RV Electrical Safety: Part II – Meters

Aug 4th, 2010 | By | Category: RV Safety

The No~Shock~Zone
Understanding and Preventing RV Electrical Damage:  Part II –  Meters
© Mike Sokol 2010 – All Rights Reserved

If you’ve read the survey we did July 2010 in www.RVtravel.com, you know that 21% of RVers who answered the survey  have been shocked by their RV.  Review the 21% report at http://new.noshockzone.org/15/.  What follows is #2 in a 12-part series about basic electrical safety 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 the HOW-TO Sound Workshops 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.

Shake & Bake

Remember when you were a child and first started to help with baking there were all sorts of measuring devices and abbreviations to take into consideration? There was a Tablespoon (Tbsp), teaspoon (tsp), Ounce (oz.), with 8 oz. in a cup, and so on. And you better not get your tsp and Tbsp mixed up or bad things would happen to your cake. The same types of rules apply when you’re measuring any electrical values. You just need to know how to use a few electrical measuring tools and then you’re ready to test your RV power.

The Meter

Now is the time to familiarize yourself with your voltmeter. Here’s a pretty typical $30 meter that you can purchase at Lowes, Home Depot or Amazon. You’ll notice a bunch of strange markings on the selection knob, only a few of which will work to measure AC voltage.  Don’t be tempted to just plug the meter leads into a  campsite socket and spin the knob. That will guarantee a burned out meter (at the least).

Note the markings on the control knob are divided up into four major groups.

  • AC V (AC voltage)
  • DC A (DC amperage)
  • OHM (electrical resistance)
  • DC V (DC voltage)

The only two groups you’ll be interested in are AC V (for measuring the AC voltage in power outlets) and DC V (for measuring the DC voltage in your batteries). For this article we’ll focus on the AC V group since we’re measuring the 120 or 240 volts AC in a campsite pedestal.

Also take a look at where the meter leads (or “probes”) are plugged into the lower right-hand connections on the meter itself. The Black COM (common) input is always connected to your black meter probe, and the red V-Ohm-mA (milliamperes) input is always connected to your red meter probe. Never put either meter probe into the 10A socket, which is designed specifically to check current flow up to 10 amps. Doing so for measuring AC voltage will blow the internal fuse in the meter, and possibly damage the meter itself.

All meters read the difference  between the two lead connections, so if the black probe is touching 0 volts and the red probe is touching 120 volts, the meter will read 120 volts. However, if both the red and black probes are touching wires with 120 volts on them, the meter will indicate 0 volts, which is because 120 minus 120 equals 0 volts. See how it works? Meters indicate the difference (what we call a differential) between any two wires or objects.  So the key to using a meter  is to connect the meter probes between the two voltages you want to measure.

Now, let’s move back to the meter settings. In the AC V area you’ll see a 200 and a 750 setting. When set to 200 the meter will read up to 200 volts, when set to 750 the meter will read up to 750 volts. Since we could be reading as much as 240 volts, we’ll always just set this to 750 and leave it alone during all testing. If you set it to 200 and connect it acrossa 240 volt outlet, the display will probably stick on 199 volts and start blinking. That doesn’t hurt anything, but it doesn’t tell you the actual voltage. Many meters of this type have a 400- or 600-volt setting, so setting for 400 or 600 volts is fine as well, just as long as it’s set for something more than 250 volts. And if you have an auto-ranging meter, just set it to read AC volts and it will figure out the proper scale for you.

The Outlet

Before you graduate to measuring the big 240-Volt, 50-Amp outlets, you need to start on a common 120-Volt, 20-Amp outlet like you might find in your living room or throughout your RV. Here’s what one looks like and the connections as standardized by the National Electrical Code. You’ll see a little U-shaped hole: that is the Ground; a taller slot on the left, which is the Neutral; and a shorter slot on the right, which is the Hot connection. Don’t be confused if the receptacle is mounted upside down with the ground connection to the top. The taller slot is always the NEUTRAL, and the shorter slot is always the HOT.

