Electric: 120 VAC Electricity in your RV

by Phred Tinseth © 2002 Reproduction Permitted

It’s not my intention here to write a book on AC electricity but to cover those aspects that are peculiar to RV use. “RV Electrical Systems” (mentioned above under Sources and References) is your must-have reference for all things AC. It goes into much greater (and well-illustrated) detail. (I’m interested in covering those things that can drive U Nutz.)

First thing first: The neutral and ground in an RV electrical load center are NOT to be bonded together.

The RV chassis is isolated from ground by definition. Metal leveling jacks and such, mounted on the vehicle frame do not make adequate ground contact. Therefore, in an RV, you always want to have a “floating” neutral, where the neutral and vehicle ground never come together. When you bond the neutral and ground together in an RV, you have a “floating” ground (rather than neutral) and this will result in nutty things happening.

Combined with ANY receptacle (at campground post or in RV) having reversed polarity, it may result in YOU becoming the path to ground in certain conditions (like standing on wet ground and touching metal vehicle parts). Usually, you just feel a slight “tingle,” but it can get serious. One result often seen is a dog, chained to a metal bumper, bouncing up and down. The above electrical faults result in breakers breaking and Ground Fault Interrupters (GFI’s) that don’t GFI (and pity the poor drunk who stands alongside the RV, with his hand leaning on the metal frame, and takes aleak)

This shouldn’t happen if there’s no reverse polarity at ANY receptacle, because the ground wire is still grounded and does not become a hot wire by the fact that current of near-zero voltage is flowing in it. So, then, everything should still work OK and normal. The dog would still be happy right up to the time you drive off down the highway — dragging the whole affair — a dog and all. (I’ve seen this happen.)

However, the whole purpose of the third ground is to be a fail-safe, so the current is not allowed to (normally) flow in this wire. Having more than one grounding point CAN make this possible. (See later.)

RoVers can come on a bad electrical receptacle almost anywhere. Frequently in a campground, where the “electrician” is the guy who mows the grass and does other “high-tech” chores. They can also have one or more reverse-wired receptacles in their RV. (Factory RV wiring is not usually done by mental giants.)

One inverter manufacturer’s manual has a note about disconnecting a certain “pin” so this “double ground” won’t happen. One inverter manufacturer now makes a “Marine” (same as RV) Model preclude this (and it’s not supposed to be sold for use in a fixed residence).

50 Amp and 30 Amp Electricity to the RV

When the four wires from a 50 amp cable enter the RV, you have two “hots,” a neutral and a ground. The wires then usually enter a dual “load center” (or two individual load centers). It’s set up just like in a “house,” EXCEPT the neutrals go to an insulated neutral bar. The ground wire goes to a separate “ground” bar.

While the neutrals are bonded together and common, the neutrals and ground wire are NOT bonded to the metal load center as a “common” (as is done in a house where there’s an independent ground).

The neutral and ground are common only at one place — the meter and load center bars on a “house” or the power receptacles in the “box” you plug into at a campground. It’s essential that the “neutrals” be floating (not connected to ground) throughout an RV.

CHECKING POLARITY — and seeing if there’s any electricity at the receptacles So far, there doesn’t appear to be a simple, automated, LED-readout polarity checker available for 50 amp receptacles. It’s no big deal though. You can do it with a simple multimeter. You’re going to be checking the 50 amp receptacle at the CG power post or similar. These things are nothing more than standard mobile home connectors and quite simple.

Here’s a simple description as any electrician and every electrical book have it. If it’s unclear, use the book mentioned above for more details and illustrations. As you face the [50 amp] receptacle at the campground hookup, you’ll see three vertical, flat slots. The outer (left and right vertical slots) are 120VAC “hot” outlets. The inner (center/lower) flat, the vertical slot is neutral. The round hole in the ground.

Set multimeter for AC volts and go to the higher range (usually 750). Put one probe in the left hot slot and one probe in the extreme right hot slot. You should read about 240VAC.

Next, move one “hot” probe to the neutral (center) slot. You should read about 120VAC. Move hot probe to another hot slot. You should again read about 120.

Next, move the probe from a neutral slot to a round ground hole. You should again read about 120 VAC. Move probe from first hot slot to other hot slot and again should read about 120VAC. The above readings indicate receptacle and polarity are OK.


IF, however, you did not read 120VAC from either hot slot to the round ground. Leave one probe in-ground and move the probe from the hot slot to neutral. If you now read volts, you have reversed polarity. Not safe.

If there is a big difference in voltage readings from hot slots to neutral and hot slots to ground, there’s a short to ground somewhere and the receptacle is not safe to use.

Here’s a (usually good) quickie option:

You should, if a real RoVer, have a 50 amp to 30 amp adapter. You should also have a 30 amp to 15/20 amp adapter and it goes without saying that any experienced RoVer will have also have a standard 15/20 amp polarity checker. Plug the string together and you will be able to check the polarity of one of the 50-amp hot legs. While you’re there, disconnect the 30A adapter and polarity checker from the 50A. Plug the 30A and polarity checker into the 30A receptacle and again check polarity. Do it again at the 15/20 receptacle.

Here’s something a lot easier: You can buy, make or have made a 50A adapter that has two adapters on short cables coming out of it. A 30A is attached to one of the 50A hot connectors (inside the plug) and a 15/20A to the other 50A hot. It makes for easier checking of polarity and is also very handy for hooking other things too without cluttering up the whole box. If you have a 30Amp RV, it’s simpler. You’ll just be dealing with one hot, one neutral and one ground. Just stick a 30A by 15/20A adapter in the 30A receptacle and plug the polarity checker into it.

