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Can I use a 20 amp breaker in the situation of having 15 amp wire connected to a 20 amp main wire?
If I can't would using a 15 amp breaker be ok? Wiring Diagram

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No, in a situation like this absolutely not. The breaker in this case would need to be 15A.

Question is, WHY? Why use #14 wire when the circuit is already #12? Stick with what is there and keep the breaker 20A. In some cases, such as in a kitchen, laundry or bath, you'd be creating a violation doing it like you show in that diagram.

And don't be confused by the receptacles being "15A rated". They are FINE on a 20A circuit and are 20A feed-thru rated as well.

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You must limit the current to the smallest gauge wire in the circuit or else rip out the smaller wire and replace with the proper size. It's all due to diameter and resistance. Small wire heats when current above its capacity is passed through it. – Fiasco Labs Jul 20 '14 at 17:44
Very good to know. The #14 wire is existing and i'm running the #12 main wire in case I change the #14 in the future. I would like to change everything to #12 but it would be a monumental task. Also, the current apartment has one 15 amp breaker for the entire apartment except for the fridge, which has a 20 amp breaker and another 20a breaker, but i don't know for what yet! It was wired in the 70's. So, I'm adding three additional breakers at this time. Any more advice? – Frank Jul 20 '14 at 18:07
I think that sometimes it is allowed, for example with motor loads (large inrush currents), or appliance cords. Could the answer be expanded to including those sorts of applications? For hard-wired devices, can a smaller wire be used? (Though, perhaps this is a unnecessary complication since the original question asked about adding outlets). While the 14 gauge wire is in place, one could install a 15 A breaker. – Pigrew Jul 20 '14 at 18:23
@Pigrew, yes, that would add unnecessary confusion to this question. That is why I said "in a situation like this...". There is NO reason to get into tap rules and motors for the question at hand. – Speedy Petey Jul 20 '14 at 22:24
@Frank If you do run a 12 AWG wire you should mark it as a 15 amp circuit until such time that there is absolutely no 14 gauge wire on the circuit. – Brad Gilbert Jul 21 '14 at 12:16

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