This is a GFCI (Ground Fault Circuit Interrupt) receptacle so there are test and reset buttons. More on this later, but pushing the “test” button should cause the “reset” button to pop out and kill the power from the outlet. Pushing the “reset” button in until you feel a click will restore power to the outlet. The job of the GFCI is to kill the power to the plug before it kills you, say from a hot skin condition on your RV. But these GFCI receptacles are only on the 20-amp campsite outlets, not the 30- or 50-amp outlets. In that case you’ll need your own GFCI breaker or outlets in the RV that will help protect you from a shock to ground. We’ll discuss this topic more towards the end of this series.

Also note the difference between the 20-Amp and 15-Amp versions of the outlets. A 20- amp outlet will have another sideways slot for the neutral connection, while a 15-amp outlet will only have a single vertical slot.

The Measurements

Since we’re going to be measuring live voltage, you need to observe the safety rules from Part I of this series:

  • Use only one hand to hold the plastic handles of the meter probes
  • Be sure you don’t touch the metal tip portion of either probe;
  • Don’t stand or kneel on wet ground. For most situations, dry sneakers will insulate you from the earth sufficiently, and if you’re doing this test in your living room then wooden floors or carpet will protect you if something goes wrong. But if you’re going to measure voltage at a waterlogged campsite I suggest standing on a dry rubber shower mat so your feet are insulated from the ground.


With nothing plugged in to the camp outlet, switch on the 20- Amp Circuit Breaker at the power pedestal, set your meter to the 600 or 750 V AC setting and using one hand insert your meter probes into the left and right Neutral and Hot slots. Remember not to rest your opposite hand on the metal box. It really doesn’t matter which side gets the red or black meter probe since you’re reading Alternating Current.

Since the Neutral connection is at 0 Volts and the Hot connection should be around 120 volts, it should read somewhere between 110 and 125 volts on the meter display. If not, then something’s wrong with the power hookup. If you measure 0 volts, then maybe you need to reset the circuit breaker, or if you have an outlet with a GFCI (Ground Fault Circuit Interrupt) remember to push the little reset button on the outlet itself. If it still doesn’t measure 110 to 125 volts, immediately contact the camp manager. If you measure 220-250 volts, then that power outlet has been jury-rigged inside the box to produce higher voltage. This is illegal and highly dangerous as you’ll surely blow up every piece of electrical gear in your RV if you plug into this outlet. So, if you read 240 volts on the 120-volt outlet do not plug in your RV, and, again, immediately contact the camp manager.


If hot-to-neutral checks out around 120 volts, then it’s time to test the ground, so plug your two meter probes into the HOT (shorter slot) and GROUND (U-shaped hole) connections. Since you’re reading from the Ground connection, which should be 0 volts and the Hot connection, which should be around 120 volts, your meter should indicate about 120 volts. If you read 0 or something strange such as 60 volts, then the ground wire might be floating (disconnected), which could cause a hot-skin condition that will shock you when touching the body of the RV.


Next, check from Neutral to Ground. That should read very close to 0 volts, but up to 2 volts is acceptable according to the electrical code. If, however, you read 120 volts from Neutral to Ground, then the polarity of the power outlet is reversed. Don’t plug your RV into this outlet. Again, this can cause a dangerous hot-skin condition depending on how your RV is wired.

Final Exam

As a final check, a $5 outlet tester from your local home center will confirm that the polarity of the outlet is correct. Plug it into the power outlet on the pedestal and you should see only the two yellow or amber lights light up. If you see any other combination, do not plug in your RV.

If you’re only using 20-Amp power for your RV, you’re just about done. At this time I recommend plugging the outlet tester into an outlet inside your RV that you can see from the open door or window. Now, go ahead and switch off the circuit breaker, plug in your 20-Amp RV connector, and turn the circuit breaker back on. But before you touch anything on your RV take a peek through the door or window at the outlet tester inside your RV to confirm it’s showing the same Yellow/Yellow pattern. If not, then your extension cord or RV plug has been incorrectly wired. If that’s the case, turn off the circuit breaker and find out what’s wrong before proceeding to power up your RV. I also like to keep an outlet tester like this plugged into a visible interior RV outlet  at all times. That way if something happens to the campground power in the middle of the night that electrifies all the RVs in an area, you’ll get warning from the outlet tester before you get shocked on the door frame while stepping out.

