RV Electricity – Just Ask Mike (J.A.M.): Dogbone adapter confusion cleared up – Part 2


By Mike Sokol

Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.

Dear Readers,

In my last JAM Session I posted a simple question about how dogbone adapters are named. (Yes, that’s me with a dogbone in my mouth.) This survey was done because I often get a lot of confusing questions about these most basic of adapters in my various Facebook groups and RV forums that I watch. Reread Part 1 of this JAM Session HERE.

The answer is not 42!

While there are lots of electrical questions to consider, the most basic one is what do we call an adapter that connects an RV with a 50-amp plug to a pedestal with a 30-amp outlet? Do we say it’s a 50- to 30-amp adapter because it lets you connect a 50-amp RV to a 30-amp pedestal? Or do we say it’s a 30- to 50-amp adapter because you can use a 30-amp pedestal outlet to power a 50-amp RV? What is the answer (and it’s not 42, like in the Hitchhikers Guide to the Galaxy)?

There are at least two ways to define this connector, such as a 30-amp male to 50-amp female adapter, etc. – but I’m looking for the simplest way to describe it. So last week I took a poll of what you would call this most basic power adapter. And here is what you all voted for.

Drum roll, please…

While 11% of you voted that I was being silly, 24% said it should be called a 50- to 30-amp adapter, while 64% called it a 30- to 50-amp adapter.

In fact, these numbers seem to correlate with my own experience where perhaps 25% of the time I get a confusing question because someone is asking about a 50- to 30-amp adapter when they really mean a 30- to 50-amp adapter. For a deep dive into how dogbone adapters power your RV, please read all about it HERE.

Which way is correct?

So which way is right? Well, I always state the pedestal outlet amperage first, followed by the RV shore power amperage. So this specific dogbone adapter shown in the survey would be called a 30- to 50-amp adapter. More specifically I would call the adapter in the survey a 30-amp male (plug) to 50-amp female receptacle, or 30P-50R, for 30-amp Plug to 50-amp Receptacle.

Is it a 15-amp or 20-amp adapter?

For just a little more confusion, please note that a 15- to 30-amp or 15- to 50-amp dogbone adapter is actually a 20-amp to 30-amp adapter. What???? Yes, there’s really no current carrying difference between the 15-amp and 20-amp versions of the NEMA (Edison) plug. It’s just a coding thing where one of the blades is rotated by 90 degrees to keep you from inserting a 20-amp plug into an outlet with a 15-amp circuit breaker.

In this example there’s a NEMA 5-15P plug, but it’s connected to a 12-gauge wire feeding a 30-amp outlet. So it’s actually capable of carrying 20 amperes of current safely. Hey, this is how the history of electricity in the U.S. evolved, so don’t shoot the messenger.

But wait, there’s more…

Here’s what an actual NEMA 5-20 plug looks like with the one blade rotated by 90 degrees. While it’s indeed rated for 20 amperes of current, that’s only totally true if it’s connected with 12-gauge wiring. And it will only be able to supply full current if it’s connected to an outlet powered by a 20-amp circuit breaker and 12-gauge wiring.

Even more…

In addition, note that nearly all Edison outlets at campgrounds have the 20-amp (sideways blade) outlet configuration, and they are required to be GFCI protected. Yet all (or at least all that I’ve seen) 15- or 20-amp dogbone adapters use the 15-amp blade orientation, but have 12-gauge wiring which is actually rated to 20 amperes of current.

So in this case, a 15-amp dogbone adapter with 12-gauge wire plugged into a 20-amp GFCI outlet is actually rated for 20 amperes of current. However, an extension cord with a NEMA 5-15 plug using 14-gauge wire is still only rated for 15 amperes of current. Make sense?

More of my experiments

As a side note, my engineering colleagues at Hubbell/Acme just shipped me a 2 kVA 120-volt to 12-volt buck/boost transformer, which has an output current rating of 166 amperes at 12 volts AC. That’s right: When powered by my 3kVA VARIAC, I can now send anywhere from 0 to 166 amperes of current through most any wire or connector on my test bench. Wires don’t really care about voltage (that’s the job of insulation), so I can save a lot on my home electric bill (90% in fact) by stepping the voltage down by a factor of 10 which steps the amperage up by 10 times. Cool trick, eh?

So I’ll soon be setting up a series of video demonstrations that show what happens when you overload an extension cord, power strip, or shore power connector with too much current. I’ve done a few casual demonstrations in the past, but now I have much more refined test gear including FLIR cameras and thermocouples in addition to infrared thermometers.

No, I won’t leave this over-current experiment running unattended. Yes, I will have a fire extinguisher at the ready. No, my wife has no idea what I’m doing back in my lab. But don’t worry, I’ll be very safe with this. I’ve seen too many electrical fires in the wild to be complacent. But when I state that I’m testing something to destruction, that’s exactly what I mean.

OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.

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.
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And you don’t want to miss Mike’s webcasts on his YouTube channel.

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Al Buckner
17 days ago

Great information and I love the pic of you with your dog bone!

23 days ago

Good stuff Mike, thanks! On another note, I got a call from a national sales manager at Southwire this morning. He was following up on a question I had about their voltage booster (which they no longer make). While we were on the phone I asked what other products they make that are oem for rv’s. Besides wire they make electric reels, connectors and bunches of other stuff. They quit making the booster for other reasons besides the N.E.C.’s position on them, even though I was told different from someone else there in the past. I don’t think they were a source of revenue for them anyway- the Autoformer is pretty much top dog in that arena.

23 days ago

Here in AZ in some rural areas there is/was a 3-wire feed that has/had a “wild” leg that was 220VAC. Without measuring the 3 legs it looked like a split 120AC feed. In reality it was 120/240/neutral. A lighting tech in an old theatre here found out the hard way while hooking up a a dimmer pak. Ouch!


23 days ago

When describing a dog bone I always state the amp ends in the direction of current flow. So the photo in the poll is 30 – 50.

Jacques Marcotte
23 days ago

Mike: how would you be saving money on your power bill? Is it not charged by WATTS, watt-hour in fact? And in a transformer, watt in is slightly more than watt out! So it should cost you slightly more!

Donald N Wright
23 days ago

So, Mike, at home the electric range and the electric dryer are both 220 volt, but the plugs and jacks are different. Why ?

23 days ago

I’ll let MIKEE give the the expert answer. However I do know that the house power is wired differently than an RV plug. If you attempt to hook up your RV to a dryer plug, you’ll get 220 volts and blow out everything in your RV. The RV plug is 120V.

Jacques Marcotte
23 days ago
Reply to  Ran

D. Wright is not talking about his RV vs dryer, but range vs dryer!