Wednesday, September 27, 2023


Do I need an “intelligent” surge protector for 50-amp shore power?

Dear Mike,
Thanks for your recent articles on why an intelligent surge protector is needed for 30A service. But would this also apply to 50A service? I have been reading these articles and did the survey, but then realized everything you wrote has been about 30A plugs and service. —John G.

Dear John,
While it does seem like all I’m writing about is 30-amp outlets, in fact I’ve done tons of articles and answered dozens of questions about why you should have an intelligent surge protector on 50-amp/240-volt pedestal outlets. So this is a great chance for me to draw up a new graphic that will explain it to you. Yes, my first job out of college was a mechanical engineering draftsman, but that was in the ’70s, which required lead pencils, a drafting board and a real blueprint machine that used ammonia to develop the prints. I really don’t miss those days….

But first we need to examine how a 50-amp/240-volt service works. Then we can define a few failures and see how dangerous they can be. Here’s the primary point of confusion – a 50-amp/240-volt outlet for an RV is really a pair of separate 50-amp/120-volt services spliced together in the middle to provide 100 amps of current at 120 volts. That’s because unlike in your house, virtually nothing in a stock RV built in the USA is actually 240 volts. Nope, your rooftop air conditioners, electric water heater, convection microwave, power converter and all appliances are 120 volts. That’s also why you can use a 30-amp/120-volt to 50-amp/240-volt adapter to plug your 50-amp RV into a 30-amp pedestal. More on that later, but for now let’s stick to the 50-amp/240-volt outlet.

Here’s a graphic I’ve posted many times before. It’s my diamond reference chart of the voltages you should find in a properly wired 50-amp/240-volt outlet. You’ll find the same Ground and Neutral wires like in a 20- or 30-amp outlet, but you’ll also see an additional Hot line, so we’ll call them Hot 1 and Hot 2 for reference – but some schematics may denote them as L1 and L2 or Line-1 and Line-2. Also notice that just like any 20- or 30-amp outlet, if you measure from ground or neutral to a “hot” you should find somewhere around 120 v0lts. Indeed, all of the appliances in your RV (or at least the ones that are expensive to fix) will be expecting this same 120 volts, give or take maybe 10 volts. With me so far? Excellent.

You’ll also see from the diagram above that if you measure between Hot 1 and Hot 2, you should find around 240 volts or so. That’s because those two Hot legs are 180 degrees out of phase with each other, which makes the voltage additive. You could guess that if you hooked up an appliance like your refrigerator, microwave or rooftop air conditioner between Hot 1 and Hot 2, it would receive 240 volts instead of 120 volts. And if you guessed that would be a bad thing, you are indeed correct. But let’s assume you’ve wired into the outlet correctly and each appliance is connected to the neutral and one of the Hots. Can something still go wrong that will create an over-voltage condition? You bet your sweet bippy it can.

So let’s go a little farther upstream of the pedestal and show how this can happen. Hold on, because this is a fairly advanced concept, but one that’s critical to understanding US 120/240-volt power. All electricity that comes to your house or campground starts on the power pole at around 11,000 volts or so, which is then stepped down to 240 volts by the trashcan-sized transformer we still can see on power poles, but which now may look like a big metal box sitting on the ground up to several feet on each side. You should also note that in addition to stepping the incoming 11,000 volts down to 240 volts, it divides it in half again to make two separate “poles” of 120 volts each, and which we’re calling Hot 1 and Hot 2 in the diagram.

Now look at Load A on the diagram. Even though we have only 100 watts of things powered up on the Hot 1 leg and 900 watts of things powered up on the Hot 2 leg, the voltage is still divided evenly into 120/120 volts and all is well. That’s because the neutral provides balancing currents which keeps the two hot legs evenly divided.

Look at Load B on the diagram and you’ll see that the white neutral wire was somehow broken (by a loose connection, broken wire, corroded contact, etc.), but because we have exactly the same loads on the Hot 1 and Hot 2 legs (1,000 watts each), the 240 volts is still divided evenly down to 120/120 volts.

However, if you look at Load C, you’ll see the most likely scenario where you don’t have equal loads on the Hot 1 and Hot 2 legs. In this case I’m showing a 100-watt load on the Hot 1 leg, and a 900-watt load on the Hot 2 leg. Without the neutral line to provide balancing current, this forms a 10:1 voltage divider with 24 volts showing up across the 900-watt load side, and 216 volts showing up on the 100-watt load side. Ouch. Whatever appliances happen to be connected to the Hot -1 side are going to be FRIED in seconds. If it’s a light bulb it will get very bright for a few seconds and then burn out. If it’s a microwave or refrigerator, the main circuit board will likely suffer immediate damage and stop working. And your converter is probably toast as well and may go up in smoke.

