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Old 04-03-2019, 04:14 PM   #1
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AC trips Onan Generator

I have a 2015 Forest River Sierra 376BHOK. MY air conditioners work fine on shore power. When generator is running and i turn the A/c on it trips the generator switch. Cummins says generator is fine and tested it on their load machine and it will handle 125% of what its rated at.

Anyone else had this problem? It does the same thing even if i turn on 1 zone or 2. Its a Cummins Onan RV QG 5500 LP.
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Old 04-03-2019, 04:20 PM   #2
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You have a 30A and a 20A leg on that generator. Once warmed up and running, it should handle one AC on each leg unless you have other major items running like WH or microwave. Does the same breaker trip each time?
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Old 04-03-2019, 04:24 PM   #3
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tell us exactly 'how' this was tested on their 'load machine'...?

are you referring to a test performed AFTER it was installed in you RV, or
just that they 'test' these under their load machine at the factory?

I suspect you have a loose wire at one of the Generator's internal breakers - when the a/c unit applies the load, the wire heats up and may lose it's connection to the breaker, even for a instant, which may trip the breaker, which ultimately trips BOTH breakers since they are tied together by a common handle, even though you may only see a single breaker on your generator.

if not, it 'could' be the connection at your ATS(automatic transfer switch) which controls whether you are on Shore or Generator power to your Main Electrical Panel. Any slight arc or loose wire at the Generator side of the ATS could cause the generator's breaker to trip.
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Old 04-03-2019, 04:38 PM   #4
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Generator

I was going to mention my mess with the Onen 5500 until I saw LP. I was considering going to LP to get away from gas just because of shut down issues I had until I replaced carb on 2 year old generator. Sorry I can't help you.
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Old 04-03-2019, 04:39 PM   #5
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I am not positive how they tested it. He just said he cut the wires and hooked it up to their load thingy etc.

That’s when he said he didn’t recommend replacing the breakers on the gen because I’m under load it ran great for a few hours. That’s when he said it could be the inverter causing the problem but I didn’t understand that at all.

The ATS seems to make sense to me. I assume this is something I can swap myself?

I have a 3rd ac on the bunkhouse which I had rewired to it’s own 30amp plug. Of course United RV says they tested everything however if they had they would have found an issue. Not to mention they over filled the oil in the gen.

Anyways, I appreciate the feedback. United RV is 4 weeks out on getting it back in and camper has to sit there the whole time to hold place in line.
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Old 04-03-2019, 05:03 PM   #6
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This from Cummings web site.

Cummins 5.5 kW LP Powered RV Generator (5.5 HGJAB-1119)
More power for medium size RVs (30A & 20A Breakers)
Runs two 13,500 BTU AC units w/ 1100 additional watts available
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Old 04-03-2019, 05:35 PM   #7
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not sure 'who' you are referring to as 'testing' the generator, or why they would 'cut the wires', or even 'which' wires you are referring to... give us more details.

and no, the Inverter has NOTHING to do with the generator, or the air conditioners.

The ATS is simply a 'junction' box, usually located somewhere close to where your Shore Cord enters the RV...sometimes even under the bed, behind a false wall, or even outside in the bay where the electrical cord is stored.
The box simply houses the connections from the Shore and Gen, and into the Main electrical panel, with a set of magnetic 'switches', which determine which power source provides the power to the electrical panel.
The wires that connect in and out of these switches can sometimes be LOOSE, as one of mine was. With no shore power and the generator not on, you can remove the cover and check the connection/tightness of each wire.
You can also at the same time do the same within your Electrical Panel, because many times these wires become loose after some usage. The factories tighten then, but the heat of the loads can sometimes initially cause them to loosen up, which can cause nuisance tripping of breakers.
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Old 04-03-2019, 05:43 PM   #8
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The “who” is the Cummins dealer that is servicing it. He said that he cut the wires to tap in and be able to add a load to the generator. To which he said it ran for hours under a heavy load.

Thank you for the info.
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Old 04-03-2019, 05:48 PM   #9
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wow, 'cutting' wires? really? What 'wires' is he referring to? Maybe, or hopefully, he just meant that he 'disconnected' the main wires from the generator breaker output and connected his 'own' wires to a load.

so, basically he 'bypassed' the ATS, which is probably where the problem is, and which is why he has seen no issues with the generator.
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Old 04-03-2019, 05:58 PM   #10
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I get your point and I agree, hopefully he didn’t cut them.

