RV Electricity – Do I need a 30- or 50-amp surge protector?


By Mike Sokol

Dear Mike,
Referring to your comment on your post “RV Electricity – An easy way to measure 50-amp outlets”:

Speaking of 50-amp service, I am looking to purchase an EMS surge protector for my new
Grand Design, when I pick it up in a few weeks. Since we will have two air conditioners, we will most often be hooked up to a 50-amp service.

My question is: When we don’t have access to 50-amp service, and use a 30-amp connection, will the 50-amp EMS still work? I think that I understand the basics of the unit, I’m just unclear about how it would work if it is hooked up to a 30-amp connection.

Thanks, and I learned a lot from your presentation in Hershey, last year. —Tom Herd

Dear Tom,
Thanks for attending my seminar in Hershey. Looks like I’ll be doing 5 more seminars there in 2019, but you’ll need to get to the show early since there’s a LONG LINE to get in, and my seminars start at 9 a.m. sharp. Here’s a link to my 2019 seminar schedule so far.

Back to your question. There’s a really simple answer which I’ll expand on a bit here. The proper way to think about this is you want to select the proper Surge Protector for your RV’s shore power cord-set (30- or 50-amp), then use a 15- or 30-amp adapter on the pedestal or home outlet that has a 50-amp female outlet to plug into your RV. I’m showing it in this diagram with a portable surge protector, but the same rules also apply for any of you with an internally mounted surge protector.

Of course, your total power available in watts won’t increase beyond whatever the 20- or 30-amp outlet can supply. But with the proper dog-bone adapter the power will be split between the two hot legs of your 50-amp cordset, so everything in your RV will receive power. Just remember that with a 20- or 30-amp service there won’t be enough amperage to run both air conditioners at the same time.

So buy the 50-amp Advanced/EMS surge protector from Surge Guard or Progressive, then add a 30-amp to 50-amp adapter for when you’re plugging into a 30-amp pedestal, and a 15-amp to 50-amp adapter for when you’re plugging into a 15- or 20-amp outlet at a house. Then you’ll be all set. I’ve talked to Progressive Industries and Southwire/Surge Guard about this, and both of their engineering departments agree with me.

Also, I’m not confused when I talk about a 15-amp adapter plugged into a 20-amp outlet. The difference is that while the contacts in all 15-amp plugs and outlets are actually rated for 20 amps of current, the 15-amp version is keyed with two vertical slots and the 20-amp outlet is keyed with one vertical and one horizontal T-shaped slot.

A 15-amp plug can be inserted into either a 15- or 20-amp outlet, while a 20-amp plug can only be inserted in a 20-amp outlet, not a 15-amp outlet. But both the 15- and 20-amp outlets are typically wired with 20-amp circuit breakers, so either one will supply 20 amps of current. That’s why we generally use 15-amp plugs in 20-amp outlets, and all is well. Clear as mud, right?

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.


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Tom Herd

Thanks again for the detailed response. Somewhat along the same line, I’ve heard stories of an EMS being stolen from the post at night or when the owner is gone. Some choose to lock it to the post in order to secure it. I’m considering another option and looking for an opinion.

What would you think of using a 50-amp cord between the pedistal and the EMS. Then a cord from the EMS to the trailer. I’m thinking that the water line access hole, in the floor of the storage compartment, might be large enough for the plugs to fit through and, if not, another hole could be cut in the floor of the storage area. That hole would be covered when not in use, to deny access to small critters.

The EMS would be secured, inside the storage compartment, protected from the weather, and still be portable if you wanted to use it elsewhere.

I understand that you would like to keep the cord run as short as possible, so I’m not sure if the trade-off of the longer cord would be worth the added security for the EMS.