic neutral mixup & connection between the breakers. He's got all neutrals tied to together at one or more outlets when they should not be, this is what is tripping the AFCI breaker(s).
    – ron
    Commented Dec 19, 2023 at 14:49
  • and he does not know what he's doing, first thing that he should've mentioned was I have AFCI breakers. So my you could call an electrician is pretty much the correct answer
    – ron
    Commented Dec 19, 2023 at 14:51
  • 1
    Instead of burying this info in the comments hours later, edit it into the answer...
    – FreeMan
    Commented Dec 19, 2023 at 14:52
  • 2
    @ron Although I do have AFCI breakers, the issue occurs with standard breakers (I swapped the CAFCI breaker with a normal breaker for testing). The two circuits in question are entirely lighting; there are no outlets on them. What part of my question leads you to believe I do not know what I am doing? I'm here for assistance and looking to learn, but that isn't very helpful.
    – Travis
    Commented Dec 19, 2023 at 18:33

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