RV Electricity – Just Ask Mike (J.A.M.): Can I run a generator and shore power at the same time?

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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 Mike,
I’ve never really had to worry about this because I’m never connected to shore power when I run the generator. But, if that were the case, is there any risk of backfeeding to the pole if the generator starts and I’m already connected to shore power? (It’s a factory-installed Onan generator on a Vengeance Touring Edition toy hauler.) I’m asking because I’m considering installing an Energy Command Auto-Start 30 system to fire up the generator and keep important things powered and running, should I leave the camper for a few hours and the shore power fails. —Brian Z

Everyone,
This had a few answers on the RV Tips Facebook page, two of which are sort of urban myths.

This first answer from Nancy is what a lot of dealers tell their customers, but the dealers are incorrect.

I know very little about this kind of stuff but I know the dealer told us to never have the generator and shore power running at the same time. —Nancy L

While this answer from Dave has a bit of validity, the truth is that transfer switches really can’t fail in that mode.

Before I use one source, I turn off the other. Transfer switch can fail. —Dave S

The answer from Tom, below, is correct.

Generator is fine with shore power. Transfer switch handles it automatically, but if it fails (rarely) then shore power stays connected. All circuits (120 volt or 12 volt circuits) are protected from mix-ups like this… Not to worry about it. (from my decades of experience) —Tom M

And here’s my answer…

The ATS (or Automatic Transfer Switch) consists of one or two mechanical relays that are interlocked so that only one power source at a time can be connected to your RV’s electrical panel, which totally isolates your generator output from the shore power line. This is not like a home generator that’s been improperly installed on a circuit breaker panel without an interlock. The backfeeding of power from your home generator into the electrical grid where it can injure or even kill a power company lineman is a real thing.

However, RVs are built with a generator transfer switch so they can’t backfeed power into the shore power line with your onboard generator. The default condition is line power, and once the generator powers up there’s a 20- (or so) second delay before the transfer switch relay switches the RV over to generator power.

When the generator shuts off, the ATS switches your RV back to shore power, so you can run them both at the same time and nothing bad will happen. If you have an automatic generator starter and the shore power fails, the ATS will automatically switch over to generator power once its running and the 20 second time delay occurs. —Mike Sokol 

For more advanced study on RV generator transfer switches and how they work, read my article about it HERE.

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.

For information on how to support RVelectricity and No~Shock~Zone articles, seminars and videos, please click the I Like Mike Campaign.

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Steve
5 months ago

I can post pictures of a fire caused by a generator when the power came on in a power outage while the generator was running which caused a major back feed. Auto transfer switch will work, but never hook direct!

Nanci
6 months ago

Thanks for the info. While in one place for a few months I still want to “exercise” the generator and usually shut down shore power first, when I remembered. This is helpful.

I already knew how to burn out a transfer switch though. Setting up in the heat of Arizona late one summer, didn’t check if the surge protector indicated full power to the rig yet. Had the microwave on (running on the inverter evidently), two AC units turned on but not starting. When the power finally hit the transfer switch from the pedestal, the transfer switch fried. So glad it didn’t fry the microwave and AC units too. $600 later we had power, AC and an expensive lesson.

Wayne
6 months ago

Interesting. The generator on our 2001 Chinook will not start if we are connected to shore power. In fact, it won’t even start if we are plugged into shore power but have the pedestal breaker turned off! The only way it will start is if we are completely unplugged from shore power.
Have never tried plugging into SP after gen start.

DAVE TELENKO
6 months ago

Hi Mike, what controls the 20 second time delay?

Drew
6 months ago

To Brian,

If your trailer has a residential fridge it may not come back on by itself if the power goes out and then comes back on. There may also be other electronic devices in your rig that act the same way. It would be easy to test and verify.

DAVE TELENKO
6 months ago
Reply to  Drew

Hi Drew, funny you should mention that about veriying the power on situation. I did check mine out on our 2017 Forest River 34QS, on all systems including running on the inverter, generator & shore, I tried all different ways of running & they all worked just fine including the residential refer!
Snoopy