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RVelectricity™ — How to check 30-amp outlet polarity

Dear Readers,
There was a lot of confusion last week on my RVelectricity Facebook Group about the proper wiring polarity of a 30-amp RV outlet. In fact, there were at least a dozen completely incorrect answers in the comments, as well as one insult to our Chief Admin Mike Zimmerman telling him he was wrong and needed to consult an electrician. In actuality, Mike Z has his Master Electricians License in 3 states as well as being a P.E. (Professional Engineer), so he knows what he’s talking about. Yikes!!!

What is this polarity thing?

Well, if you look at the pictorial diagram of any single-pole outlet, you’ll see that it has a hot, a neutral and a ground. The ground wire is pretty obvious because it’s connected to the contact at top of the receptacle. But the other two contacts are the neutral and hot wires (aka poles).

So if you somehow get the hot “pole” and the neutral conductor swapped, that’s known as Reverse Pol(e)arity. Get it?

Is this dangerous?

Not immediately, since if everything else is wired correctly it will not cause a hot-skin voltage. However, it can be dangerous for anyone working inside of an energized electrical panel in your RV since the white/neutral conductors will be energized with 120-volts AC, and the black/hot conductors will be at 0-volts AC. Seems a little crazy, but that’s how it works.


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Why does this happen?

With a standard 15-amp outlet you’ll see that the taller slot is always the neutral. And if it’s a 20-amp outlet it’s always the T-shaped slot which is the neutral. ALWAYS! But for 30-amp outlets the slots are exactly the same size, so you only have orientation to guide you.

And some DIY maintenance crews don’t realize that the back of the receptacle is a mirror image of the front. So looking at the front of the outlet, the white/neutral wire will be on the right if the ground it at the top. But when looking at the back of the outlet, the white/neutral wire will be on the left with the ground wire at the top. Mirror, mirror!

What about color coded screws?

Now, most NEMA 5-15 and 5-20 outlets have color-coded terminal screws, with a silver-colored machine screw for the white/neutral wire, and a brass-colored machine screw for the black/hot wire. But that’s not guaranteed.

That’s because nothing in the NEC or UL requires color-coded terminating screws. It’s completely optional on the part of the manufacturer. And to add insult to injury, I’ve occasionally found outlets with the colors of the screws reversed during assembly. So never rely on the color of the terminating screws to determine correct polarity. I know of one state campground that miswired 500 brand-new pedestals with reverse polarity because they didn’t have silver and brass terminating screws.

Any other markings?

In many of the 30-amp receptacles you’ll find an imprint of the letter “W” for the white/neutral conductor. But don’t count on it being there because many receptacles are not marked with any lettering at all.

And to keep the party going, sometimes they use black-colored screws for everything except the green ground. So for any 30-amp outlet wiring you really need to refer to a wiring diagram or commit the proper polarity orientation to memory like I do. Yes, I really do have all this stuff in my head.


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How to test for proper polarity…

While you can certainly use a digital multimeter to check for proper polarity, a much simpler and foolproof method is to use a 3-light outlet tester plugged into a 30-to-15-amp adapter, plugged into the 30-amp outlet on the pedestal.

This will cost around $10 to $15 for the 30/15-amp adapter and a 3-light outlet tester. So plug it in and look for two amber lights and no red light for the correct polarity. If it’s not correct, then contact the campground manager and have them fix it.

And yes, I just made a video showing several ways to test a 30-amp campground pedestal for correct polarity, which you can watch by clicking on the picture above or HERE.

Let’s play safe out there….

Send your questions to me at my new RVelectricity forum here.

Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.

##RVT1036

 

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John Olson
3 months ago

Thank you for all your information. Tested our pedestal at a State Park in Nebraska last year and found it to be reverse polarity. Checked 10 sites and 4 of them were reverse. Talked to the camp host and he moved us to another site. The next day they had a service technician come out and check all of the pedestals, Many of them were wired incorrectly. Thanks again… we always test pedestals before hooking up our RV.

Mike Sokol
4 months ago

And I’ll show how to test a 50-amp pedestal outlet with a combination wiring/voltage tester next week.