I didn’t see anything about phase protection in your last article. I realize on a 30-amp service it’s not an issue, but people should know on a 50-amp service it’s a big deal – because without it (phase protection) a miswired shore power pedestal can cause a serious overload on the neutral leg. How do you know when you have an incorrectly wired shore power pedestal? … —Larry McGaugh (from CheapHeat Systems) [See the rest of the comments and Larry McGaugh’s answer here.]
Thanks for setting up the next article for our readers. Here’s the follow-up.
As you may know, Larry is the designer of the CheapHeat System for RVs, and he’s always on top of the power situation. So I’m going to use a few graphics he sent me as well as my own to explain what he’s talking about.
What Larry is referring to is 120/240-volt common-phase wiring at the campground pedestal that could lead to a fire or damage in your RV’s electrical system.
Larry drew a nice graphic with the proper hookup on the left, and something we’ll call a bootleg or common-phase 240-volt hookup on the right. To be wired correctly, all 50-amp pedestal outlets need to have both a red and black “hot” wire connected to the appropriate side of the circuit breaker and outlet. You’ll note that even though there are two separate “hots,” there’s only one neutral “return.” And that’s all that’s needed because in a properly wired pedestal the two “hots” are actually 180 degrees out of phase with each other. That means two things happen. If you meter between these two “hots,” then you should read 240 volts. That is because when one hot leg is swinging positive, the other hot leg is swinging negative. And that causes the voltage to be “additive,” which gives you 240-volts from leg to leg. See my diagram below.
In your home that’s important since you likely have a number of appliances that need 240 volts, such as your oven and electric water heater. But typically in an RV you really don’t have any 240-volt hookups (with the possible exception of Larry’s CheapHeat System).
But what about the one neutral wire that has to carry up to 50 amps of return current from the red/hot leg, and up to another 50 amps of return current from the black/hot leg? Doesn’t that add up to 100 amps of current that could overheat the wire or plug? Happily, not in a properly wired 120/240-volt system. That’s because the neutral current in a properly wired 120/240-volt outlet is “subtractive.” So if you’re pulling 40 amps on the red/hot leg and 20 amps on the black/hot leg, the neutral will only see 20 amps (40 – 20 = 20). And if you’re pulling 50 amps on the red/hot leg, and 50 amps on the black/hot leg, then the current on the neutral will actually be 0 amps (50 – 50 = 0). That’s exactly why the designers can use a single neutral to carry the total return current from two hot wires.
But what happens if the campground miswires the pedestal with a single 120-volt hot leg which they jumper to both sides of the circuit breaker? In that case the voltage between the hot legs will measure 0 volts, but the voltage from the neutral to each hot leg will measure a confidence-inspiring 120 volts. However, this creates a really dangerous common-phase condition because now the neutral currents are additive instead of subtractive. So if you’re pulling 50 amps from the one hot leg and another 50 amps from the second hot leg, the neutral wire and contacts will be passing up to 100 amperes of current. Since it was only sized for 50 amps of current, that 100 amps will cause the neutral contacts and wiring to overheat with the potential for starting an electrical fire right inside of your RV.
How to test for this dangerous common-phase condition? Well, just metering between the two hot legs is the best way check. So, in my diagram if you measure 240 volts or 208 volts (3-phase power) between Hot 1 and Hot 2, and close to 120-volts from the neutral to each Hot, then all is well. However, if you measure close to 0 volts between Hot 1 and Hot 2, then you’re on a common-phase 120/240-volt pedestal, which you should avoid and report to someone other than the campground manager/electrician (explanation next week).
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Are there any “smart” surge protectors that warn you of a “common-phase” condition? And who do you report this condition to if not the campground manager? More on that next week, so stay tuned.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 40 years in the industry. Visit NoShockZone.org for more electrical safety tips. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.