The old 110/220 volt standard has been replaced in recent decades with the modern 120/240 volt standard for single
phase services. You're unlikely to find 220 volts anywhere in the real world, apart from a
Variac on your bench, anyway.
Well, yes and no. There is actually very little standard to this "Standard." Your
voltage is whatever the
power company delivers to your door. Sometimes, it's 110, sometimes 115, sometimes 120, sometimes 130. If you complain the likely response will be "Yea, well."
Line voltage in OUR area is pretty tight in the 125 to 127 range. Each time the
power company upgrades equipment, it is likely to climb a
bit.
120, 208, 240, 277
etc., Good numbers to generalize on, but it is more the ratio between them that is important. If your 120 tracks high, so will your 208 or 240. (If 130v then 225 / 260) Yet, we still have to use SOME number!? So, why not use 120/208/240. If you average it out over the whole US, it is probably close to that.
Still, although off topic, the biggest problem with all of that is lamps. Generally, theater lamps come in 115 and 120 flavors. Dimmers are about 97% efficient, so 120 volt lamps are acceptable on 125 volt sources. Dimmers can often be calibrated to max out at 120 volts even if the
line is up at 130.
As for magnetic ballasts, the saturation of the core provides some regulation. Usually there is a 208 and a 240 tap. I would still use the 208 tap even if it was running high at 225. Because of core saturation, you are not going to reduce the lamp life that much, where as on a fixed tap
ballast, low
voltage will cause drop-out problems. Auto-range E-ballasts eliminate the problem.
208 single
phase: If all you are getting is two hots and no
neutral, then there is no
phase relationship to anything else and it could be considered "single
phase." If you are getting 208 with two hots and a
neutral, then you actually do have a
phase relationship as there is 120 between each hot and the
neutral, but 208 between your two hots. Does it matter? Not really. Even though dimmers are connected to multiphase
power, they are single
phase devices. Any 208 movers are single
phase as well. Just be sure of your
headroom on your
neutral capacity.