RV Electricity Issue 13 – Forum Q&A – 50-amp pedestals

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Q&A’s from other forums

I spend a lot of time on dozens of other RV forums answering questions about electricity. Here is one really interesting topic that keeps cropping up. Just how many amps can a 50-amp shore power outlet supply?

From the Holiday Rambler Group:

Q: Dear Mike,
Saw this page on the web describing a 50 AMP service. I have a problem with some of the wording saying that each leg is 50 amps at 120 volts. That has made some RVers think they can run 50 AMPS on each leg. I think the wording should be clearer that there are only 50 AMPS total available not 100 AMPs. What are your thoughts? Dick K. / 2015 Holiday Rambler Ambassador 38DB

A: There’s just no easy way to put this, so I’m going to rip off the bandage fast. You are wrong and they are right. That is, a properly wired, 50-amp, 120/240-volt campground pedestal does indeed have 100 amps of current available at 120 volts as long as you don’t exceed 50 amps on either leg. That’s because a properly wired 50-amp service includes two separate 120-volt legs (Hot 1 and Hot 2) of up to 50 amps each. So you can draw 30+30 amps, or 40+40 amps, and up to 50+50 amps and still be within limits. See my diagram on the left that shows how this works.

Note that if you add 50 amps plus 50 amps you get 100 amps total, which is 12,000 watts of power (120 volts x 100 amps = 12,000 watts), and power is the only thing that really counts. Now don’t confuse this wiring diagram with the 240-volt power like you would use in your house to run an electric stove. In that case you definitely only have 50 amps of total current at 240 volts.

But because nearly every appliance in an RV is wired for 120 volts (with only a few exceptions), then you have up to two times the amperage available on a single leg. And even if you do have an RV with 240-volt appliances (yes, the larger Cheap Heat systems use 240-volts), then you should be able to read the amperage supplied to a hard-wired 240-volt heater on both of your amperage meters. So you don’t have anything to worry about until you get over 50 amps of current on either leg.

Electrical Code Alert!

One important thing to know is that according to the NFPA 70 National Electrical Code, a 50-amp circuit breaker and associated wiring is only rated for 50 amperes of non-continuous power. In reality, you have to derate it to 80% for continuous duty appliances such as electric heaters. So a 50-amp RV outlet is really closer to 80 amps (100 amperes x 0.80 = 80 amperes) if you’re in constant power draw mode. And space heaters can definitely demand constant power. Just one more thing to know about electricity.

And yes, your house wiring has the same derated amperage percentage. You just don’t normally have to think about it because a 200-amp panel in your house actually has 400 amps of current available at 120 volts or 200 amps at 240 volts, or any combination thereof. So even a 50-amp, 120/240-volt pedestal can’t keep up with the power available to your house, no matter how much you want it to.

As a side note, also be aware that the 20-amp wall outlet in your house is only rated for 16 amps of continuous current draw, which is why domestic space heaters typically top out at 1,800 watts, because 1,800 watts divided by 120 volts equals 15 amperes, and that’s below the 16-ampere continuous duty rating for appliances such as space heaters and hair dryers.

 

Email me at mike (at) noshockzone.org with your questions.

Mike Sokol is an electrical and professional sound expert with 40+ years in the industry. Visit NoShockZone.org for more electrical safety tips. 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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Tommy Molnar

I have a basic understanding of electricity but much of this just flows over my head. I try to keep up because it is of significant interest to me. I’ve actually spent more time zeroing in on 12-volt power since we mostly boondock and that’s where all the power comes from.

Mike Sokol

Tommy, I’m going to be dedicating more articles in 2019 to 12-volt DC power for RVs, which is why I’ve been contacting manufacturers about RV batteries with different chemistries, inverters, converters, 12 volt refrigerators, solar panels, and replacement LED lighting. Of course I know all the basics of 12-volt DC electrical systems, since I was technical support for the forklift battery charging shop where I worked in the 70’s. But RVs have special technologies and usage patterns that are unique to the industry. Once I get my head around it I’ll be qualified to write and teach about it. I… Read more »

Shipp

So now the question becomes, how much current does an RV use? I do not have one of the newest, but I do have a Fifty amp service. I also have a Progressive industries EMS protector which gives a reading for each leg. One leg mostly supplies current to the Air-conditioner. Every thing else is supplied form the other leg. When I run on a Generator, I am running two 110 volt 20 amp outlets , each running one of the legs. I never actually have a 20 amp load on either leg. Every thing runs just fine and the… Read more »

Mike Sokol

Shipp, sorry but it’s not going to happen. There’s not only one less wire in a 3-wire 120-240-volt feed, there’s also 1/2 of the voltage drop compared to a 2-wire system of the same gauge. That’s because the voltage drop on the neutral of a 3-wire 120/240-volt feed cancels out to zero as both poles are loaded. And the 50-amp plug shouldn’t be confusing at all since it’s exactly how every electric oven in the USA is wired. As for power requirements, many of these all-electric RVs use a LOT of current, and there’s even talk of adding a second… Read more »

Steve

Mike, I think more people should get your book. Electricty is actually straight forward, but you need to understand it. I think one of the confusing things when people talk 30 amp vs 50 amp is how much “power” is available. So folks use the simple formula. V x A = watts (power). 2 legs x 120 vac x 50 amps = 12,000 watts, as does 240 x 50 amps = 12,000 watts. Where 120 vac x 30 amps (one leg) = 3600 watts. Straight forward. Understanding this will help folks know what to look for going forward when buying.… Read more »

Mike Sokol

You’re very welcome. Yes, once you know the basics of voltage and amperage, then all this starts to make sense. However, lots of people who claim to understand RV electricity can’t even solve the most basic formula of volts x amperage = watts. If they can’t do that much, then there’s no way they should be wiring anything at all.

Darrel

in 2015 I had a “electrician” at Sigsbee Key West Naval Air Station replacing our 50 amp breaker that kept popping on minimal draw. He swore up and down that the 50 amp only provided 50 amps split over the two legs, and became highly agitated when I told him that was incorrect – and that there were 50 amps per leg.

He told me all about his “years” fixing campground pedestals. I finally went along just to get it fixed.

Mike Sokol

Sorry, but he was wrong. This is my 40+ years of electrical engineering talking, plus I’m currently studying to update my 2018 Master Electrician License that I originally earned back in 1978. There’s simply no internal electrical connection between the two sides of a 2-pole/50-amp circuit breaker. There’s only a mechanical connection that causes both sides to trip if EITHER breaker pole trips at 50 amps. So you can indeed draw up to 50+50 (100-amps total) of current at 120-volts. It’s a little too cold outside for this demonstration right now, but as soon as it warms up a bit… Read more »

Drew

Mike,

I think that would be an excellent demonstration. I loved the expression “Ripping the bandage off fast.”

Mike Sokol

One of my jobs around the home for spring will be to dig a trench to my workshop building and run a new 60-amp/240-volt service and panel. That will allow me to do these type of demonstrations in my backyard. Sometimes I have trouble explaining to campgrounds exactly what I’m trying to do with my experiments and convince them I’m not going to damage their wiring. If all goes well I can make a lot of videos on shore power over the summer.