RV Electrical

Battle of the dog bones

Are all RV power adapters created equal?   This post is about two “dogbone” adapters —  one of which I sent back! 



Anyone familiar with RV electrical systems knows that the shore power provided by the park doesn’t always match the coach.  Our 35′ fifth wheel (an Alliance Paradigm 310RL) is designed for 50A service, for example, but my wife and I like to frequent National and State Parks where the shore power is often only 30A.  Some are even 15A, but that’s a subject for another day. 

Enter, the RV “dog bone”  

The answer is to use one of the adapters, above, but which one would you choose?  Both of them will allow a 50A RV to plug into 30A shore power. How do they do that?   To answer that question,  I’ll first outline the electrical system of a 50A coach and how that differs from 30A shore power.  I’ll then show how these dog bone adapters are wired —  It will feel a bit “geeky” at first but you’ll see right away how it all works. 

The 50A RV

Modern 50A RV electrical service is a simple 240V double-pole circuit with a neutral wire, which means you have TWO 50A  (120V)  legs.  A coach with two air conditioners, for example,  will wire one of them to “leg 1” and the other to “leg 2” at the service panel. Such a service requires FOUR wires at the pedestal:   Two  “hot” legs, each with 50A capacity at 120V, a neutral and a ground.    That’s right:  True, 50A service is really 100A (at 120V) total capacity.  They just call it 50A because it really is 50A at 240V.  

30A shore power

30A shore power at the park consists of ONE 30A (120V) leg. For the electricians among us, this is a single 30A breaker on one side of a 240V panel.  Such a service requires THREE wires at the pedestal:  One  “hot” wire with 30A capacity,  a neutral and a ground.

How can we connect a four-wire 240V 50A shore power cord from the coach to a three-wire 120V 30A supply?   In a nutshell, the dog bone binds “leg 1” and “leg 2” together on the 50A side (which the pedestal doesn’t know or care about)  and connects this to the single 30A leg at the pedestal.  That way the single, 120V 30A supply is available to the entire coach, so that you don’t have to worry about “leg 1” vs “leg 2” (you do, of course, have to limit your electricity consumption to avoid tripping the breaker at the pedestal).  Below find a diagram showing how this is accomplished. 


In this photo, the “30A Male” end of the dog bone plugs into the 30A pedestal, and the 50A female end of the dog bone provides the “50A” connection for the coach’s shore power cord.  This is the correct way to power a 50A coach from 30A shore power.  



Now then, what about the two dog bones I am holding in the first photo?  I verified that  both are wired correctly (L1 is tied to L2 as in the diagram), but only the yellow one (a Camco #55185)  possesses that little black label from “Intertek”.

Who is Intertek?  Intertek is a nationally recognized and OSHA-accredited, independent test lab who has certified that the product meets the Canadian standard known as “CSA 22.2 No 21” (see the small print in the lower right-hand corner of the black label).  The standard itself (the documentation describing it) is very expensive to purchase, so I haven’t read it.  I have, however, seen a change log, which reminded me of reading through pages and pages of  ISO-9001 documentation, years ago!  Lets just say my impression is that CSA 22.2 No. 21 is quite large and detailed.

I reached out to Intertek for additional information and learned that CSA 22.2 No. 21 itself contains the test criteria they follow, when certifying the Camco dogbone.    They even audit Camco from time to time to insure ongoing compliance.  In addition, should Camco make any substantive change to the product (“substantive” would also be defined by the Test Standard),  Intertek would have to re-qualify (re-test) the product.   My own editorial opinion here is that Intertek probably tests a sample of each Camco production run, but without actually reading CSA 22.2 No. 21,   I can’t say what the sampling procedure actually is.     

It is interesting to note that the green dog bone in the above photo (RVGUARD “30Amp to 50 Amp RV Adapter Cord”), possesses no Intertek certification.  That doesn’t necessarily mean its a bad product;  its just means the product has not been independently tested against any standard  (this is perfectly legal in the United States, as the use of an independent test lab is optional for RV power cords).  As I mentioned before, the RVGUARD is wired correctly and no doubt will work, but I decided to send it back because I’m convinced that the Intertek certification means something.  RVGUARD did not return my request for additional comment.  

In addition to looking for the Intertek certification, I perform a few tests of my own to verify component quality.  With my Fluke 179, I test for connector resistance to make sure that the crimps inside the cord are tight, and that there is no corrosion or other compromise.  Resistance measurements will even tell me if the manufacturer used Aluminum wire instead of Copper wire! I also perform some simple load tests by putting the cord into service and watching for heat build up on the exterior insulation (Heat build-up is the biggest enemy of electrical cords, and the reason is as simple as ohms law:  P=I^2 times R.   Even a small amount of degradation in a connector can cause it to get hot).  

I perform such tests on all of the RV cords and plugs I purchase, just because I’m nerdy that way — and if I find anything out of the ordinary I send it back, because safety is my first priority.   Thankfully. I’ve never had to do that, no doubt because the cords I purchase have already been tested. 

That little Intertek label means something important!

Here’s to safely powering your adventures, 


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