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.
I realize this is an old post but I just stumbled upon it today. On the top drawing, the neutral and hot are switched on the 30 amp outlet.
Three things I would recommend to people that have a meter and want to check the pedestal voltage.
1 inspect the leads for condition first, no broken wires, loose connections and etc.
2 use insulated gloves rated for the voltage you expect to test. I purchased mine at an electrical distribution supplier.
3 make sure you have prongs long enough to make contact, avoid putting a screw driver in the outlets.
Turning the breakers off before testing is advisable and try to stay clear of the meter is best do not hold it in your hand. I have seen expensive meters blow up and the idea of someone using one from the dollar store scares me. I always check the base out first and then use an expensive surge protector that also. verifies the condition of the pedestal. Joe Bulger retired power plant electrician
Just below the correctly and incorrectly wired 50 amp pedestals there is a paragraph and in it there is a sentence about the two hots being 180 degrees apart. I think that is 120 degrees as all 3 phases would be 360 degrees and any two are 120 degrees from each other. I enjoyed the read. Keep the information coming.
3 phase has 3 hots that are 120 deg out of phase, 2 phase has 2 hots that are 180 deg out of phase
Yes, 3-phase service has 3 hot legs that are 120-degrees apart. And standard 120/240-volt service is actually 2 phases that are 180 degrees apart. But you dare not call that 2-phase to any electrician or engineer. That’s because they consider a 120/240-volt service to be a single-phase that is “split” at the transformer into 2 hot legs of 120 volts each. There was indeed a 2-phase service in the early 1900’s with 2 legs that were 90 degrees apart. But that produced too much vibration in a poly-phase motor. Wait till you look at Delta vs. Wye service, and High-Leg Delta service will drive you crazy if you’re not careful.
Have never had a problem with a 50 amp plug (other than low voltage) but I did try to use a “cheater box” on two 30 amp plugs and blew the breaker and fried the box. I think the neutral and hot wires were reversed on one of the plugs, and so when I plugged in the cheater I made a dead short between the hot wire on one and the neutral on the other.
Cheater plugs can be very dangerous if the pedestal isn’t wired exactly right. And that’s why I won’t recommend them to the casual user who doesn’t know how to measure every aspect of the shore power before plugging in. But I see many RV’ers using them simply because the campgrounds don’t’ have enough 50-amp equipped pedestals. As RVs get more power hungry and campgrounds refuse to invest in improving their infrastructure, I expect to see even more electrical problems and damage to RVs. So protect yourself and your investment with a smart surge protector.
Hi Mike I keep seeing all the great stuff your writing, I hope that the readers take heed to your advice, I know I have. I’ve actually dry camped for the last 45 years or so, at best my generator in the last 18 years saw about 350 hours on the meter & that was mostly running it once a month with the ac on to keep the carburator & generator stuff cleaned up, I think in time span I hooked up maybe 6-8 times & didn’t know that I should be checking the pedestal. But now older & getting a more modern ac everything, we sometimes go to (yuck) RV parks & hook up. I bought your book & it’s very informative. Hey I’ve even up graded my VOM from Harbor freight to a Sears multi function one thats a bunch better. I used it while on our 4 week vacation & HAD to (yuck) stay in RV parks. I never found anything out of line, but its good to know how to check it out before you ever plug in.
Thanks again for all your info
I can not emphasize enough the importance of a quality EMS in any RV plugged into strange Joe’s pedestal. I actually had a 30 amp cord melt the end when plugged into an RV pedestal at an RV park in Wallowa,Oregon.Of course the owner denied any problems with the park’s wiring.It is not if,but when,a big costly surprise will meet you head on.
I agree. The cost of a smart surge suppressor is probably less than the deductible on your RV insurance policy. Plus insurance won’t pay for a lost vacation if your RV’s electrical system takes a hit and melts down. It’s just a smart investment.