Originally Posted by
alanh
However, 120V is provided using one of those conductors (called "hot") and "neutral", which is tied to earth at the service entrance. A third conductor also tied to earth is provided at outlets. Ideally, the neutral line and earth should both be at 0 volts, but for various reasons they might not be.
So the hot wire will always have 120VAC between it and the neutral line and earth.
I am glad you said that, it is a common misconception that the 110V mains supply in the US is balanced. Here's a snippet from the
electrical wiring FAQ which sums it up;
One thing where things might get a bit confusing is the
different numbers people bandy about for the voltage of
a circuit. One person might talk about 110V, another 117V
or another 120V. These are all, in fact, exactly the same
thing... In North America the utility companies are required
to supply a split-phase 240 volt (+-5%) feed to your house.
This works out as two 120V +- 5% legs. Additionally, since there
are resistive voltage drops in the house wiring, it's not
unreasonable to find 120V has dropped to 110V or 240V has dropped
to 220V by the time the power reaches a wall outlet. Especially
at the end of an extension cord or long circuit run. For a number
of reasons, some historical, some simple personal orneryness,
different people choose to call them by slightly different numbers.
This FAQ has chosen to be consistent with calling them "110V" and
"220V", except when actually saying what the measured voltage will
be. Confusing? A bit. Just ignore it.
One thing that might make this a little more understandable
is that the nameplates on equipment often show the lower (ie: 110V
instead of 120V) value. What this implies is that the device
is designed to operate properly when the voltage drops that
low.
208V is *not* the same as 240V. 208V is the voltage between
phases of a 3-phase "Y" circuit that is 120V from neutral to any
hot. 480V is the voltage between phases of a 3-phase "Y"
circuit that's 277V from hot to neutral.
In keeping with 110V versus 120V strangeness, motors intended
to run on 480V three phase are often labeled as 440V..."