If all the above checks out OK, plug RV cable into the receptacle. Go into RV. Once there, with circuit breakers on, simply plug standard polarity checker into any AC receptacle. Reading on the polarity checker will tell you if it’s OK.

I have no idea how your RV is actually wired. It might have been miswired at the factory (often happens because somebody doesn’t know what they’re doing). Some previous user might have tinkered with the system as well.

In essence, when you plug in an RV, you are really just plugging in an extension cord and the things in the RV (at the end of that cord) are the equivalent of plugging an appliance into an extension cord in a house (electrically, no more than plugging a microwave in a wall socket).

As above then, there should be only ONE connection where neutral and ground are bonded. And that is at the house or RV park hook-up, and BEFORE it ever gets to the RV (unless you want to pound in an eight-foot ground rod every time you park).

The simple hardware store polarity checker referred to will do what you need, but others will do far more. See the book above “RV Electrical….” for a range of more sophisticated checkers and how to use them. The book also includes details on what to do if you find a fault.


Less than full output. Most of the less expensive generators as used by RVers and for emergency use by homeowners do not actually put out “peak” voltage. Some only provide enough power to run a battery charger at about 50% efficiency. Increasing the speed of the generator just a bit can sometimes overcome this. If not, there are voltage boost transformers that can restore full charging amps. Check with “Backwoods Solar” for expert info on these.

Here’s one that screws a lot of people up:

If you have a permanent onboard generator, many have the neutral and ground at the generator bonded together. That’s a no-no. Some generators can cause problems — depending on how they’re wired to the RV. Onan is especially sticky. In my case, I wired the gen only to a receptacle in the RV umbilical cord compartment just like the one on the commercial 120VAC campground power source. In order to use the gen, I had to disconnect from commercial power first. A bit clumsy, but idiot-proof and safe. I did this with a PowerStar inverter (with no bat charger) also. Since then, I’ve used Heart, Trace and Statpower inverters with battery chargers. I’ve not disconnected (or otherwise trifled with) anything. I’ve had no problems.

If using an inverter, for example (you don’t want to invade the interior parts of and “screw around with them”), and it has a neutral/ground bond, then that item (inverter) should be the single “bond to the ground” for the RV. This is the answer from the “experts.” Duh…?

CONTINUITY CHECKS Checking ohms (resistance) to track your AC problems isn’t really going to do you a lot of good unless you disconnect EVERYTHING from the specific item you’re checking. This can be one hellova job (if you do it right) and often won’t accomplish much anyhow because of the “hidden” things it will never occur to you to check (or disconnect).


You turn off the power and start checking resistance. However, one of your lights was turned “on” when you turned off the power. The resistant element in that light is still there, power or none. So, when you do your ohm-meter number, you’ll get a reading.

Any of a number of appliances plugged into the AC line, be they turned on or off, will give the same erroneous resistance readings because:

They might have (like a microwave) a clock that runs whether on or off, or a phantom load (as from an instant-on TV, stereo, radio, clocks and similar).

ANY line that has ANY kind of transformer in it, be it on or off, such as one of those plug-in thingies used to recharge shavers (or anything else).

Anything that has a transformer or “inductor” such as a “filter” or “coil” in the line as in a line filter for stereo, CB, etc. These can be hard to find if you don’t know what you’re doing, because a radio filter, for example, can be buried in the innards of your converter –and if you aren’t very expert, you’ll not know what it is, let alone how to find it.

The key component of a GFI or GFCI (Ground Fault [Circuit] Interrupter– same thing), be it in a receptacle or circuit breaker, is a very small transformer. This will confuse you when measuring ohms also.

If you had a line with a break in it and bypassed it with another, are you sure there are no “tapes” to anything else still on the old line?

You MUST have good ground connections all the way through all the circuits, or you won’t get dependable readings.


Keep in mind, the slightest amount of moisture (or bug poop, etc.) can set off a GFI. Not only if in the GFI, but if in any receptacle “downline” from the GFI (if so wired).

There are two ways to wire GFI’s:

In one, common in RVs, the GFI is the FIRST receptacle in a circuit from the breaker at the distribution panel. Subsequent receptacles are then also protected. This is usually done so that an outside receptacle will be protected. There’s nothing wrong with this if everything is done properly. A better way (I think) is to use individual GFI receptacles where needed (bath, kitchen, outside). Each is installed as a normal receptacle, but the “load” wires from GFI are capped. The GFI then protects only its own two outlets. This isn’t overly expensive and makes troubleshooting a lot easier.

I’ve seen many GFI outlets wired up backward, where input goes into “load” and output is wired to “line” (or line-in).

Pushing the “test” button on a GFI momentarily shorts “hot” to “ground.” This is the way it’s supposed to work.

The best way to check a GFI is with power on. If it doesn’t work properly after all the above possible faults have been corrected, start physically disconnecting things one-at-a-time. Eventually, you should solve the problem (find the culprit).

An experiment: Hook up a GFI temporarily (with “load” wires capped) to an extension cord. Plug it into a non-protected receptacle in the RV. Measure volts from plus to neutral, neutral to ground, plus to ground. Record them. Plug the thing into a (correctly wired) receptacle at your hookup.


What are the differences?

Push the test button. Try it again.

If you’re interested, this will entertain you for a day or two.

The above checks are something you really ought to do with a new RV.

Once is enough if you write it all down (make a chart).