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On Thu, 9 Dec 2010 10:27:58 -0800 (PST), Borrall Wonnell
What you suggest is probably safe (#10 to the stat #12 to a 1500w
heater) but it is not legal.
Arguing on the safety side I can say the 1500w load is well within the
ampacity of the 12ga wire and a short will still safely be cleared by
a 30a breaker.

A short circuit will trip breakes with much smaller wire than the code
requires. That is only part of the problem. If there is a wiring problem
that causes the smaller wire to draw more current, but not enough to trip
the larger breaker, the wire can over heat and causes a fire.