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Old 07-19-2019, 07:31 AM   #1
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Using AC with Generator

Own a 2011 390QS. Just recently came off a 5 day trip hooked up to a 50 amp service. Both AC units worked fine. Am now going on a 4 day "dry" camping trip & decided to try generator with AC units to be sure they worked. Seems that only one unit will work on the generator, you can switch from front to rear but can only get a single unit to run. Is that the way the AC works on generator power?
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Old 07-19-2019, 07:40 AM   #2
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Own a 2011 390QS. Just recently came off a 5 day trip hooked up to a 50 amp service. Both AC units worked fine. Am now going on a 4 day "dry" camping trip & decided to try generator with AC units to be sure they worked. Seems that only one unit will work on the generator, you can switch from front to rear but can only get a single unit to run. Is that the way the AC works on generator power?
How big is your jenny, and how big are your AC units?
Itís quite possible that your generator, because of its size can only power one or the other.
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Old 07-19-2019, 08:22 AM   #3
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As a rule of thumb, a 3000 watt generator is the minimum for a single A/C and a 5500 watt generator is the BARE minimum for dual a/c (no microwave, no heavy draw items). I am pretty sure most RV people will tell you a 7k is about the right number for a dual a/c.

Also, the watt rating on a generator is for reference only, each manufacture has different ratings, hp, amperages, and quality. If you buy a cheap 5500 watt generator, you may not have enough power to run both a/c.

Do your homework and read up on the ratings of your generator.


Also, one last thing. Make sure to use a surge protector with a generator, especially cheaper ones. As the generator peaks in power draw it works harder, which can cause the generator to surge up and down on power as it works to keep up. A lot of times the cheaper ones don't regulate the power like you would hope.
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Old 07-19-2019, 08:27 AM   #4
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that doesn't seem the norm for most larger diesel coaches - the generators are usually sized to accommodate both a/c units operating together, and everything else, too...

- what happens when you turn on one, then use the second thermostat to turn on the other?

- 'how' are you determining that one work, but the other won't, when you 'switch' these - are you simply turning the thermostat OFF on the working one, and ON on the second, then suddenly it comes on?

- does your coach have an 'EMS'(electrical management system), where it possibly sheds loads when there is 'too much' power asked for?
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Old 07-19-2019, 11:42 AM   #5
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My thought is that your EMS is seeing the genny as 30 amp power (one phase) and limiting running both A/C's.
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Old 07-19-2019, 12:26 PM   #6
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My thought is that your EMS is seeing the genny as 30 amp power (one phase) and limiting running both A/C's.
Which is very possible. The last time this occurred on the forum, there was an issue with the transfer switch or something.
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Old 07-19-2019, 01:36 PM   #7
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My Georgetown with a Onan 5500 Gold could run both AC's and a few other items at the same time. They were the 1300 AC units though not the big ones.
Bill
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Old 07-19-2019, 02:02 PM   #8
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My Georgetown with a Onan 5500 Gold could run both AC's and a few other items at the same time. They were the 1300 AC units though not the big ones.
Bill
My Onan 5500 can run 2 A/Cs - 15k and 13.5k and has a bit left over for other stuff.
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Old 07-19-2019, 06:32 PM   #9
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Originally Posted by DFW Rusty View Post
As a rule of thumb, a 3000 watt generator is the minimum for a single A/C and a 5500 watt generator is the BARE minimum for dual a/c (no microwave, no heavy draw items). I am pretty sure most RV people will tell you a 7k is about the right number for a dual a/c.
My Onan 5500 will run both air conditioners fine. We turn one on and walk back and turn the second one on. I think the units with three air conditioners come with a 7000 watt generator.

I looked at the Progressive EMS and it shows that running both air conditioners comes nowhere close to pulling the 45 amp limit on the 5500 when on shore power.

Perhaps a soft-start kit would help the OP if they truly are hitting the generator power limit.

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Old 07-19-2019, 07:12 PM   #10
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Originally Posted by Chester Hill View Post
Own a 2011 390QS. Just recently came off a 5 day trip hooked up to a 50 amp service. Both AC units worked fine. Am now going on a 4 day "dry" camping trip & decided to try generator with AC units to be sure they worked. Seems that only one unit will work on the generator, you can switch from front to rear but can only get a single unit to run. Is that the way the AC works on generator power?
on our Berkshire, we have a Oman 7000 generator, most likely same as yours. We run both ACs with out a problem. Might be the transfer switch gone south. Try tapping the switch from the outside. Skip
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