RV Electrical Safety – Part 3 – Measuring outlets

By Mike Sokol

Everyone,
Here’s Part 3 of my 12-part series on RV Electricity which I originally posted in 2010. I’ve updated it with the latest information on 3-phase campground power and improved shore power connections. But first you should re-read Parts 1 and 2 so you’re familiar with the terminology.

Read Part 1 on basic electrical concepts HERE.

Read Part 2 on how to use a digital meter to measure AC voltage HERE.

If you read the survey we did way back in July 2010 at RVtravel.com, you know that 21% of RVers who answered the poll have been shocked by their RV. What follows is Part 3 of my 12-part series about basic electricity for RV users and how to protect yourself and your family from shocks and possible electrocution.

This series of articles is provided as a helpful educational assist in your RV travels, and is not intended to have you circumvent an electrician. The author and RVtravel.com will not be held liable or responsible for any injury resulting from reader error or misuse of the information contained in these articles. If you feel you have a dangerous electrical condition in your RV or at a campground, make sure to contact a qualified, licensed electrician.

Reruns

Last week we learned how to read a basic digital volt meter and test a 15- or 20-amp standard outlet such as you might find in your living room or RV interior. Now it’s time to move up the ladder to testing 30- and 50-amp campsite outlets. Again, you’ll be handling live voltage so take all safety precautions. As usual, you can click on any graphics to see them full size.

• Only use one hand to touch the meter probes or campsite pedestal. Electricians are taught to put their other hand in their back pocket so they don’t lean on anything.
• Don’t stand on wet ground while testing outlets. If the ground is perfectly dry you should be safe wearing dry sneakers. If not, then put a dry rubber shower mat down on the ground to stand on while checking voltages. Standing in the rain while measuring live voltages with a meter is dangerous and to be avoided if possible.
• Always make sure to turn off the circuit breakers on the power pedestal before plugging or unplugging your RV from campsite power.
• Safety or standard prescription glasses are highly recommended. These don’t have to be anything fancy, but if something goes wrong you’ll be glad you were wearing glasses. I owe my eyesight to the fact I was wearing glasses when an electrical panel shorted out and blew up right in front of my face. It’s cheap insurance.

Bigger outlets

Today’s RVs have much greater power requirements than those of even 10 years ago. You have lots of appliances, so that a single 20-amp outlet like you find in your house can’t provide nearly enough current. This is when you need to step up to 30-amp outlets at the campsite. Let’s see how they’re wired.

Here’s a side-by-side comparison of a “home-style” 15- or 20-amp outlet on the left and a special “RV” 30-amp TT-30 outlet on the right. Next week we’ll get into how the amperage rating affects the number of appliances you can run, but for now we’re just measuring voltages.

The first thing you need to note is the orientation of the ground lug on both sockets. Click on any picture for a full-size illustration you can print out.

Last week we used a 20-amp outlet for an example with its U-shaped ground lug at the bottom, while in this picture the ground lug is at the top. That was not a mistake, as most home outlets are wired with the ground at the bottom, and most electrical pedestals should be wired to more recent campground code with the ground at the top. It doesn’t really matter which way the socket is wired as long as you keep your own head right-side-up. So if you’re looking at an upside-down outlet, turn the illustration upside down to match the outlet. But note that if the campground wires their outlets “ground down,” then you’ll have a difficult time plugging in your shore power cord or portable surge protector. All campgrounds should really have the ground pins oriented at the top of the electrical outlets.

While most casual RVers will recognize the little U-shaped hole as the ground, it’s the other two slots that get confusing. As we learned last time in Part 2, on a 15- or 20-amp outlet the taller slot is always the Neutral wire, and the shorter slot is always the Hot wire. Here’s a little more info: If you peek inside an electrical panel or extension cord plug, the ground wire is always GREEN, the Neutral wire is always WHITE, and the Hot wire is BLACK and sometimes RED. This holds true for both the pedestal outlets as well as any extension cord you may have.

However, on a 30-amp outlet, there’s no difference in size between the Hot and Neutral slots when viewed from the outside. So here’s where you just have to remember this fact or print this page out. Think: While looking at the front of the outlet, if the ground is at the top, then the “white” is on the “right.” That’s how I remember it. If you’re looking at the back of the receptacle you’ll see two different colored screws, and the white wire goes under the white screw while the black wire goes under the brass screw — but we’re getting ahead of ourselves as that’s a future article on testing extension cables.

Settings

Remember to set your voltmeter to an AC voltage scale more than 250 volts, typically 400, 600 or 750 V AC. Plug your meter leads into the Black COM and Red VOLTS connections on the meter, and get ready to poke the meter probes into the receptacle. Now turn on the circuit breaker in the campsite pedestal and push in the reset button on the GFCI if it’s popped out.

You can see from the red triangle diagram in the middle of the illustration that from Ground to Neutral you should measure close to 0 (zero) volts, but up to 2 or 3 volts is expected on heavily loaded circuits. From Hot to Neutral you should expect to measure around 120 volts (actually, 108 to 128 is generally acceptable), and from Ground to Hot you should also measure around 120 volts. Probe between each two slots and note whether the voltages are correct.

Be especially careful that the Neutral to Ground reads close to 0 (zero) volts and the Hot to Ground reads around 120 volts. Anything more than the expected voltage drop on the Neutral suggests a poor connection or overloaded wiring. I should note that code suggests 3 volts maximum is expected and safe, but if you measure more than 5 volts between Neutral and Ground something is very wrong. If it measures full voltage (near 120 volts) then the outlet is wired with a reversed polarity. Generally that’s not immediately dangerous, but it does indicate the pedestal was not tested after it was installed, so other things could be miswired as well.

