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Old 06-21-2017, 01:15 PM   #1
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Running Air conditioner

Never done this before so just curious. When we are leaving a campground where we have had our motorhome plugged in to AC. Is it ok to start the generator with the AC still plugged in and run the air off the generator? Or do we need to shut the air off unplug from ac start the generator and then start the air back on?

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Old 06-21-2017, 01:19 PM   #2
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It's generally recommended that high draw appliances (AC, washer/dryer etc) be turned off any time you switch power sources. Either from shore power to generator, or from generator to shore power.

Some will say that it doesn't make any difference.

I figure it can't HURT to shut off the AC for the length of time it takes for things to switch over. And it MAY prevent an issue somewhere down the road. I always turn off the AC before switching power sources.

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Old 06-21-2017, 01:28 PM   #3
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Most transfer switches and A/C units have a built-in delay between start-up and full load so what you are asking to do shouldn't be an issue.....but to be safe, you can shut off the A/C before starting the genset. Biggest issue in this area is allowing the auto transfer switch to transfer power with large amp draw/load in place/on line.....
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Old 06-21-2017, 01:39 PM   #4
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Start the gen let it get up to speed and run it for a few minutes. Then go unplug. Drive off.
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Old 06-21-2017, 01:43 PM   #5
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Most transfer switches have a 20 second delay before switching the power contactor from shore power connected mode to generator connected mode. This allows the generator to start and run with no load a few seconds before being hit with a load.

The motor in your AC compressor starts best with coolant pressure equalized on both sides of the compressor. This takes 1-2 minutes after compressor stops running.

Best to have the AC units off 2 minutes when switching from one power source to another.
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Old 06-21-2017, 03:34 PM   #6
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Our tech said always turn off before switching power sources
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Old 06-21-2017, 04:28 PM   #7
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Hi Ron & Kerrie,
Technically it is okay to leave the A/C on, start the generator and then go unplug. No muss and no fuss. The components are engineered to handle the switch. But I would not do this. If done, one is putting stress on the generator, the HVAC units and the transfer switch. It is a minor inconvenience to shut down the HVAC, start the generator, go disconnect from shore power and then turn on the HVAC, it would be best to do it this way. This minimizes stress on the components.
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Old 06-21-2017, 07:48 PM   #8
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Originally Posted by wildtoad View Post
Start the gen let it get up to speed and run it for a few minutes. Then go unplug. Drive off.
The transfer with will prioritize the generator in most setups. As soon as the genny is making power, the transfer switch will use it regardless of whether you are plugged in or not.

I think best practice would be as described above. Minimize the load by shutting off high draw devices, then start the genie, then restart the devices. A pain, I know. I am told that this reduces arcing and pitting of contacts on the transfer switch.
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Old 06-21-2017, 07:50 PM   #9
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I agree with turning off AC before starting the generator. If I remember correctly the generator is the preferred source of power by the transfer switch even when shore power is available. By that as soon as the generator is started and up to speed the transfer switch will auto switch. I'm not sure if the transfer is smooth/fast enough that the AC wouldn't notice the source change and cycle.
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Old 06-21-2017, 07:59 PM   #10
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I agree with shutting off the heavy loads anytime you are transferring power. I do the same when shutting power off at the pole and unplugging. Or switching to generator. The transfer switches can handle the load but repeatedly doing it will mean the transfer switch can fail sooner.
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Old 06-21-2017, 08:21 PM   #11
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No heavy loads when transfer switch is switched, to minimize the switch contacts arcing. While it will not cause a instant failure, the cumulative arcing over time can cause failure.
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Old 06-22-2017, 03:40 AM   #12
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we've experienced the GenSet main breaker tripping when we have the ACs on and crank up the generator (too much load) and have since been told (by Newmar) that the correct procedure is to turn the ACs off before switching to generator power then, turn them back on. Since doing it this way we've not have the generator's main breaker (on top of the genset) trip.
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Old 06-22-2017, 06:02 AM   #13
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Thanks for all the help and advice Everyone appreciate it
Ron: 1999 Dutch Star: 38' 2 Slideouts
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Old 06-23-2017, 03:29 PM   #14
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I always turn off the A/C before switching power sources. I always wait about 5 minutes before turning the A/C back on again because it takes a LOT of amperage to start an A/C compressor and with my motorhome, both A/C units come on at the same time. Also, it's good to let the jenny warm up a bit before putting a heavy load on it. The starting amperage of an A/C is about 300 percent of the running amps. Let's say your unit's running amperage is 10 amps, 120vac. So, starting amperage would be 30 amps which would taper down to 10 amps once the unit is running. But that's a strain on your power source. If for some reason the capacitor has not finished recycling when you turn it back on, then the starting amperage can go out of sight... like 80-90 amps. That's why the lights in your house dim way down if you try to turn on the A/C after a brief shut down... and the A/C won't restart in that situation. Newer units have a fail-safe for this situation... but... they can fail too!

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