By Mike Sokol
Today’s RVs have much greater power requirements than those of even 10 years ago. You have lots of appliances, so that single 20-amp outlet can’t provide 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 both a “home-style” 15- or 20-amp outlet on the left and a special “RV” 30-amp outlet on the right. (Click on any picture for a full-size illustration you can print out.) In another article 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. Sometimes you’ll see me use 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, while most electrical panels and commercial buildings are wired to more recent code suggestion 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.
While most casual RVers will recognize the little U-shaped hole in the outlet as the ground connection, it’s the other two slots that get confusing. As I’ve showed you previously, in 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 always BLACK (or RED in a 2-pole circuit). 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. While looking at the front of the outlet, think: 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.
Remember to set your voltmeter to an AC Voltage scale more than 250 volts, typically 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 (0 to 3) volts. From Hot to Neutral you should measure around 120 volts, 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 0 (zero) volts and the Hot to Ground reads around 120 volts. If not, then the polarity of the plug is reversed and your RV could exhibit a Hot-Skin condition or trip an internal circuit breaker when you plug it in, depending on how it’s wired.
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 drop at the pedestal outlet when you draw any amperage, plus 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 electronic appliances suffer from brown-out problems, especially your air conditioner. (See RV Electricity Newsletter #8 for my article about voltage drop). Plus a green ground wire could measure up to 2 or 3 volts between itself and neutral or itself and the earth, but any more voltage on the ground wire means 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 up the pedestal box yourself and poke around inside. That could get you killed.
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 a previous article on volts – 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 in this case is 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 120 volts.
So 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 left. You’ll see a standard 120/240-volt, 50-amp receptacle as found in many campgrounds. Look at the illustration on the left and you’ll see that 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 approximately 120 volts, and finally from 3rd base back to home you should read approximately 120 volts. Now, from home plate to 2nd base you should read close to zero (0 to 3) volts, and from 1st base to 3rd base you should read between 230 and 240 volts.
If you measure zero volts between Hot 1 and Hot 2 of the 50-amp outlet, then the campground has wired a single 100-amp pole of 120-volts across both sides of the outlet. That’s a really big code violation since now the return current on your neutral won’t be subtractive. Instead, the currents will be additive, so the wiring and connector designed to carry only 50-amps of current could be forced to carry up to 100 amperes. That will result in overheating and meltdown of the plug and outlet, with an eventual loss of the neutral connection entirely. Here’s a quick way to remember all of this.
As you move your meter probes around the bases of the diamond, 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 220 and 250 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 click on the image to see it in full size and print it out. Put these pages in a notebook and you’ll always have a power plug reference for when you roll into a new campsite. And after a few times it will seem quick and simple, so don’t become complacent. 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. Don’t take a chance. Always test before plugging in. And, as always, 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 to make the repair.
Finally, if you ever feel a shock from your RV, immediately get away from it and shut off the circuit breaker in the campsite pedestal. Then notify the campsite electrician and refuse to hook up power until the problem is resolved.
3-phase voltage readings (geek alert)
Since the latest revision of the National Electrical Code now allows RV parks to use two legs of 3-phase power for 50-amp/240-volt outlets, there’s one additional voltage reading possibility that’s perfectly safe and acceptable. This is something called a 120/208 3-phase WYE service. In that case, you will still read 120-volts from the neutral to any of the hot legs, but 208 volts between the hot legs, instead of 240 volts which you would normally expect. Seems crazy, but 3-phase AC power was one of Nicola Tesla’s greatest inventions, and it’s how the world’s electrical grid is powered.
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 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 allowed by code. I’ll do an entire article on this topic later with a video showing how 3-phase power works.
Email me at mike (at) noshockzone.org with your questions.