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I have a microwave that works great except when the temperature outside is something like 15 degrees F or cooler outside. When this is the case, it more or less reliably trips the GFCI breaker in my basement, requiring me to turn it back on to try to get a few more seconds of heating out of it.

This never happens when the weather is warmer.

I’m only able to do proper tests when it is cold like this. What i did today was after the breaker tripped and I flipped it back on, I plugged the microwave in with a kill-a-watt meter in order to monitor the voltage. I conducted the same test (heating one hot pocket) and it completed a full minute without incident. So it may be possible that the kill-a-watt meter introduced some difference that prevents it from tripping now. Although, this is only one data point.

I just wonder what are some other things that I can test. And if I plug the microwave on a different circuit (having a regular breaker) it runs fine.

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    Since it’s a GFCI circuit breaker it could be tripping for two reasons 1) ground fault 2) over current. Since it only does this when it’s cold out, the first question we should ask, is there anything else on this circuit? Possibilities that immediately come to mind that you might not think of include: outdoor outlets with heat related loads such as gutter heat, dog dish heaters, block heaters. – Tyson Jan 31 at 12:26
  • There should be an indicator on the breaker saying whether it is tripping for GFCI or overcurrent. – Harper Jan 31 at 16:13
  • To add to @Tyson comment, pipe heating tape. – DrMoishe Pippik Feb 1 at 2:37
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I looked at this again now that it has tripped once more with 38 degrees F outside temperature. It is an AFCI breaker, not a GFCI (or possibly more correct it is both), and when flipping it back to on, the light labeled ARC FAULT comes on (not the other one labeled GROUND FAULT).

Upon more reading, simply ensuring the microwave isn't underloaded (e.g. add a cup of water to it while heating something small) should prevent this.

Except that what is funny about this is that when it tripped today it in fact already had a bit of extra water inside. Although it was a shot glass of water, not a whole cup. Now that I'm barking up the right tree I should be able to get somewhere with this though.

  • An unloaded/underloaded microwave is known to generate internal cavity arcing, so using a full water glass as a dummy load should keep this problem from recurring provided the microwave's still in good shape. – ThreePhaseEel Feb 24 at 14:22
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Remember, extreme temperatures expand and contract metal, like copper conductors in a witing circuit. This is usually not a problem when there are good connections. Check for loose connections in the circuit, especially any exterior devices or wires and anything close to an exterior (cold) wall.

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