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Wire gauges vs breaker sizes


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Cory, you have that backwards. A 20 amp circuit requires 12AWG (if it's copper) but there is nothing wrong with protecting a 12AWG circuit with a 15 amp breaker. The only down side is that it it will cost more than it needs to. When I designed the circuits in my house I ran 12AWG for the bathrooms and the dining room with 15 amp breakers. When we entertain there is usually a crock pot or electric roaster running in the dining room but when the wife decided to run both 15 amps wasn't enough, and with 24 people over and dinner half done it was very easy to upgrade the circuit and get the party going again. The little electric heater that the wife runs to take the chill off the bathroom at the begining an end of the heating season and her hair drier also overloaded the 15 amp circuit there. Anticipating potential loads like these is a good reason to run 12AWG for circuits that would normally be 14AWG 15 amps.

A 14AWG circuit on a 20 amp breaker is wrong because the wire isn't rated to handle 20 amps.


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Absolutely nothing wrong with oversized wiring on a breaker, using 12 awg on a 15 amp breaker is perfectly fine, using 10 awg on a 20 amp is fine, etc. Undersized wire, now that's usually bad, except as has been noted about the A/C units (and heat pumps). And, I think it's not appropriate to just change out the breaker to a lesser rated breaker because you don't know what damage has been done out on the circuit, but I leave that up to the licensed electrician.

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