Do you think the ATS is near the panel in the hall or back by the shore power plug? I would think you would want the shortest run as possible so meeting in the middle at the breaker panel makes the most sense. I’m gonna have them change the gen breakers since the camper is already at their facility.
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Old 04-03-2019, 08:45 PM   #11
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it could very easily be a loose connection that is heating up and drawing more amperage. go into your electrical distribution panel and turn every branch circuit breaker off. make sure you are disconnected from shore power. now start up the generator and let it stabilize. then go turn on one circuit breaker for one a/c and get it running. see what happens. if all is well turn on the circuit breaker for the other a/c and get it running. see what happens. keep track of the which circuit breakers you flip and you continue to turn them on one at a time.
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Old 04-04-2019, 12:40 PM   #12
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You need a Hard Start Capacitor. Check with Cummins for the exact capacitor. I had the exact same problem. Once installed. No more issues. Easy to install. See youtube for install method.
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Old 04-04-2019, 06:48 PM   #13
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Ok, the Cummins service tech said they hooked it up to a “load bank” and it ran fine for 2 hours at 95% capacity. He didn’t recommend changing the gen breakers as he said they didn’t trip under load bank. However again, when he turned on the air conditioners it tripped the gen breakers. Did the same thing even with everything off and then turning on an air conditioner. He keeps mentioning the inverter or converter I don’t recall which one.

Any other suggestions? Could the ATS still be the problem even though everything runs off the generator fine until the ac is turned on?

Thank you all for your feedback, I appreciate it.
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Old 04-04-2019, 07:48 PM   #14
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i'm stumped! there have been many many posts about this generator running two a/c's successfully.

may i ask some questions for clarification?
have you owned this trailer for some time or did you just purchase it?
have the air conditioners ever run using generator power?
do you have 1, 2, or 3 a/c's and what type?
when the onan circuit breaker trips is it always the same one?
does only one circuit breaker trip on the generator or both?
by any chance to you have an EMS in the shore power path that shows the amperage being used on each leg when connected to shore power? if so, what does it show when you are running the a/c's from shore power?
are you trying to start multiple a/c's at the same time?
are the a/c's all turned off until the generator starts and stabilizes?
can you turn all of the distribution panel breakers off and then only turn on one a/c at a time to see what happens?
can you look at the distribution panel and confirm that the a/c's are actually wired to separate power legs and not both on the same leg?
if you just turn on the a/c fans and not the compressors what happens?

onan says they tested it with a load bank. i am not sure of this but i think a load bank is a resistive load (like a heater) while the a/c's are a induction type load that generates larger start up loads than running loads.

i wish i could offer a suggestion as to what is wrong.
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Old 04-05-2019, 10:57 PM   #15
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Quote:
Originally Posted by CHICKDOE View Post
i'm stumped! there have been many many posts about this generator running two a/c's successfully.

may i ask some questions for clarification?
have you owned this trailer for some time or did you just purchase it?
have the air conditioners ever run using generator power?
do you have 1, 2, or 3 a/c's and what type?
when the onan circuit breaker trips is it always the same one?
does only one circuit breaker trip on the generator or both?
by any chance to you have an EMS in the shore power path that shows the amperage being used on each leg when connected to shore power? if so, what does it show when you are running the a/c's from shore power?
are you trying to start multiple a/c's at the same time?
are the a/c's all turned off until the generator starts and stabilizes?
can you turn all of the distribution panel breakers off and then only turn on one a/c at a time to see what happens?
can you look at the distribution panel and confirm that the a/c's are actually wired to separate power legs and not both on the same leg?
if you just turn on the a/c fans and not the compressors what happens?

onan says they tested it with a load bank. i am not sure of this but i think a load bank is a resistive load (like a heater) while the a/c's are a induction type load that generates larger start up loads than running loads.

i wish i could offer a suggestion as to what is wrong.
Is that all you got?
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Old 04-05-2019, 11:04 PM   #16
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Update

I got the camper home and turned all the breakers off. Started the generator and let it warmup. Then I flipped one of the ac’s on. No problems. After about 30 minutes I turned the second one on. No problems. I’m thinking the Cummins dealer tried to run both ac’s, both fridges and whatever else was on all at the same time. I still think the gen should handle that but being the fridges were hot and I assume had to be in overdrive as well as both ac’s being on, plus lights etc that could potentially be to much for the gen?

Thoughts?
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Old 04-05-2019, 11:15 PM   #17
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Quote:
Originally Posted by frankc7037 View Post
You need a Hard Start Capacitor. Check with Cummins for the exact capacitor. I had the exact same problem. Once installed. No more issues. Easy to install. See youtube for install method.
I just noticed your reply. Thank you, that makes perfect sense. Iíll look into that. I noticed when the air conditioner compressor kicked on it sounded like it needed some more oomph.
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Old 04-06-2019, 09:08 AM   #18
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i was wondering about when the breaker was tripping. if both a/c's, both refrigerators, the converter, and possibly the water heater had all been left on that could be a pretty heavy startup load put on the generator as soon as the transfer switch connected it to the loads. i do not know if this was the case or not. just a guess. but it does appear that the generator is handling the loads as long as they are not flipped on simultaneously. again, there have been many many posts about that size generator handling 2 a/c's just fine. i'm glad you were able to get as handle on the problem.
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