Kevin Aylward said:
It doesn't because its wrong. The official standard in the US *IS* 120V.
It is also interesting to look at the standard voltages as you go up from
low voltage to medium and high voltage. For single phase, the standard seems
to be 120/240. For three phase, it is 120/208. There is also 240 V three
phase, which would have 138 VAC Phase to neutral, but this is rarely used.
Then 480 V three phase, which is 277 VAC phase to neutral. The 277 is often
used in lighting circuits. You sometimes see 416 VAC (208*2), and there is
also some 600 VAC, which is the limit for "low voltage".
I have also heard that power sources are rated higher, as 240 or 480 VAC,
while the equipment that uses these sources is normally rated lower, as 220,
230, 440, or 460. It may also apply to 120, 115, and 110. It is interesting,
however, that I have never heard of anything other than 208 VAC for that
type of three phase, even though it typically varies from 200 to 216 VAC.
In the medium voltage area, from 600V to 5000V, very common voltages are
4160, which is 208 * 20, and also 2400, which is just the line-to-neutral of
4160. There is also 4800 V, which is about the limit. These are all based on
ratios of 2, 4, and 5, as well as, of course sqrt(3) to convert L-L to L-N
for three phase. Medium voltage is commonly used for high power motors and
generators, and for residential and small commercial and industrial
distribution lines, in cables and overhead.
I don't know as much about standards for high voltage AC. I think 13.8 kV
and 14.4 kV are fairly common, and possibly 57.6. Of course, these are also
the same numbers as common baud rates! I think 100 kV or 120 kV is also
common. The really big transmission lines I think are in the order of 500kV,
750kV, 1MV, and maybe up to 2MV or so.
Many long distance transmission lines are now DC, because of about 50%
better efficiency due to less radiated emissions and impedance losses. There
are some interesting articles on the effects of having a high voltage field
of DC with respect to the earth, where there is a constant "wind" of ions
traveling from one conductor to another. There are supposed health benefits
from negative ions, and feeling of impending disaster from positive ions
(which is evidenced just before lightning storms in behavior of animals,
including people). This strays a bit from the original question, but it is a
matter of interest.
The actual conversion from AC at the generator, to DC for transmission, and
back to AC for distribution, is also interesting.
