USA Split Phase considerations?
Much of USA residential is split phase, 120V x 2 and neutral. It’s my understanding the phase difference is 180 degrees of time domain. How are the split phase legs combine to equal 220V?


Assume the neutral is at 0V.  Bear in mind that voltages are always measured between two points.  Often, you measure the voltage between one point and another point that you have declared to be 0V.  But you don't have to - you can measure the voltage between any point in the circuit and any other if you want to.

The first phase is alternating between about +170V and -170V, relative to neutral, with an RMS (root-mean-square) voltage of 120V. (170V is 120V times the square root of 2).  The second phase is also doing the same thing.   But since it is out of phase, when the first phase is positive, the second is negative, and vice versa.  So the voltage measured between one phase and the other is going between +340V and -340V, with an RMS value of 240V.

The supply voltage is actually around 110 to 120V, so the phase-to-phase voltage is around 220 to 240V.  There's always a tolerance on the supply voltage.


  Related: Do water heaters, dryers or stoves just use half the legs in separate circuits operating at 120V or are the legs somehow combine?


That's up to the appliance manufacturer.  In many cases, it's possible to connect a load between one phase and the other, ignoring the neutral.