On Dec 18, 11:44*pm, wrote:
* I would like to run some new outlets in my garage. *There is
currently a subpanel located there, the run will be fairly short. *I
have a section of armored cable which was left over from work at my
office that I'd like to use. *I'm thinking of using a setup which I
have seen used in other houses, but want to run past you first.

* The armored cable has two 10-gauge (white & gray), 4 12-gauge
"hots" (various colors), and two 12-gauge grounds. * I'd like to use
it to run 4 20-amp circuits. *Circuits A and B would come off of
opposite phases, and share one of the neutrals and one of the
grounds. *C and D would be run similarly. *That way, neither neutral
would ever carry more than 20 amps.

* I know that I would never exceed rated ampacity for any conductor,
but I do not know the NEC well enough to say whether that will meet
current code or not. *Anybody want to chime in?

Two hots (opposite each other) using the same neutral is commonly
called an 'Edison circuit'. AFAIK it is still OK to to wire new
circuits this way but there are problems.

I'm not an electrician but as I understand it if one breaker is shut
off the neutral can still carry current if the other breaker is on.
The outlets should be wired in such a way so that if an outlet on
circuit A is disconnected (for replacement or other reson) the neutral
will not be broken.

Do some googling on 'edison cicuit energized neutral' and you will
come up with many examples of how this type of circuit can be very
dangerous. Here is just one link:
http://www.hanford.gov/rl/?page=968&parent=334

Ganged breakers can prevent problems but think about the guy down the
road or yourself in 10 years when you're not exactly sure what you
did. Wire is cheap don't take the chance.