Quote Originally Posted by John Coloccia View Post
It's 120V per leg, and when you use two legs together (out of phase) you get 240V. That's RMS, not peak of course, because it's an AC measurement, and that's the most logical way to measure it so that you can figure out power. For whatever reason, and I'm not sure it's a good reason to be honest, in the US we've decided that the "normal" lines use one leg, with a return at ground potential, for 120V, and we reserve 240V for ovens, water pumps, air conditioners, power tools, etc. Seems kind of crazy to me, to be honest.

110V is a historical value. If memory serves, that may even go back to Edison's DC power distribution system.
There's a good reason for having the system with a ground. Suppose we had an ungrounded system, with the secondary of the power transformer as it is today, center-tapped to provide both 120V and 240V. If everything was working properly, if you touched a hot wire, you would not get shocked because there would be no return.

The problem is an inadvertent ground. Suppose one side of the secondary of the transformer gets grounded, maybe in the underground feeder, or maybe a wire rubbed against a metal pipe enough to get copper to metal contact. The system would now have a ground and you wouldn't know it. If you touched one of the other wires, you would see 120V to ground if you touched the center tap wire or 240V to ground if you touched the other wire of the transformer.

It's safer to put a ground in the system where you want it, in this case the center tap, which limits the voltage to ground to 120V.

Mike