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.