Voltage of 120/240 comes from a standard single phase system, with two hot legs coming from the transformer, each being 120 volts to neutral, where the AC sine waves are 180 degrees out of phase.
But many industrial facilities use 3-phase power because it’s more efficient. A 3-phase Y system (there’s also a 3-phase Delta system) has 3 hot legs, each of which is 120 volts to neutral, and each of which is 208 volts to one of the other legs. Each leg’s AC sine wave is 120 degrees out of phase.
Because 3-phase power is more efficient, some utilities use it to supply residential properties too. So there will be a central transformer with a 3-phase Y system, and each home will have 2 of the 3 legs of that system, plus the neutral. To keep things balanced the missing leg will alternate among the properties served by that transformer.
There are some downsides to the homeowner with this type of system. An air conditioner is only going to see 208 volts instead of 240 volts, so it will have to work harder at the lower voltage. But pretty much all equipment is rated for either 240 or 208 volts, so it will be OK. Also, on a multiwire circuit the neutral wire will always see some current (you need to understand what a multiwire circuit is to make sense of this issue).
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