In Mark Lloyd writes:


[snip]

I have heard of:

110/220
115/230
117/234
118/236
120/240
125/250

There is always a 1:2 ratio (higher voltage is twice lower one). Older
people often say 110/220. I measure almost exactly 120/240 here.

Except where there isn't.

Many places in NYC are... 120/208

This is what happens when you take two hot legs
of a "three phase" circuit.

Because of the housing density (especially with
all the apartment office buildings) it's quite
common to have three hot legs (plus neutral) come
off the local transformer.

Apartment A will get legs 1 and 2 (plus neutral),
apt B gets legs 2 and 3, C gets 1 and 3

Rinse, lather, repeat.

All three legs (plus neutral) go to the
elevator motor and the HVAC system, etc.

This is also the case with private homes,
where the first one on the block gets
legs 1 and 2, etc., etc.

And yes, despite manufacturer claims that their
higher voltage equipment can handle the lower
voltage, a LOT of the time it doesn't.



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