On May 13, 10:30 pm, wrote:

Is this the system where the voltage is 133 volts relative to ground and 230
volts between phases (and formerly 127 volts relative to ground and 220 volts
between phases)?

Since I'm posting from GoogleGroups I can't respond to Phil, but the
rest of you can be enlightened.

In 120/240 or similar systems there is not the freedom to choose this
ratio. The wiring of the source transformer determines it. As others
have noted, in the "Edison" U.S. system the source is a center tapped
transformer with the center tap grounded. This makes a two phase
system with each 120v "leg" 180 degrees out of phase with the other
one. The ratio of the high voltage (240v) and the low voltage (120v)
is always therefore 2:1.

In a three phase system there will be three transformers with
secondaries (one for each phase) wired in a "star" or "Y"
configuration. This is necessary because you need the center point of
the "star" or "Y" to be ground for each low voltage phase. If you wire
with a "delta" configuration there is no central grounding point
available for the individual phases. IN three phase circuits the
relationship between that individual phases to ground (say 120v) and
the voltage measured between phases is not arbitrary. It is always
determined by the square root of 3. Hence the between phase voltages
being sqrt 3 x 120 = 208V. Just like the two phase system these
ratios are determined by physics and can't be arbitrarily set.

Of course there is the issue that electric companies often will name a
voltage one thing while actually supplying an other for small
variations about the "standard" voltage.