"In the United States, we use alternating current at a 60 Hertz rate to distribute power, because only alternating current can be transformed easily with transformers from one voltage to another.   This alternating current, is like a child on a see-saw, it goes up and down in "polarity" at a sixty times per second rate, which is called 60 Hertz, in honor of Heinrich Hertz, a famous scientist.  So, the socket on the wall instead of having "DC" or direct current on it, where one terminal is positive in relation to the other terminal at all times (the negative terminal), is actually going positive to negative to positive to negative smoothly at a sixty times per second rate. The maximum voltage achieved is actually about 155 volts.  Since the voltage changes follow the "sine" function as the generator rotates through a complete cycle of 360 degrees, which represents one complete rotation of the armature of the generator where the voltage originates, it is called a "sine wave" and measures 120 volts root mean square (rms) for convenience. 

Now, understand that most "single phase" (as opposed to "three phase power," not discussed here), feeds from power mains are two hundred forty volts rms, or twice the normal socket voltage of 120 volts, which originates at a transformer with the center of the transformer secondary winding becoming the return path for current from the "top" of the transformer and the "bottom" of the transformer. The neutral is usually grounded for safety reasons.

Here is the key phrase-- "when the top of the transformer is positive, the bottom of the transformer is negative by the same amount!!  Just like kids on a see-saw.  So the TOTAL voltage is the sum of the voltages taken from the "top" and "bottom" of the transformer is TWICE the voltage across either the top to center (neutral) or bottom to center of the transformer.  The fact that the bottom is just opposite in polarity to the top is mathematically stated as "180 degrees out of phase"-- 

Now, getting to RV's, here is the story.   THIRTY AMP SERVICE is EITHER THE TOP OR THE BOTTOM of the transformer feed to neutral (one leg) limited to 30 amps, versus 50 AMP SERVICE which provides the RV a whopping 50 amps from BOTH the "top" leg and the "bottom" leg of the feed transformer.  This means if you are only  using 120 volts either leg to neutral, and not using the "top" and "bottom" in series, where the neutral doesn't come into play, you have 100 amps at your service!!  You can see that 50 amp service is very expensive to provide, and is therefore usually limited at most RV parks to a select few sites.  In any case, 50 amps is the limit on either leg.   This service really had its roots in mobile home parks, and now extends to luxury coaches. Of course, with 50 amp service, you can easily run both ac's from shore power, which most RV's do not allow with 30 amp service, as in my Fleetwood Coronado.

By:  Ed Cohn, elect. contractor and electrical trades instructor, Corinthian College, Long Beach Calif.