RV Electricity: Power principles – 50-amp shore power

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By Mike Sokol

Hello Everyone,
Things are really cranking up on my RV Electricity Group on Facebook, which is providing really great questions for me to answer at length in my other publications, like this Saturday column in RVtravel.com. So if you’ve stumbled on this article via a search engine, please sign up for all of my publications at RVtravel.com/subscribe.

There were a lot of comments and confusion this week about pedestal power, specifically how many amperes and watts were available from a 50-amp RV pedestal outlet. I’ve written about this topic a number of times, so this will be a review for some of you. But the basics bear repeating, and I’ll go into a little more depth on measurement techniques here. However, I’ll save the more hard-core explanations for my RVelectricity Sunday Newsletters, as well as my 3-hour advanced seminars for consumer RVers, and 8-hour training clinics for RV Technicians.

With that in mind, Chuck Woodbury and I had an interesting discussion this week about technical levels of my articles and videos. As I’m sure you realize, I have readers whose technical backgrounds vary from complete beginners about RV electricity, full-timers who have a few decades of experience dealing with RV power, and all the way up to Master Electricians, Power Company Linemen, Electrical Engineers, and Design Engineers for the manufacturers. I’m comfortable teaching at each and every one of those levels, but probably not at the same time.

So at Chuck’s suggestion I’m going to attempt to implement a trial “difficulty system.” My J.A.M. (Just Ask Mike) columns will be Easy Level, the weekly RV Electricity column will be a combination of Easy and Intermediate Levels, and my monthly RV Electricity Newsletter will be a combination of Intermediate, Advanced and Expert level articles. Expert only levels will be reserved for RV Technician training clinics. Of course, we’ll include cross-links where possible for more information on a particular topic.

The following article has an Intermediate difficulty level, so many of you should be able to understand it with just a little effort.

The Power Principles: How 50-amp shore power works

Tons of confusion on this topic, from manufacturing level on down to beginner RV owners. So I’ll cover the basics of 50-amp pedestal power at an Intermediate level, and create future articles at Easy and Expert levels. I’ll save the Expert level instruction for my RV Technician clinics since that will necessarily deal with potentially deadly level troubleshooting, and how to protect you all from dangerous situations. So, as always, if you don’t feel comfortable doing any of these measurements or basic troubleshooting techniques, DO NOT PROCEED. Find someone that can help you get better acquainted with electricity, and not as a forced introduction.

So what is a 50-amp RV outlet, and how is it actually powered?

Technically it’s called a “receptacle,” not an “outlet,” by any professional technician, electrician or engineer, but I let it slide most of time when I’m discussing it with entry-level RV owners. However, its official name is a NEMA 14-50R receptacle, and it’s exactly the same 240-volt “outlet” often found in your kitchen to power an electric oven. And that’s why if you go into your local big box store you should easily find them on the rack. The only real difference between one for your kitchen and one for your RV is the RV version needs to be mounted in a rainproof enclosure rated for outside exposure, while a kitchen version won’t be waterproof since in should be mounted on an interior wall behind the stove or oven.

How are they wired?

Well, it’s called a 125/250-volt AC 2-pole receptacle, with the 125/250-volt rating describing how much max voltage it can have between the neutral wire and each pole (125 volts), as well as how much voltage it has between the two poles/legs (250 volts). It’s also rated for a maximum current of 50 amperes … but much more on that in a future advanced article.

As many of you have seen in my previous articles, I’ve drawn up a diamond measurement chart showing all the voltage levels you’ll find in a 50-amp outlet. Now, this isn’t just a suggestion, it’s actual code. So if someone deviates from this wiring configuration either by accident or on purpose, it’s a code violation and should never have passed inspection.

You’ll see that I’ve called out possible voltage measurements you should find with a digital voltmeter. More on this in an advanced article, but know that it’s possible to measure a nominal 208 volts between the 2 hot legs (poles) and still be perfectly acceptable since campgrounds are now allowed by code to run 3-phase power to the pedestals (yes, they really are). This won’t impact you at all except to know that if you read around 120 volts from the neutral (bottom contact) to either of the hot legs (poles), it’s completely normal to measure 208 volts from leg-to-leg. However, you must NEVER measure 0 volts between the two hot legs on a 50-amp outlet since that will allow the neutral wire to carry too much current, overheat and possibly burn up.

Where does this power come from?

