On 11/7/2009 11:51 AM Robert Green spake thus:

I believe the problem most people have with shared neutrals is that
it seems like they are adding 1 plus 1 and getting 1 as a result.
However, since they are out-of-phase, you're really adding 1/2 plus
1/2 and coming out with 1. When the first circuit's amplitude is at
its peak, the other phase is at the bottom, and the two phases cancel
each other out. It doesn't seem to be common sense that by adding a
load to the other half of unbalanced load that you'd actually be
reducing the current in the shared neutral wire, but that's how it
works. I think. (-: At least that's how my friend who designs 240VAC
gear explained it me.

That's 'zactly right.

Being 180° out of phase, any current running in one leg of the circuit
will be cancelled by any current in the other leg. The amount of
cancellation depends on the amount of current being drawn in each leg.
The highest current possible in the shared neutral will be when only one
leg is drawing maximum current. If both legs draw maximum current, then
the current in the neutral is close to zero. In this case, the majority
of the current flow is through the two "hot" wires.

Very clever idea. However, for reasons given here many times, I think
Edison circuits are to be avoided, on account of the potential problems
they can cause.

Spend the extra 25 cents and use paired hots and neutrals.


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