50 Amp shore powerby Tom Jones
This subject had been bugging me for several years. I'd even asked electricians, technicians and engineers and never received a straight answer. My understanding was also clouded by my background in UK power distribution systems which are quite different from those in the U.S. So I did a little research of my own.
Our power distribution system is called split phase (a single phase supply fed by a center-tapped transformer), which provides two legs of 125V power. This applies equally to our domestic electrical service and that of 50A service at campgrounds.
Here's the wiring diagram(14-50R is the receptacle used for 50A service at a campground):
Diagram from Leviton's catalog.
Note: The line labeled "W SYS GR" is the neutral, which is often connected to earth ground back at the power source. This should not be confused with the line labeled "G EQUIP GR", which is the true earth ground connection.
When we plug our RVs into 50A service, our appliances and a/c units are shared between the two 125V legs; Some are wired between one leg and neutral, and some are wired between the other leg and neutral. Some high-end coaches have 250V appliances which will be connected across the two legs, rather than between one leg and neutral.
A little light reading (no pun intended):
Warning Wiring of shore power should only be conducted by a licensed electrician.