Once you’re familiar with the procedures, all this can be done in a minute or two. It’s a very small inconvenience that will help ensure the safety of you, your family, friends and pets. Stay safe!

Quick Tips

  • Always set your meter to read AC volts using the 400-, 600- or 750-volt scale
  • Hot (short slot) to Neutral (tall slot) should read approx 120 volts
  • Hot (short slot) to Ground (U-shape) should read approx 120 volts
  • Ground (U-shape) to Neutral (tall slot) should read approx 0 volts

Part 3 of this series will cover how to check 30-Amp and 50-Amp circuits at the campsite pedestal before plugging in as well as using non-contact AC tester to check for an RV hot-skin condition, so stay tuned.

Mike Sokol is the chief instructor for the HOW-TO Sound Workshops (www.howtosound.com) and the HOW-TO Church Sound Workshops. He is also an electrical and professional sound expert with 40 years in the industry. Visit www.NoShockZone.org for more electrical safety tips for both RVers and musicians. Contact him at mike@noshockzone.org.

6 Comments to “RV Electrical Safety: Part II – Meters”

  1. noah says:

    My male ground is broken off the extension cord.
    Do I need to replace the male end or is it ok to use?

    • Mike Sokol says:


      Never use an extension cord with a broken off ground pin. That could cause you to get shocked or electrocuted from your RV. Replace the male plug immediately being sure to double-check the polarity of the wires. The white wire should connected to the white-colored screw which is the wide blade, the black wire should connect to the brass-colored screw which is the narrow blade, and the green wire should connect to the green colored screw which is the U-shaped blade. Check both ends of the extension cord for the correct wiring just to be sure.


  2. Jonathan Johnson says:

    Just a quick note on voltages. In the United States, the standard nominal voltages are 120 and 240 volts. This is the voltage that electric companies aim to deliver to your outlets. However, there is a tolerance: the voltage can safely range from 110-125 ( and 220-250) volts. Nearly all electrical appliances are capable of operating within this voltage range without damage, though they are usually most efficient at 120 volts.

    If the voltage is outside of this range (110-125 volts and 220-250 volts), do not plug in — your appliances could be damaged or, worse, it could present a shock and fire hazard.

    • Mike Sokol says:

      Exactly right. The general accepted voltage tolerance is plus or minus 10%, which would imply 12 volts above and below the nominal 120 volts. So a power company could be delivering as low as 108 volts and as high as 132 volts and still be considered “legal”. In the pro-audio world we tighten that up a bit to plus and minus 10 volts, so it’s a low of 110 volts and a high of 130 volts. We have a big output control on our rock concert generators which I usually set to 122 volts or so which makes the sound guys happy.

      However, everyone should remember that 110 volts is the minimum that should be delivered to the appliance power supply itself. So if you’re only getting 110 volts from the campsite power pedestal and have 50 ft of extension cord feeding your RV, you can easily get another 6 volts of drop with any significant load. Now you’re down to 104 volts at your RV panel and there will be additional voltage drops within the RV itself as current is drawn from various other power hungry appliances. That’s why it’s not unusual to find below 100 volts at the input of a appliance.

      Now many modern appliances have intelligent switching power supplies rated for as low as 95 volts, so they’ll automatically boost their output voltage to compensate, but that comes at a price as well. That particular piece of gear will start drawing more current to make up for the lower input voltage which can cause even more of a voltage drop to other appliances on that same AC circuit. So it’s a vicious cycle when you begin with low voltage and use a light-weight extension cord. The key is to keep any extension cords as short and heavy as practical, and don’t plug into pedestal power below 110 volts or above 130 volts as something bad is bound to happen.

      Mike Sokol

  3. J says:

    I found this page VERY useful. I would like to confirm that a 30AMP RV plug is also at 120 volts. Also, as this is the 2nd part of a series, can you update this site to link them all together? I found it challenging to find this from an article later in the series.

    • Mike Sokol says:

      Yes, all 30-amp RV receptacles/plugs (in the USA) are wired with 120-volts. I’ve not spent the time linking articles together since the entire series is now available in a both Kindle and paperback versions. That’s what ties everything together int one publication with a table of contents.

Leave a Comment