Dave Solberg 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. Today he discusses running a generator when the RV is plugged in to shore power.
Dear Dave,
I own a Class C motorhome. Can I start the generator when I am connected to 110v AC power at a campground or at home? Or will that create a problem? Thank you for your help. —Sam
Dear Sam,
You did not give the make, model and year of your Class C rig. However, you should be able to start the generator while plugged into shoreline power without an issue. Your rig either has a “J” box that the shoreline plugs into or an Automatic Transfer Switch (ATS).
Automatic Transfer Switch (ATS)
An Automatic Transfer Switch is a box located typically in the service center that has a line coming from the generator and the shoreline cord coming out of it, as well. The ATS will switch internally to provide power to the distribution center whenever the generator is turned on. Power from the generator is the default or dominant power source when the ATS senses it is on. So, even if you are plugged into a shoreline power source, when the generator is running, it will provide power from that and not both sources.
What’s a “J” box?
A “J” box is typically found in travel trailers and economical motorhomes. It has a wire coming from the generator to the service center with a plug in. The shoreline cord comes off the distribution center into this compartment. It has to be manually plugged into a campground source or the “J” box inside the compartment to supply power from the generator. If you are plugged into shoreline power in this configuration and start the generator, no power goes through the shoreline cord.
However, there is one more item to cover! Even if you have a “J” box there are typically two circuits coming off a generator as shown in the photo above. The 30A circuit goes to either the “J” box outlet or the ATS. The second circuit, either a 20A or 30A one as shown here, is wired directly to the rear roof air conditioner, if so equipped. This is designed to allow an owner to run two roof AC units while plugged into 30 Amp service by using the front one through the distribution center and supplying the rear roof AC unit with 30A power by the generator.
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Is there a reason to have a generator running while plugged into shore power? Also, I’m pretty old school, so it wouldn’t bother me to have to manually switch between generator and shore power.
The reason I do it is that I’m staying long-term in a park and need to exercise my generator regularly. I learned this lesson the hard, very expensive way many years ago. I mark it on my calendar for the 15th of every month so I won’t let it slip by again. If/when I need to go on the road again, I can be sure that the generator will work.
You should also have a load on it too. Just disconnect the shore cord and run the rv off the gen for an hour or so.
I would caution not to cause a Transfer of power sources while under load, such as ACs running. All potential loads should be idle. If you’re running a lot of amperage through the transfer switch when it executes a power change it could/would cause arching at the contacts that could be damaging. Ask me how I know!