So you can now see why a neutral wire opening up on a 50-amp/240-volt pedestal  can wreak all kinds of expensive havoc in your RV’s electrical system. And no, a basic surge protector CANNOT protect you from this. That’s why you need an intelligent surge protector with a relay that will monitor neutral-to-line voltage on each side and disconnect your RV from shore power if the measurements go out of normal range. Both Progressive Industries and Surge Guard make intelligent surge protectors for 30- and 50-amp circuits that will do the job. But as I noted in an earlier column, these active, relay-based surge protectors cost on the order of $250 to $300 for 30-amp models, and up to $400-$450 for 50-amp models. The basic $100 entry-level surge protectors from ANYONE will not monitor or disconnect you from an open neutral or rogue 30-amp outlet miswired with 240 volts, no matter WHAT they tell you.

Just so you see how important this concept is, I had an email last week from a 5-rig caravan that took off on a vacation trip together. However, the first campground they stayed at had a power problem AFTER they all tied into shore power. Somewhere between the incoming service panel and the first pedestal in the “loop” there was a conductor failure which caused the neutral to open up on the entire campsite loop. I believe that at least 3 of the 5 RVs were on the same loop and suffered extensive electrical damage, including one that was full of smoke and ashes from the burned electrical components that were destroyed. They moved on the next day and did get their electrical systems back up and running in a few weeks, but had to schedule several unintended stops along the way with a few days’ layover here and there to fix the various things that were damaged. Not exactly the vacation they planned, but they were lucky that the campground’s insurance company paid for the damage to their RVs. In many cases you’re on your own for the repair costs and your own insurance company or wallet takes the hit.

Next week I’ll do a detailed article on the real differences between basic and intelligent surge protectors with features and prices. See you then….

In the meantime, let’s play safe out there….

Mike Sokol’s excellent book RV Electrical Safety is available at 


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Eddie Harris
5 years ago

We have a 2015 Winnebago Tour and I bought the Hughes Autoformer 50amp. My questions is this enough protection also there is a factory installed Surge Guard?

5 years ago

Mike, I’m trying to determine the order in which to use the following when hooking up at a home (friend’s house for visit). Our RV cord (30amp) does not reach the power. Using 3 items when RV cord will not reach the outlet – is this the correct order if they have a 3 prong plug outdoor receptacle? EMS-PT30X, #10 extension cord (25′), RV 30 amp #10 extension cord to rig? Next using just two items – EMS-PT30X (receptacle) and RV cord? AND what is the order if the EMS-PT30X will not fit the house receptacle in either of those situations? Also, in relation to the first situation, is the order of the 3 items correct if hooking up to a campsite pedestal if your RV cord does not reach and you use the #10 extension cord? We are minimalists when using electricity but could have fridge, hot water heater and 2 lights on at the same time. The next biggest load might be fridge and AC on at the same time. Thanks for your advice on this as I can find little about use of ‘regular’ extension cords when an RV cord does not reach a campsite post OR the same situation at a ‘house’.

John Whitney
5 years ago

Ii have a progressive ind 50 amp hard wired smart surge protector. I just returned from a brief trip where everything was fine. Plugged into my 50 amp plug at home, and the device showed loss of line 2. Because everything had been working I assumed there was a problem with house wiring. Wrong! After lots of checking it appeared to be a broken conductor in the power cable. I had replaced one end last year, so I checked the plug. It was ok. I thought the RV end was a molded plug, but it had been replaced. Turned out whoever did it didn’t install it properly and one of the hot wires had pulled loose. It could just as easily been the neutral conductor.

Sherry Dawson
5 years ago

I will be starting out in a small RV that uses 15 amp or 110V connections. Am I correct that when I have hookups I should use the 30-amp service with a smart surge protector stepped down to 15 amp?

What do I use for protection when I’m plugged in to regular household current, like at home or when that is the only outlet available? I have searched for a 15-amp smart surge protector and I don’t think there is such a thing.

Bob Beeres
5 years ago

Well, 89% of those responding have never had a problem. I have been RV ing since 1962 and since 1976 in motor homes and have never had a power problem. While leading a 65 day, 10 unit rally to Alaska we encountered several places where those with the $500.00 +/_ protectors did not have power when the rest of us did with no problems. We saved the $500.00 and enjoyed having power.

Richard Palmer
5 years ago
Reply to  Mike Sokol

These sorts of comments crack me up Mike. “I’ve never had a problem in 50 years!” I deal with these every day at my job when I tell someone they should have had surge protection on their expensive electronics that just let the smoke out. I’ll never understand folks that would rather chance having thousands to tens of thousand of dollars worth of damage rather than spend a few dollars on the simple insurance policy of having a surge protector. I may be without power, but I sure won’t be buying an RV full of electrical components because of poor shore power.

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