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My shiny new subpanel has a shiny new Eaton BR AFCI/GCFI 15 amp breaker feeding two receptacles.

I plugged a rather disreputable looking 12-amp electric mower into a receptacle with no other load. The breaker tripped. The breaker passes its own test button and a GFCI outlet tester plugged into it is happy. The mower works in another outlet that is a 15 amp outlet on a 20 amp circuit.

A heat gun plugged into the same outlet is perfectly happy; of course, it's probably not drawing anything like 12 amps.

Is this a question of transient load, and if I wanted to be able to run such a thing would I need to upgrade the wire to 12 gauge and the breaker to 20 amps?

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  • What happens if you try powering the mower with a different 15 amp circuit? Mar 26, 2022 at 0:06
  • Is the other 15A outlet on a 20A circuit where the mower works fed by a GFCI breaker or a plain old simple breaker?
    – brhans
    Mar 26, 2022 at 0:12
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    It could be the motor tripping the AFCI(arc fault) section of the breaker. Old motor produce arcs sometimes at the brushes.
    – crip659
    Mar 26, 2022 at 0:13
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    How quickly does the breaker trip, and what fault codes does the breaker yield when you trip it with the mower? Mar 26, 2022 at 1:07
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    Can you plug it into a Kill-a-Watt and find out what it's really drawing? Gosh, another data collecting meter might be better, since it's hard to mow and read a Kill-a-Watt at the same time. Are you giving the electric mower a real challenge? Also, can you follow the procedure to get the breaker to report whether it is tripping on GF, AF or overcurrent? Usually with Eatons it's a simple blinking light with no manipulation needed. Is the panel Eaton, Cutler or Bryant? Mar 26, 2022 at 2:58

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Probably the circuit is undersized for your mower. You say your mower is 12A. The NEC calculations say that in order to size a motor circuit, you need to add it's load and 25% for start up. That would be 12*1.25 or 15A. So our rough calculations is showing that the load is right on the maximum load allowed before trip. Since you can put you mower on a 20A circuit and that breaker does not trip, I would assume that your mower is a little over the allowed amperage for a 15A circuit, thus the tripping.

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