On 2008-02-03, Terry wrote:

12 guage wire on a 30A breaker a code violation and can be fire
hazard.

This is 98% true. The one exception I know of is for motor loads.
Motors can have a very high startup current, so in a dedicated motor
circuit an alternate arrangement is acceptable, as follows. The motor
itself or a dedicated motor controller or relay provides the
overcurrent (overload) protection. The breaker is just for short
circuit/ground fault protection. The breaker is sized so that it
doesn't trip due to the startup current. So you could end up with a
30 amp breaker on a #12 wire. For further details, see Article 430 of
the NEC.

Cheers, Wayne