RV Electricity – An easy way to measure 50-amp outlets


By Mike Sokol

Dear Mike,
About that 30-amp electrical tester in RV Daily Tips on 2/12/19 (review it here): That is a great idea rather than trying to figure out a volt meter … but what about 50-amp rigs? Is there a simple plug-in to test the power pole at a campsite? This would be so easy and could easily check out several near where you are parked with your rig. Would think if you are doing this at several poles that others would want to know what is going on and may purchase to check out poles on their RV excursions.

We love RV Travel and it’s helpful hints and stories. —Wanda K

Dear Wanda,
All you have to do is ask, and I have a solution for you. Yes, digital voltmeters can be intimidating to casual users. In fact, I teach an audio electrical class to my college students every year, and meter usage one of the most challenging parts of the course. That’s why I think the Prime Products Outlet tester is such a great tool for basic pedestal testing, as long as you add the correct outlet adapters. In the RV Daily Tip you mention I show how to use this neat little tester on a 30-amp outlet. But here’s how you can use it to test a 50-amp pedestal outlet using the correct outlet adapter and one more step.

If you look at how the campground pedestal is wired, you’ll see that the 50-amp/240-volt outlet isn’t only 240 volts (as usual, clink on any image for a full-size view). It’s actually two separate 50-amp circuits of 120 volts each measured from each line to neutral. Now when you measure across the two hot lines you should get 240 volts, but only rarely do RV appliances use an actual 240 volts. So it’s important to treat this as two separate tests looking for around 120 volts on each side of the 50-amp outlet.

What’s needed is the proper splitter Y-Cable that has a pair of 15-amp female outlets connected to the two sides of the 120/240-volt/50-amp outlet. And lucky for you, there is such an outlet adapter. Now I think this adapter is terribly dangerous to use for actually powering something, because it would allow you to draw 50 amps of current down an extension cord rated for maybe 12 or 16 amperes. And that could cause a fire in minutes.

Take a look at this video I made awhile ago where I purposefully overload a 15-amp extension cord with 30 amperes of current, and its temperature approaches the boiling point of water in just a few minutes.

But while I think this Y-adapter cable would be dangerous to power electric griddles on the picnic table (yes, it really is), I consider it perfectly safe for voltage testing the pedestal using a smart outlet tester such as the Prime Products AC Line Meter. Or even a more advanced Ground Loop Impedance Tester such as the Ideal SureTest Analyzer, which at $300 is WAY out of the price range for most users. However, I think it’s the best tool available for doing yearly campground pedestal inspections.

Here’s what you do. Plug your 50-amp to 15-amp Y-cable adapter into the 50-amp outlet on the pedestal. You really should shut off the circuit breaker first just in case you get your fingers on the plug contacts, but there’s no current draw at this point so no contact damage will occur. Now, turn on the circuit breaker and plug in the AC Line Meter to the first 15-amp outlet on the Y-cable and confirm that the polarity shows as correct and is measuring between 105 and 128 volts. Much more or less than that voltage on an unloaded pedestal and you’re asking for trouble once you start drawing power.

Now test the other 15-amp outlet on the Y-cable to confirm polarity and voltage. If the voltage measures more than 10 volts difference between the two sides, that’s a good sign that the neutral wire powering that part of the campground could be compromised and you don’t want to plug in. So measuring 125 volts on one side and 115 volts on the other side (10 volts difference) is the maximum difference to be expected in a campground with unbalanced current draw from a bunch of 30-amp RVs.

But if you measure 105 volts on one side and 125 volts (a 20-volt difference) between the two sides of your Y-cable adapter, then DO NOT PLUG IN. Something could be very wrong with the neutral, and the voltage on one side of the outlet could easily swing to 140, 150, or even 180 volts or more. If that happens it will be time to get out your checkbook to pay for the damages to your RV, which can easily reach thousands of dollars.

Of course, I highly recommend you also get (and use) an Intelligent/EMS surge protector from Surge Guard or Progressive Industries, since they constantly monitor the shore power for any voltage problems and will disconnect your RV from the pedestal if something goes wrong. Any electrical problems in the campground (or even your home electrical service) can cause extensive electrical damage to your RV in just a few seconds.