You can accept as low as 108 volts and as high as 132 volts on a 120-volt feed using the 10% plus or minus rule, but realize there will be additional voltage drops at the pedestal outlet when you draw any amperage, and you’ll also have a few volts loss in your extension cord(s). So it’s better to start with at least 115 volts on an unloaded pedestal since it really needs to stay above 110 in your RV to guarantee that none of your compressor motors (air conditioner or residential refrigerator) suffer from brown-out problems. (Voltage drops will be covered in part 6 of this series.) Plus a green Ground wire could have up to 2 or 3 volts between itself and Neutral,  but any more voltage on the Ground wire suggests the outlet is wired incorrectly so DO NOT PLUG IN YOUR RV. Notify the campsite manager immediately and get this checked out by an electrician. Do not open the pedestal box yourself and poke around inside.

What’s this 240-volt thing?

Perhaps the most confusing part of hooking up an RV is that some plugs are 120 volts while others that look similar are 240 volts. Just how do they manage to get two different voltages out of the same wires? Glad you asked.

If you remember our water tank example from Part 1: the taller the tank, the greater the pressure. And since voltage is really electrical pressure, the same idea holds true. Look at the tank on the left and imagine you have a pressure gauge that reads the difference between two pipes. So if you read between the red pipe at the top and the black pipe at the bottom, your gauge (or meter) will indicate the full pressure, which is in this case 240 PSI (Pounds per Square Inch). However, if you hook up the gauge (or meter) from a center pipe to either the top or bottom pipe, it will indicate exactly half the pressure, which in this case is 120 PSI.

The exact same thing happens at the power transformer on the pole feeding into your house or RV site. You really have 240 volts available, but there’s a center tapped transformer rather than a pipe. So if you connect a meter (or appliance) between the Red and Black wires, it will receive the full electrical pressure, which is 240 volts. But pick only the Black or Red “hot” wire and hook the other side of your meter or appliance to the center-tap White “neutral” wire, and you’ll have exactly half of the full voltage, which will be around 120 volts.

50-amp outlets

If your RV has a 50-amp, 120/240-volt plug for its power connection, you really have to understand what you’re hooking into and testing is critical.

Take a look at the illustration on the right. You’ll see a standard 120/240-volt, 50-amp receptacle as found in many campgrounds, and the slots are placed like a little baseball diamond. If it’s oriented according to code with the U-shaped ground at the top, then follow along. If you plug your meter probes from home plate (Neutral) to 1st base (Hot 2) you should read around 120 volts. From 1st base to 2nd base (Ground) you should also read about 120 volts. From 2nd to 3rd base (Hot 1) should read approx. 120 volts, and finally from 3rd base back to home you should read  approx. 120 volts. Now, from home plate to 2nd base you should read close to zero (0 to 2) volts, and from 1st base to 3rd base you should read between 230 and 240 volts.

So, as you move your meter probes around the bases, every slot to the next slot should read about 120 volts. As you read sideways across from the left side (Hot 1) to the right side (Hot 2) you should read between 200 and 260 volts. And, as you measure from top (Ground) to bottom (Neutral), you should read close to zero (0 to 3) volts.

If your meter reads anything else, STOP IMMEDIATELY, shut off the circuit breaker and notify the campground electrician. DO NOT plug in your RV or any other gear as it could be damaged or you could be electrocuted.

Thanks for the memories

You don’t have to remember all these connections we’ve discussed as each of the plug diagrams above has been scaled for printer output. Just print out each illustration and put it in a little notebook. After a few times it will seem quick and simple, so don’t become complacent. Just remember you could test 99 campsites as perfect, but it could be number 100 that has a wiring issue that could electrify the skin of your RV or destroy every electric appliance you have plugged in. So don’t take a chance, always test before plugging in.

If you think there’s an electrical problem with your RV or campsite outlet, don’t try to fix it yourself. Get a licensed electrician or RV technician to make the repair. And, if you ever feel a shock from your RV, immediately get away from it and shut off the circuit breaker in the campsite pedestal and unplug from shore power. Then notify the campsite electrician and refuse to hook up your RV to the pedestal until the problem is resolved.

UPDATE – 3-phase voltage readings

Since the last revision of the National Electrical Code allows RV campgrounds to use two legs of 3-phase power for 50-amp/240-volt pedestal outlets on standard 50-amp NEMA 14-50 outlets, there’s one additional voltage reading possibility that’s perfectly safe and acceptable. This is something called a 120/208 3-phase WYE electrical service. In that case, you will still read 120-volts from the Neutral to any of the Hot legs, but nominally 208 volts between the Hot legs, instead of 240 volts you would normally expect. Of course, this could be a little higher or lower depending on the nominal voltage the campground is getting from the power company.

So in a campground with a 50-amp/240-volt outlet, once you determine that the voltage from the Neutral to each of the Hot legs measures around 120-volts, if the Hot-to-Hot voltage measures 208 volts (instead of 240 volts) that’s perfectly safe and normal. That just means that the park is distributing 208-volt/3-phase power, which is now code compliant and perfectly safe.  I’ll publish an entire article on this later with a video showing how 3-phase power works.

Hope you’re having a great camping season so far. See you tomorrow, Sunday, June 30, for my latest RVelectricity Newsletter Issue #20. In the meantime, 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.

©Mike Sokol 2010/2019 – All Rights Reserved

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