Geek Alert: If you don't know or care to know about 3-phase power, then skip what follows since it gets a bit deep. But trust me, it's definitely campground related.
What most of you encounter is single phase 120/240 volt power. That's what you've got in your homes and in most campgrounds. And while electricians never call this 2-phase power, indeed it is two opposite phases of 120 volt AC with 180 degrees of phase shift. So as most of you know, grabbing one hot leg (phase) of 120 volts AC plus the other hot leg (phase) of 120 volts AC will add up to 240 volts AC.
However, all industrial buildings run something called 3-phase power, which is done for two simple reasons. You can run the same amount of power over less copper wires, and it makes electric motors really easy to start and run clockwise or counterclockwise, just by reversing any two of the phases. That's because the 3 legs are at 120 degrees of phase shift to each other, and this forms a natural magnetic rotation caused by the 3-phase voltages swinging up an down in perfect synchronization. It's a beautiful thing.
And that same 3-phase power has two main versions, the first one being a Delta transformer (shaped like a triangle on schematics) and the second one being a Wye transformer (shaped like a Y on schematics). However, back in the 60's it was common to see a hybrid version called a high-leg Delta transformer in manufacturing facilities. See
http://en.wikipedia.org/wiki/High-leg_delta for a tutorial on it. The trick is that in a standard 3-phase Wye distribution panel you'll measure 120 volts from the neutral to any of the 3 hot legs, and 208 volts between any two legs (phases). But in a high-leg Delta panel, any hot-leg will measure 240 volts to any other hot leg, but only two of the hot legs measure 120 volts from the neutral. The third "high-leg", "red-leg", or "wild-leg" measures 208 volts to neutral.
This was originally done to save the cost of an additional transformer at a factory, and since most everything in a factory that draws a lot of power can use 240 volts, it was a reasonable design. However, you had to be careful when hooking us a single-pole circuit breaker for 120-volt office power not to pick the third "high-leg" phase or you would send 208 volts AC to all of your 120 volt wall outlets.
The reason I relate this is that last year while in Texas teaching a sound seminar, my load-in crew came in a little late because they were working on expanding a campground area by wiring in a bunch of new campsites. However, the "electrician" had found the 3-phase high-leg delta panel with only two rows of circuit breakers populated on the 120-volt phases. He just assumed he could plug the new 30-amp 120-volt breakers into those empty spots, which then sent 208 volts AC to all the new campsite's 30-amp 120 volt pedestals. Several campers had just hooked up and were burning up their RV electrical systems because of 208 volts rather than 120 volts on the 30 amp outlets. The campground owner had blamed the RV electrical problems on "lightning", but after they burned up half a dozen RV's, finally got out a meter and figured out why the third "high-leg" phase of that panel had not been used up to this point. That phase only puts out 208 volts. What a mess.
This all could have been avoided if the electrician had simply metered the panel before plugging in a dozen new circuit breakers. And if he knew anything about 3-phase power he should have seen the orange markings on the high-leg phase and been warned.
Interestingly, the latest version of the NFPA 70E (National Electrical Code) will allow campgrounds to run any two phases of 208 volt 3-phase WYE transformer power to campsite outlets normally marked as single-phase 120/240 volt power. I expect a whole lot of wiring mistakes at campgrounds very soon.
Mike Sokol
mike@noshockzone.org