Now for any of you DIY folks that are going to tell readers, “You can build your own Y-adapter cable for less than $15, so don’t waste money on a $37 premanufactured cable,” please keep those comments to yourself. Unless you’re already adept at wiring electrical outlets and following schematics while using a voltmeter, then you have NO BUSINESS trying to wire one of these Y-cables yourself. So telling someone else to do that is irresponsible, and if you post a comment to that effect I’ll delete it immediately. My class, my rules. Get it?

Here’s where you can buy the 50-amp/15-amp Y-splitter on Amazon.com.

And here’s where you can buy the Prime Products outlet tester on Amazon.com.

And here’s a great source for these Surge Guard Products: 30A Surge Guard; 50A Surge Guard.

Let’s play safe out there….


Mike Sokol is an electrical and professional sound expert with 40+ 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.


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27 days ago

The test that you described wouldn’t tell you if the 2 hots on the 50 amp receptacle were being fed by opposing poles or a jumper wire from a single pole which wouldn’t give you 240 volts.

Randy Shrimplin
1 year ago

I would think that a true 50 amp tester would be available as the test you describe, while WAY better than nothing, does not look at the 2 hots to make sure they are not jumped from the same leg which would cause a neutral overload. I propose adding one of those little neon voltage testers to be placed across the hots to make sure you are getting 240
Like this one:
Seems to me that someone could come up with and market tester for a 50 amp outlet that would test voltage on both legs and combined, neutral, and ground.

Tom Herd
1 year ago

Speaking of 50-amp service, I am looking to purchase an EMS for my new Grand Design, when I pick it up in a few weeks. Since we will have two air conditioners we will, most often, be hooked up to 50-amp service.

My question is: When we don’t have access to 50-amp service, and use a 30-amp connection, will the 50-amp EMS still work? I think that I understand the basics of the unit, I’m just unclear of how it would work if it is hooked up to a 30-amp connection.

Thanks, and I learned a lot from your presentation in Hershey, last year.

Tom Herd
1 year ago
Reply to  Mike Sokol

Thanks, so much. You are a wealth of information, and I truly appreciate the quick response.

Brian Jensen
1 year ago

I use a 20 amp circuit tester plugged into a cheap 20 amp to 30 amp adapter to check the campground pedestal before I plug in. It doesn’t catch everything but it catches a lot and is very easy to use. I keep it in the power cord compartment.

Mike Sokol
1 year ago
Reply to  Brian Jensen

Brian, I’m curious as to what 20-amp circuit tester you’re using?

Bill Klaes
1 year ago

Because “digital voltmeters can be intimidating to casual users” could you do a column(s) outlining the common settings found on voltmeters that are used when doing pedestal testing? Please.

Mike Sokol
1 year ago
Reply to  Bill Klaes

That’s a feature article in my RV Electricity Newsletter tomorrow. Are you already subscribed?

Dave Telenko
1 year ago

Mike, I take it that they don’t have a tester for 50 amps at the pedestal? i have one of those “Y” connectors, actually the dealer gave it to me when I bought my RV. Mine had the female 50 Amp end & the “Y” had a 30 Amp & a 15 Amp female on it, as I recall I plugged the 50 Amp into my motor homes 50 Amp cord & the 15 Amp into my house 15 Amp extension cord, well that didn’t work, I didn’t know why. So the dealer tried it & with my plug got the same results. So they replaced the transfer switch under warranty. Only to find out it still didn’t work, but worked Ok when you plugged it in to a normal 50 Amp connection. So I guess you can’t do what I had done or the “Y” was broken! So what do you say Mike?

Mike Sokol
1 year ago
Reply to  Dave Telenko

You’re going the “other way”. You can’t combine a GFCI protected outlet with any other outlet since the unbalanced currents is a guaranteed trip. I’ll do an article in a few weeks showing you why this won’t work.

Mike Sokol
1 year ago
Reply to  Dave Telenko

Dave, there used to be a 50-amp/240-volt tester made specifically for pedestal testing, but it hasn’t been available for years. If someone knows of an existing product, please let me know and I’ll do a review of it.

5 months ago
Reply to  Mike Sokol