What happens if you...?

A lot depends on other things.

As the first responder said. If the ATS is working, then a delay period after the generator starts the switch transfers from SHORE to GEN, and everything keeps working, no harm, no foul, no damage.. Same when you shout down switching back to SHORE.

But if you have some big ticket items running then.. Well

Water heater is a big current item.. That much current may cause FLASHOVER on the ATS contactors.. This can weld them so it kills the ATS. Does not harm the generator.

Microwaves likewise are big current draws, and may also blow an internal fuse.

Air Conditioenrs (motor driven stuff) not only big current draws, but there is an INDUCTIVE Kick, so they will spike the voltage as well when the ATS breaks (The ATS is a "Break before make" meaning it disconnects from shore power BEFORE it connects to generator.. NEVER are the two connected together) This is far more likely to cause Flash over and damage the ATS.


If you don't have an ATS.. Then you have a generator powered outlet somewhere you plug into to use the genny. So nothing bad happens.

HINT. always shud down A/C for 3 full minutes when transferring.

We don't have a transfer switch, so I am either plugged into the Genny or shore power. There is no way to be hooked up to both at any time. This type of setup is much preferred because it's not uncommon for the transfer switch to fail.

i always recommend against doing this
because it was not designed for that purpose
it is there so you don't have to switch something manually before applying power
Not to switch between HOT circuit power choices

transfer switches on EMG generators and equipment
activate when utility POWER is lost, and are much heavier rated and tested on a monthly basis ( i used to work at a major airport )
not willy nilly switched during working loaded conditions

a pitted sticky contact, cause by arcing when switching loads, could ruin your whole day, or cause sever electrical damage to the RV

yes minimal loads such as battery maintenance charge should not cause problems
but why risk problems that can be avoided

bad habit to risk getting into
once its a habit, you do it with the A/C and hot water on, with out stopping to think about, do that a few times and the contacts in the switch will be collecting arc marks and pitting

If you are hooked up to a 20 amp outlet with a GFI, the GFI will trip out. But nothing goes wrong at the generator.

I also have a automatic transfer switch and nothing would happen if I did what you described.

K. Charles,
We too have one of those 'automatic' switches! You plug the power cable into the generator output or you plug the cable into the park power post! Seems OK for me!! :)

Nothing would happen unless the transfer switch (whatever kind it is) has been physically broken in a very serious way. They are designed such that it is physically impossible for both power sources to be connected simultaneously by having some sort of mechanical interlock. It's not good for the switches to switch under load, but if there is no (real) load, it's perfectly safe to start or shut down the generator while connected to shore power.

Should shore power be connected to the generator, unless they were perfectly in phase the shore power would drive the generator into phase with great force. This can cause things like generator rotors being ripped out and sent flying.

We have an automatic transfer switch and can start the generator from inside the motorhome, which means we do not have to unplug. Not sure what you have - do you have a switch inside to start the generator with? If not, then you should probably unplug.

In our MH a little box in the back unhooked the shore power and hooks up the generator.

assuming your automatic transfer switch is working correctly, then nothing should happen. That is assuming you have an automatic transfer switch. ATS should always recognize the generator as the primary source of electric over the shore power cord.