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Old 08-14-2010, 11:55 AM   #1
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Transfer Switch Operation - Shore vs. Generator Power

It's been triple digits around here for a few weeks, and when driving I run the generator for the house A/C to run while going down the road. Seems leaving the air conditioner off for even a few minutes causes it to warm up quickly inside. So, now the question....

Is it harmful to start the generator while connected to shore power? If the generator is running while connected to shore power, which power are you using?

I'm wondering if I could leave the a/c on, start the generator, then disconnect from shore power when leaving, and vice versa when parking for the evening. Turning off the a/c results in a high head circumstance, and the a/c won't restart for several minutes. Not a big deal either way, just curious.
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Old 08-14-2010, 12:14 PM   #2
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HI BattChief--I have researched that question before and I think you will find most owners don't run the gen set while plugged into shore power. The transfer switch is designed to prevent problems--BUT--it's that 'Murphy’s law' thing. I did turn on the generator on my 2002 Journey by mistake (senior moment) and it passed the smoke test (nothing burned up) but I shut it down soon after. I will also be interested in others responses. I just don't like taking chances with all the electronics on board. Take care and happy travels. John
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Old 08-14-2010, 12:25 PM   #3
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I was told by our Winnebago shops Service Manager to make sure the AC is turned off when starting the gen set or plugging in to elec. He said it is very hard on AC units to plug in or start the gen set with them turned on.
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Old 08-14-2010, 12:42 PM   #4
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My theory is to reduce the load as much as possible before starting the generator and therefore activating the transfer switch. I think the switch will last much longer if there's only 2 or 3 amps going through it versus 20 to 25 amps.

I usually turn off the AC, water heater and other high draw items and then start the generator. It takes a little while for the genset to get up to speed and settle down before the transfer switch does its thing. Then I disconnect from shore power. When the genset is activated I bring the loads back on gradually, AC, water heater etc. The actual time for the AC to be off is only 1 or 2 minutes so it doesn't take too long to recover.
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Old 08-14-2010, 12:52 PM   #5
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Page 10 of my Onan generator manual states:

Before starting the generator, turn off air conditioners and large electrical loads.
It doesn't really say why, I'm assuming it's hard on the a/c compressor if it short-cycles, so I'd follow the directions.

The manual also states:
When using an appliance with a motor, turn it on and let it reach normal operating temperature before starting appliances.
Page 15 has a Q/A reading:
Q: Can I run my generator for extra power while I'm hooked up to shore power at a campground? A: Only if the RV manufacturer has supplied a separate circuit for a second air conditioner....By the NFPA code you cannot connect both power sources, generator power and commercial power, together.
Hope this helps answer some questions.

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Old 08-14-2010, 01:33 PM   #6
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Generator power takes priority over shore power so when you run the generator it always supplies power to the motorhome. NEVER turn the generator on or off when the motorhome is running a load such as air conditioners or the water heater. Many bad things can happen.

The automatic transfer switch has contacts the power passes through and if there is a load it will spark big time and can burn up the contacts.

It you are running the air conditioners and start the generator it will attempt to take over the load and can either stop running or damage the generator.

When the generator takes on a sudden load the voltage will drop and can damage the air conditioner.

Any time you are connecting or disconnecting power through either shore power or the generator you need to shut off all big loads - air conditioners, water heater, washer/dryer, hair dryer, microwave, etc. After you have made the power switch then you can turn the loads on one at a time.
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Old 08-14-2010, 02:02 PM   #7
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I agree with Norm about removing loads before starting or stopping the gernerator, and to answer your question, you do not need to unplug from shore power when running the the generator.
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Old 08-14-2010, 03:03 PM   #8
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Thanks for all the quick responses. I currently don't start the generator while connected to shore power, and I turn off loads before starting the generator. My thought process with the transfer switch was when the actual transfer occurs. I'm assuming for the transfer to occur when starting the generator, the generator has to be generating power before it will transfer (if it's not generating power, it won't be detected). Once it's generating power, activating the transfer switch would be the same as turning on the a/c. That being said, the convincing argument to me is the load going through the transfer switch. Never considered that part, but makes sense to me. With that in mind, the only concern I would have with the air conditioner would be the momentary loss or reduction of power during the transfer process.

That's another reason I like this forum... Keeps me on my toes! 8-)
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