"John Moorhead" writes:
Nope - I'm in Northern California... Well, I *do* want alternate
circuits... The reason for using a single run of 12/3 is that I wouldn't
have to run an additional separate length of wire - everything would be in
the one cable. Sorry if my post didn't make that clear...

What you want to do is legal and safe only so long as each of
the two circuits are a different phase (i.e. a different leg of
the 220v). It is also considered wise to handle-tie both
breakers (or use a 220v breaker) so that you cannot have a
situation where one conductor is live and the other isnt.

If the current carrying conductors are on different phases the current
in the grounded conductor approaches zero. If they are on the same
phase, the current in the grounded conductor will be the sum of the
current in each of the current carrying conductors. This would be
considered bad (e.g. 40 Amperes through a AWG 12 conductor).

scott