Putting a 12awg on a 15amp breaker is not the problem. Reducing the wire
size downstream is!!
"toller" wrote in message
...


You absolutely cannot do that. Any person working in your panel would
have no way to know that the 12 awg had been reduced somewhere downstream
and in doing a service upgrade for example would install the wire to a 20
amp breaker

Actually, code specifically allows it. 210.3 "where conductors of higher
ampacity are used for any reason, the ampere rating or setting of the
specified overcurrent device shall determine the circuit rating".
This is frequently done where there is a long run from the breaker box to
the first outlet and there might be a voltage drop problem.

But unless you have a good reason to do it, it probably is best to be
consistent.