> > > IIRC, The National Electric Code specifically prohibits using a common
> > > neutral between the two phases.
> >
> > I don't believe this is true.
> >
> > When I wired my woodworking shop I installed several 12/3 w/gnd lines
from
> > the service panel to outlet boxes containing  2 GFI's each.
>
>
> The neutral is rated at one size smaller than the power carrying
conductor.
> So, if the two 110 outlets are rated to carry power equal to what the
> neutral is rated than this configuration will work (30 amp neutral rating
on
> 12/3 basically rating the outlets at 15 amps each). I am surprised that
the
> inspector allowed this on an industry outlet such as a woodshop that
> normally requires a full 20 amps per outlet. I use this configuration all
> the time when wiring houses. The bedroom circuits are under rated because
> they may carry a clock, TV, or a lamp... normally all low power items.
> Pulling a three wire saves money because the bedrooms are normally the
> furthest from the panel.
>
> Jim

12/3 neutral is 12 gauge (the one-size-smaller rule doesn't kick in until
you get to higher sizes, typical AL 4 (maybe AL 6) or bigger) and thus is
rated for 20A.

Also, neutral current worst case is when you have an unbalanced load: ie:
drawing 20A on one leg and nothing on the other.

I guess the thinking is that the probability of a badly unbalanced load goes
down when the guage goes up. This makes sense as long as the load is the sum
of a relatively large number of small loads. The idea breaks down if you
have just a few high power single phase loads. A (partially) contrived
example (something similar to this actually happened to me!):

A showerhouse at a summer camp.

5 20A outlets in the boys half on one leg

5 20A outlets in the girls half on the other leg

All the girls turn on their 2000W hair dryers at the same time, while the
guys are shaving with their 5W shavers....

Now you got quite a big neutral current!

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

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