On 18 May 2006 04:15:54 -0700, wrote:
OK. So what you're saying is that the more balanced the two circuits
on the double pole breaker are, the less voltage drop I will have.

That's WRONG. Look, there is no free lunch. Your fixtures and appliances will
draw their rated current, and incur the same voltage drop no matter how well
balanced the multiwire circuit is.

It is true that, if perfectly balanced, there will be no current in the
neutral wire in a multiwire circuit. But in this case, the entire load is
borne by the two hot leads. Any imbalance results in a current in the neutral
lead equal to the magnitude of the imbalance (assuming pure resistive loads).
But at ALL times, the total current is the same.

The multiwire circuit is just a "trick" to enable having two hot lines share a
single neutral that is undersized by 50%. It works because the worst-case
imbalance, where 100% of the load is on one of the hot leads, can only result
in the same current flowing in that neutral as is flowing in the hot wire.

Theoretically then, if I had a 250' run with two outlets at the end of
the dual pole 20A breaker (one on circuit 1 and the other on circuit 2)
and had a 15A load on BOTH of them I'd be OK. But, if I removed ONE of
the 15A loads, I could be in trouble because the voltage drop would go
up and drive the load way up. Correct?

Nope.

Think of it this way. You have a pair of 100W incandescent lamps, one
connected to each circuit, at the end of 250 feet of wire. (For this example,
I assume these bulbs are ideal constant resistive loads.) A 100W light bulb
looks about like a 144 ohm resistor, so it will draw about 0.83A when the
voltage across it is 120V. When both lamps are on, that current is flowing
through both lamps. No current is flowing in the neutral. The total length of
wire feeding this circuit is 500 feet (the two hot leads in series).

Now turn off one of the lamps. What happens? Now the same 0.83A is flowing
though the one hot lead, and the neutral lead. The total length of wire
feeding this circuit is exactly the same, 500 feet (one hot lead and the
neutral lead). So the total voltage drop is the same.

I'll leave the thought experiment for an partial imbalance to you. Try it with
one 200W and one 100W lamp.

--
Art