120 plus or minus 5%.

There are many stories about where the other values come from; I haven't
heard one that sounds credible.

http://nonstop.compaq.com/TechPubs/P...nts/TPAPPA.pdf

In the USofA, You can always get three-wire service, with 2 feeds
that are nominally 120V to ground, and 240V from each other, at 60
Hertz.
The voltage tolerances vary by jurisdiction from 2-8% of the nominal
voltages. In many places, you can also get 4-wire residential service,
still with 120V on each hot to ground, but with each of the three lines
at 208V from each other.


Most of Eurasia appears to use 2-wire service with 220V to ground,
or variations thereof.



--Goedjn