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RVelectricity – Just Ask Mike (J.A.M.): How do I set my meter for 120 volts?

Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM. This week I explain how to set a volt meter to check outlet voltage.


Dear Mike,
I just bought a cheap meter and want to know how to set it to check my outlet voltage. I’m paying an electrician to hook up a 30-amp outlet on my house. But I’m a little paranoid to plug my trailer into it without checking first.

I asked the guy at the store and he said to set my meter to the ~200V AC scale and and that would work. Is he correct? —Diane

Dear Diane,
First of all, it’s not a good idea to take electrical advice from a guy in a big box store. Some of them will know the correct answer, but many of them will not. So let’s look at the basics.

You may have seen this diamond voltage chart on the dozens of articles I’ve written over the last 10 years. U.S. residential and campground power is typically 240/120 volts.

That is, if you measure between the two hot legs of the 50-amp outlet, you should see a nominal 240 volts. If you measure from neutral to either hot leg you would measure a nominal 120 volts. All good so far?

For a 30-amp outlet, you should measure around 120 volts from the hot leg to neutral. You should never measure 240 volts. That would indicate that your electrician thought this was a clothes dryer outlet and made a huge wiring mistake.

Volt meter settings

Now, if you were to set your meter to the 200-volt AC range and measure 120 volts, then all is well. That’s because you’ve told the meter that you don’t expect anything more than 200 volts, so it faithfully indicates 120 volts.

Note that you do have to set it to the AC scale (usually a squiggly line) and make sure your meter probes are plugged in the common (black) and V/Ohm (red) inputs.

Danger, Will Robinson!

But what happens if you have your meter set on the 200-volt scale and encounter 240 volts either on a properly wired 50-amp outlet or a dangerously miswired 30-amp outlet? Well, the meter won’t be harmed, but the display will normally indicate an OL, for Over-Limit.

If you’re used to reading a meter you’ll probably know that you exceeded the voltage range. But if you’re not used to meter reading, you may not realize that your electrician accidentally wired up your 30-amp outlet with 2-pole 240 volts, instead of 1-pole 120 volts. Either way, it can be confusing.

Set it to 600….

So that’s why I recommend you always set a multi-meter to the next highest scale above 240 volts, with 400-volt and 600-volt ranges being common. And always set your meter to the proper range before you touch the meter probes to any circuit.

Make sure you don’t have the Hold button pushed, which will lock the meter to display 0 or 120 or 240 volts, even after you removed the probes from the circuit.

OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.

Let’s play safe out there….

Mike Sokol is an electrical and professional sound expert with 50+ 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.
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And you don’t want to miss Mike’s webcasts on his YouTube channel.

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##RVDT1631;##RVT1007

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Crowman
3 months ago

Mike. you’re absolutely right about taking advice from people at big box stores. Most of them were saying two weeks earlier “Would you like fries with that order?”

volnavy007
3 months ago

Additionally, when you put in an outlet for RVs, put in wire to handle 50 amp. That way, if you change RVs, and get one rated for 50 amp, all you will have to do is change the recepticle or get a pig tail adapter.

Mike Sokol
3 months ago
Reply to  volnavy007

I agree. Most of the cost is in the trenching and permits and installation labor. You’ll never be sorry that you put in a 50 amp pedestal.

Bob
3 months ago

Mike, your explanation is right on. However, the question was about an inexpensive meter. Most of the low end meters do not have the multiple ranges of the one you show in the article.
The Southwire you show is a very good multipurpose meter, but most of the cheap ones do not have the same number of ranges.

Dr4Film
3 months ago
Reply to  Bob

Cheap meters are exactly what you get especially the ones that Harbor Freight gives away free. Use once and throw away. Choose AC making sure the scale is above the range you want to measure, in this case greater than 120. If you want quality then spend some bucks and get a good one. If you are a RV owner having a good meter is one of the most important pieces of equipment to have when needed.

Mike Sokol
3 months ago
Reply to  Bob

She sent me a fuzzy picture of her meter and it does indeed have 200 and 600 volt AC volt ranges.

Tom
3 months ago

Mike, Your 30 amp box with breaker. How about an Amazon reference?

Bob M
3 months ago
Reply to  Tom

Condition
New
Quantity
12 sold
3 available
Manufacturer Part Number
U013CP
MPN
U013CP
Brand
GE
UPC
Does not apply
Item Number
303450668554
Item Ending
Jul 17, 2021, 10:05 PM PDT
Item description”The Temporary RV Power Outlet is factory-assembled and factory-tested. The single, heavy-gauge NEMA 3R rainproof enclosure protects against weather, vandalism and nuisance tripping. It is lockable and is engineered for maximum protection of receptacles, cords, plugs, breaker