From what I'm reading on the list it sounds like the NEC definitely allows
sharing a neutral between two 120V branches with opposite phasing.


>>  I have seen many homes wired with a 12/3 supplying 2-20 amp circuits,
opposite phases.  This does save wire but if the neutral is lost the circuit
goes to 220 and you can say goodbye to any equipment plugged in that is
rated for only 120.  I owned an apartment building years back in Chicago and
a tenant called to tell me that some of her electrical appliances were
smoking (clock radios, etc.)  Sure enough, the small apartment was fed by a
single 12/3 with 2 circuits.  The neutral had come loose in the breaker
panel and poof!  The previous owner was an electrician who had rewired the
building.  He saved time and money with this wiring job, and I had to fork
out to cover the blown equipment.

This is a tempting way to save money, but not for me, not with this object
lesson in my past!

Another critical part of NEC is that circuits have to be derated 80%.  So a
20 amp breaker should not be loaded to more than 16 amps, or 1920 watts.
Some electricians also state that for a load that is on under 3-4 hours you
can run it at 20 amps, but they still usually derate their circuits at 80%.
In the home automation industry, we always teach this basic rule.

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