Wednesday, March 29, 2023


Ask Dave: Why do I need to keep the shore power cord plugged into the onboard electrical box?

Answers to questions about RV Repair and Maintenance from RV expert Dave Solberg, author of the “RV Handbook” and the managing editor of the RV Repair Club. This column appears Monday through Saturday in the RV Travel and RV Daily Tips newsletters. (Sign up for an email reminder for each new issue if you do not already receive one.) Today Dave discusses an RV’s electrical junction box and shore power cord.

Dear Dave,
When I purchased my (used) Coachmen Prism, I was told to keep the shore power cord plugged into the onboard electrical box except when plugged into shore power. It’s difficult to access and I really can’t understand why it’s necessary unless I’m using my onboard generator. It’s a 2008/2009 Coachmen Prism Class C with Freightliner diesel power. Any info would be appreciated! —Doug

Dear Doug,
What you have is called a “J” box (junction box), which is wired back to your generator. Your shore power cord comes directly off your distribution center and you have to manually plug it into the “J” box to get electricity from the generator to the distribution center.

This photo shows an older model from an Itasca Suncruiser. You can see the junction box to the left goes up to the distribution center and the shore power cord is wired into it. The junction box to the right has a plug-in and it is wired to the generator.

Upgrade to an Automatic Transfer Switch

This is the economy version that you need to manually plug into the “J” box. The upgrade is an Automatic Transfer Switch (ATS) that does just what the name says: automatically transfers or switches from the generator power to the shore power.

This photo shows the upgrade we made to the Suncruiser with the ATS that we got from Progressive Dynamics. The shore power cord is coming out the bottom and goes over to the next compartment. The ATS senses the power coming from either the shore power or onboard generator and automatically switches inside the box. It is also time-delayed so the generator has time to get to proper operating speed and is providing stable power. This delay can be disabled if you are using an inverter; however, this unit did not have one. With this model, the generator overrides the shore power.

With the Automatic Transfer Switch, there is no need to plug or unplug the power cord – which is nice when it’s raining and the campground power goes out!

Read more from Dave here

Dave Solberg worked at Winnebago for 15 years developing the dealer training program, as marketing manager, and conducting shows. As the owner of Passport Media Creations, Dave has developed several RV dealer training programs, the RV Safety Training program for The Recreation Vehicle Safety and Education Foundation, and the accredited RV Driving Safety program being conducted at rallies and shows around the country. Dave is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club.


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1 year ago

Mike Sokol should address this item as it is a safety issue with a running generator and a live male plug-in. Also, the shore power and generator, as I recall, should not be plugged in or operated at the same time. Only one or the other! Verify Mike? Also, on older motorhomes or Rv’s with an older style ATS should not run the generator and the shore power at the same time! An over-voltage condition would exist. (?)

Newer ATS units have an auto shutoff or priority switch from or to the generator. (I replaced mine after “forgetting” to unplug shore power – burned a lot of ATS wires!!!) (Could have been worse! (I upgraded to a new ATS with surge protection).

Mike Sokol
1 year ago
Reply to  DW/ND

Actually, I believe that all of Dave’s comments are safe and correct. However, they could use some graphics and schematics along with a deeper explanation of how this works. I’ll work up a detailed explanation and publish it in a few weeks. Stand by…

1 year ago

Instructions for my unit say Do Not plug into the jbox until after you start the generator. And when you are finished with the generator… unplug from the jbox. Seems like their are so many ways to accomplish things, the problem is will you damage your equipment in the process? Will someone please write a clear cut manual with clear cut instructions of does and don’ts!?

1 year ago

I agree the question was not addressed … and yes, when not hooked to shore power keep it plugged into the jbox .. but turn the generator off before unplugging for any reason. If you unplug from the jbox while the generator is running you most likely will create an ‘arc’ at the plug ends which could result in a damaging voltage spike.

1 year ago

Although this article is good information it did NOT address the actual question. To answer the poster’s question, NO, you do not need to keep the power cord plugged into the onboard electrical box. The only time you need to have the power cord plugged into the onboard electrical box is when you choose to use the on board generator to power the RV.

1 year ago

If I understand it correctly, the only reason to have the power cord constantly plugged in to the generator outlet (except when plugged into shore power) is so you can start the generator and have power without going outside.

1 year ago
Reply to  Larry

That’s right. I think it’s a good idea to have it plugged in there all the time when traveling or off-grid. Saves you a trip to the box when it the weather isn’t good or it’s just a pain. In my opinion the ats is a maint. item- so there’s a trade off with the convenience. If the contacts or relays fail you might have to replace it also.

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