I have a home built in 2010.

I installed some monorail lighting (150W) in my home office last year. When this light is on, it always trips the breaker for the Washer (when it's running). Not instantly, but within a minute or few.

The washer is on it's own 20A circuit. I drew power for my monorail lighting from the garage opener motor outlet (also 20A, three stall garage, each motor has its own outlet/circuit).

The odd thing, is that the 20A circuit the monorail/garage motor are on, does NOT trip. Never has.

The wiring method was all new. I bought 50 feet of 12AWG, pig-tailed the garage motor outlet and brought the line over to my office ceiling. Here I create a new ceiling outlet (for my monorail lighting) and a switch in the wall. Folded and fed the 12AWG down the wall to the new hole for the switch. Once through, I cut the loop (one for source, the other for destination). Wired the switch, then wired the ceiling fixture.

I have enclosed a picture of my electrical panel with the two circuits highlighted (blue is the lighting/garage motor, red is the washer).

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My issue seems to be the same as this unresolved issue

  • 1
    Picture is a bit far out to be sure, but the washer breaker is GFCI? Certainly all the tan breakers have extra buttons, which usually is GFCI or AFCI. A GFCI trip could be due to improperly cross-connected neutrals.
    – Ecnerwal
    Nov 13, 2020 at 14:26
  • They are AFCI, the outlets handle the GFCI where applicable. I am comfortable pulling the panel, yes. I would need to look-up cross connected neutrals; my assumption is that you're thinking the neutral (whites) of both those circuits are mapped incorrectly?
    – Napoli
    Nov 13, 2020 at 14:30
  • Use a meter to measure voltage between each panel bus bar and the neutral/ground - both "legs" should read the same (nominally 120v). If they differ, check the connection to the earth ground. If the panel ground isn't functioning properly the neutral can "float" and the two 120v phases become imbalanced.
    – jbbenni
    Nov 13, 2020 at 14:39
  • Are you using cfl or led lamps in your lights or a dimmer? AFCI circuits don’t play well with harmonics generated by switching supplies or driver circuits, dimmers create similar issues and are a known problem , I would verify the correct neutral was connected to the breaker for this load.
    – Ed Beal
    Nov 13, 2020 at 14:42
  • 1
    @Napoli - yes, you might. But this type of problem can go unnoticed easily. And it can give subtle and surprising symptoms. For example: In a kitchen of a house with this problem, the lights got brighter when the disposal ran. Turns out a car had scraped the utility pole and the ground at the pole was broken. The house Neutral was fluctuating. Checking the voltage between each phase and the neutral is quick and can be determinative.
    – jbbenni
    Nov 13, 2020 at 14:58

1 Answer 1


You have crossed neutrals

First, crossed neutrals are always bad. After all, neutrals don't have breakers: the whole idea of neutral protection is that the neutral can't possibly carry more current than its partner hot (or hots in the case of an MWBC). If you cross/share neutrals, now a 20A neutral wire can return current for 40A worth of hots, and none will be the wiser.

If an AFCI has a neutral pigtail, it includes a GPFE - a weak GFCI that is sensitive to crossed neutrals like a GFCI is. The easiest way to detect arc faults which are ground faults (i.e. hot-ground or neutral-ground) is simply detect ground faults.

So, whoever wired the monorail lighting, not naming any names, tapped off a place maybe they should not have. They picked up "hot" off the garage motor circuit, and "neutral" off the laundry room circuit.

A "switch loop" for a switch brings only always-hot, and switched-hot to the switch -- no neutral. So you can see where that's a recipe for trouble - a novice installing a new lighting string in an existing switch box might be tempted to grab hot off circuit 1's "switch loop" and for lack of a companion neutral, grab neutral off circuit 2.

99% of the time this fiasco goes undetected. And obviously if the AFCI or GFCI had supplied the hot, then no joy, since that would trip. But the odd case of an AFCI supplying the neutral is what you have. AFCIs don't necessarily interrupt neutral.

To give specifics on how to fix it, we'd need specifics about how it was wired. The above is one guess.

  • Thank you. It will take me a bit to digest what you're saying. I have edited my original post to include my method of wiring. It was all new (and real basic, pigtail off the source, run to the switch, then to the ceiling fixture), I only fed off of the garage motor outlet (i.e. the only appliance on that circuit). The washer is also on it's own circuit and inaccessible.
    – Napoli
    Nov 13, 2020 at 21:07
  • I have also included a picture of my switch wiring. I can go show you the outlet in the garage if you'd like.
    – Napoli
    Nov 13, 2020 at 21:17

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