On 2005-03-20, Doug Miller wrote:

Yes, that's true, but you're talking about resistances measured in
tenths of ohms, in series with resistances measured in hundreds of
ohms. As a practical matter, except on *very* long runs, the
resistance of the conductors is not really a concern.

Very good, I agree with this. Voltage drop due to the conductors only
matters for long runs. But it is still a reasonable question whether
sharing a neutral between two circuits will reduce that voltage drop,
even if it only matters for long runs.

But you're missing the fact that the voltage drop between the source
and the load is almost completely independent of the resistance
*elsewhere* in the circuit.

True, the voltage drop between the source and the load only depends on
the resistance of the hot. But the "available voltage" at the load
depends on the voltage drop of both the hot and the neutral.

Consider an example: exactly 120V available at the panel, a single
circuit with (long) wiring with a resistance of 0.1 ohms each for the
hot and the neutral, and an electric heater with a resistance of 10
ohms (nominally a 1200W heater). If the heater were wired directly to
the panel, the total resistance is 10 ohms and the heater would draw
12 amps. With the (long) wiring, the total resistance is 10.2 ohms,
and the heater draws 11.76 amps (120/10.2). The voltage drop across
the hot is 1.176 volts, the drop across the heater is 117.6 volts, and
the drop across the neutral is 1.176 volts.

Now suppose we have two 10 ohm heaters on an Edison circuit (shared
neutral). This represents the best case of totally balanced loads,
and the current on the neutral will be 0. So this is equivalent to
having the two heaters in series across a 240V supply. Each hot
conductor has the same resistance of 0.1 ohms. The total resistance
is 20.2 ohms, and the current is 11.88 amps (240/20.2). The voltage
drop across the first hot is 1.188 volts, the drop across each heater
is 118.8 volts, and the drop across the second hot is again 1.188
volts.

In the single circuit case, the "voltage drop" the single heater saw
was 2.4 volts (120 - 117.6). In the shared neutral case, the voltage
drop was only 1.2 volts (120 - 118.8). This illustrates the best case
scenario of using a shared neutral: with perfectly balanced loads, the
voltage drop due to the wiring with be half as much.

Cheers, Wayne