Well, if you’ve ever looked at a power pole with transformers and wondered what it was all about, here’s a basic diagram. Those wires up overhead generally carry between 7,000 and 11,000 volts of potential, which the transformers step down to 240 volts. And there’s a center-tap on that 240-volt transformer winding that further divides it into 120/120-volts on each leg. So if you have a 240-volt appliance you connect it between the two hot legs on the outlet. And if you have a 120-volt appliance you connect it between one of the hot legs and the neutral. It’s really that simple.

How much power can I get from a 50-amp outlet? 

That’s the most relevant question, and the answer is 12,000 watts. Here’s why. While it seems confusing to think about a 50-amp outlet as having 100 amperes of total current available at 120 volts, that’s actually correct. That’s because there’s a 2-pole circuit breaker in the pedestal (at least, there should be), and each pole is rated for a maximum current of 50 amperes before it trips. So you can pull up to 50 amps from one pole, and another 50 amps from the other pole.

Now, neither pole can exceed 50 amperes by itself, but you can indeed get 50 amps from each pole (or leg) and that totals up to 100 amperes of current at 120 volts.

If you look at how your 50-amp shore-powered RV is wired, every outlet and most appliances (except for some large coaches) are connected to the neutral wire and one of the two poles. So everything is being fed 120 volts, including your electric water heater, microwave oven, rooftop air conditioner, DC converter and every wall outlet you plug your computer, hair dryer, television and anything else into. A well-designed electrical panel in your RV will try to balance these loads, so the water heater and microwave will be on different legs, and if there are two rooftop air conditioners, each one will be connected to the two different shore power legs. That’s so as not to overload either of the two 50-amp circuits, which will trip both poles on the breaker.

Now for a little quick Ohm’s Law, but don’t get worried as the math is really simple. To calculate wattage all you have to do is multiply the Voltage (Electrical Potential) by the Amperage (Current Flow) and you’ll come up with Wattage (Power). So each of the two legs of your 50-amp outlet calculates as 120 volts x 50 amperes = 6,000 watts. And since you have two identical 50-amp legs, you can find the total wattage by simple addition of the two legs. Thus 6,000 watts + 6,000 watts = 12,000 watts total power from a 50-amp, 120/240-volt pedestal outlet. 


RV Electricity Facebook Group Update

Here’s a quick update on my new RV Electricity Facebook Group. We already have more than 1,500 members, and it’s growing by a steady 50 additions per day. And we’re getting some really interesting new members, including Jon Cain, who was a journeyman electrician for a power company for 16 years. Yes, that’s him stringing some really big (500,000 volt?) power cables. Great view, but I’m sure that the room service is a little lacking.

I’ll also be doing a Live Video Stream on the basics of campground pedestal operation this Sunday, April 28, starting at 9 p.m. East Coast and 6 p.m. West Coast Time. When I say I don’t have enough computers on my desk, you can see what I’m talking about on the right. Yes, I’m planning on adding a 4th computer soon, along with a pair of Sony PTZ HD Cameras and Blackmagic ATEM switcher and Hyperdeck recorder. See, I’m not just an audio geek… I’m a video geek as well!!!

This Webcast will be at the most basic technical level, so if you’re new to the RVing lifestyle, or just want a refresher course on how campground pedestals and dog-bones hook up, then tune in to my live webcast. And yes, you’ll be able to watch this video anytime later if you happen to be busy at that time. But I’m going to be actively taking your questions during the Live Video Stream, so please join the group HERE. You don’t have to join Facebook to watch the live stream, only if you want to pose your questions live. Or you can simply send your basic pedestal questions to mike@rvtravel.com with the subject line “Pedestal” and I’ll try answer as many as I can.

Let’s play safe out there….

 

Mike Sokol is an electrical and professional sound expert with 40+ 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.

##RVT894

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JTT

Hi Mike,

In regard to 50 amp power, if I’m understanding this correctly I can draw 50 amps on EACH leg for a total of 100 amps? Based on your other articles on RV electrical hazards, I’m also going to install a hardwired EMS from Progressive Industries which I believe will show the draw on a line 1 and a line 2.

Wolfe

Re: the rating system, not a bad idea, but maybe self-evident? If someone doesn’t understand something, it’s too expert for them, eh? On the other side, I hope you won’t squelch your “expert” articles since they are the more uniquely valuable — there are millions of places to find out how to plug in your RV, only a few to learn (eg) grounding nuances or how to fix your inverter guts…