Tuesday, December 6, 2022

RVelectricity: Don’t build this DIY dogbone – It can kill you

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By Mike Sokol

I saw this home-brew dogbone adapter on another RV Travel Facebook page the other day, and immediately stepped in to warn everyone of the dangers. The poster pulled it off the site in a few minutes, but not before I snagged a few screen shots. I’m worried that as RVers are displaced from their normal campgrounds due to the COVID-19 pandemic, we’ll see more highly dangerous wiring that can either blow up your RV’s electrical system, or kill you, or both. Here’s one you DO NOT want to do, as well as why it’s a code violation.

First, he posts a picture of a DIY adapter he built with a pair of standard male 15-amp plugs. That’s feeding into a box with a TT-30 RV outlet, which is exactly what you would plug an RV’s 30-amp shore power cord into.

So after posting this on my RV Electricity Facebook group, there were a bunch of questions about why exactly it was dangerous, how he might have wired it, and why it was a code violation. I promised an explanation to my group, so here it is. And this gives many of you newbies a chance to learn how to read a basic schematic, so follow along.

Here is how an electrical box is wired up in the good ol’ USA. As you can see, there’s a transformer on a pole (or pad) which converts the 11,000 volts or so into 240 volts. That’s further split down the middle into 120/120 volts with a neutral divider. Every home in the USA is wired exactly like this, which we call a split-phase 120/240-volt service.

If we connect a meter between either of the two poles (legs) and neutral we get more or less 120 volts. But connect a meter between both of the poles (legs) and we’ll measure 240 volts. I think what his plan entailed was connecting the two 15-amp plugs as I’ve illustrated in the first diagram. If that’s the case, and he does succeed in plugging into outlets on two different poles, then you can see that the TT-30 outlet is now connected between them and will send 240 volts into an RV electrical system only designed for 120 volts. Yes, it will be a total disaster in seconds.

Now, the second example is also dangerous, but in a different way. I’ve assumed that he’s wired this DIY adapter to combine the amperage of two different outlets into a common TT-30 outlet. If he manages to connect those plugs into outlets on the same pole (leg), then it will provide 120 volts to the TT-30 outlet. But pull out one of the 15-amp plugs, and its contacts will be energized by the plug that’s still connected. If he connects this to two outlets on different poles (legs), then it will be a direct short across the 240-volt service, and the fireworks will be spectacular.

In any event, it’s all dangerous, it’s all a code violation, and you never want to brew up something like this on your own. It will either kill someone or blow up your RV’s electrical system, or both. So DO NOT DO THIS.

BTW: My new RVelectricity Webstream is going live this Thursday, April 2, starting at 1 p.m. Eastern time, with additional 20-minute modules at 2 p.m. and 3 p.m. You’ll want to subscribe to my YouTube channel HERE, but I promise it will be painless. Yes, it’s free and available to watch anytime later.

See you on my RVelectricity YouTube channel this Thursday, April 2.

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.
Join Mike’s popular and informative Facebook group.
And you don’t want to miss Mike’s webstreams on his YouTube channel.

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Red Kilowatt
2 years ago

Gotta disagree with your dire warnings here. It theatre, it’s SO common to use the two hots to make a 208vac connection. (Like two edisons to a L-6 20 connector. I have done this for years to get 208 where there is none. If you don’t know how to meter circuits or have little knowledge, don’t. But is it a death warrant? Absolutely not. It should be used temporarily, not as a permanent install.

Bob Godfrey
2 years ago

Once again Mike, thanks for some great, useful information!

Ralph Williamson
2 years ago

I agree none of this should ever be done, but I disagree that it is a code violation. Correct me if I’m wrong, but the NEC is for electrical installation requirements, not for what you foolishly plug into those installed outlets.

Bill Kaupe
2 years ago

Ralph, I guess you need a chapter and verse from the NEC Codebook? Why don’t you ask Mike for that? “Correct me if I’m wrong” usually means you are wrong.

John Goodell
2 years ago

I like reading this stuff because I always learn something, and more importantly it INSTILLS A HEALTHY RESPECT FOR ELECTRICITY! I am a somewhat knowledgeable RV’er and can trouble shoot 12v and 110v circuits and equipment, and this article humbles me and keeps me from getting overconfident and killing myself! My Dad was a career electrician who was very hesitant to teach much of the trade to us kids, and this is a good example of why. Just because I own a volt meter and some hand tools doesn’t make me a competent amateur electrician, because there is no such thing!