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Old 08-05-2015, 04:44 PM   #1
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Running both the A/C and microwave while on generator

We have a Cummins Onan QG 4kw model in our 2010 Fleetwood Encounter it will run the A/C or the microwave just fine. But if we have the A/C on and then start the MW, the generator will slowly start to bog down at 15-20 seconds, then die after about 30-40 seconds.

The genny should produce 33amps, the A/C (I believe) is 15k btu, and the microwave is probably about 900-1000w. The compressor is running, not cycling on and off when this occurs.

Sometimes I have noticed that the genny will hesitate slightly when the A/C is running, then resumes normal RPMs. I exercise it at least once a month and it always starts right up, unless it's very hot and it has vapor locked, when I turn it off, then try to restart it before its cooled off.

Does anyone have any ideas or solutions? Or is this just normal, and its not made to handle both the A/C and MW at the same time. Thanks in advance for your input!


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Old 08-05-2015, 05:59 PM   #2
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You are pushing your limit on amps. I trained my wife what I call the Lisa Douglas theory from green acres. Always watch your amp gauge and stay below 30 amps. I have a 5.5 and it works mine real good but not like yours. Check around and see what else you have plugged in.

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Old 08-05-2015, 06:02 PM   #3
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You generator as a 4kw, at 120 volt output it can produce ~33amps of power less ~10% efficiency so say 30 amps of available power. The AC unit will probably pull ~15 amps, possibly more. The microwave probably 8 amps.

Are you using any lights? I assume you have a converter or inverter on you RV. What else might you be running, exhaust fans, On my coach there is about 4 amp draw . If the refrig is running on electric that could be another couple amps. Even with this additional load you are still only at ~30 amps?

There could be a possibility that the AC unit is starting to fail pulling higher amps. You might have to get a load meter and test.

My blower motor was starting to seize up causing the breaker to trip on a more regular basis. When I replace it and had it out on the workbench I could barely turn the shaft by hand.
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Old 08-05-2015, 06:37 PM   #4
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You need to check the watts input on the microwave. A 900 watt microwave is the cooking wattage.

A 900 watt microwave will pull pulling 1450 watts, per Hamilton Beach.

That puts you right at the border of your generator output with a little over 12 amps for the microwave and 15 amps or more with a dirty condenser coil.

You could try changing the gas filter to see if it helps.

I would see if the 4000 watt output is surge or continuous output.
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Old 08-05-2015, 06:50 PM   #5
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Just get in the habit of turning off the A/C before you run the Microwave. No good reason to cause issues with a good Generator. Issues with Generators are expensive.

PS- A/C on and Hot water heater on electric, plus a few lights, converter,etc, NOT counting the Microwave--shows a 30AMP draw on my Progressive Industries Electric Mgt system (Winnebago view 2014). If the Power Pole is not in good shape, with plenty of power,it will kick a breaker on the Pole
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Old 08-05-2015, 08:20 PM   #6
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Yes, I have gotten into the habit of turning the A/C off, but then it will be about 3 to 4 minutes before it kicks back on, even if I only use the MW for 45 seconds.

I will sometimes have the vent on over oven, the one thing I didn't think about was the refrigerator being on auto and switching over to AC when the genny comes online. I'll have to check that next time.

I have about 400 watts of solar on the roof and will usually encounter this problem when on the road and stop for lunch, so the batteries are typically fully charged.

I hope the A/C unit isn't going bad! Last thing I want is an expense like that. Maybe it is just right at the limit of the generator. I'll have to check on phantom loads next time we're out.

Thanks for advice so far!
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Old 08-05-2015, 08:24 PM   #7
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have started puttin refig on gas when cooking
Rick & Barbara
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Old 08-05-2015, 08:29 PM   #8
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Borrow a good Fluke meter and check the voltage and frequency of the generator output. If low Hz or v, then your inductive loads like the AC and MW will draw higher current than at full voltage.
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Old 08-05-2015, 08:36 PM   #9
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Something doesn't sound right here. If you exceed the load capacity of the generator you should pop the breaker, not kill the generator. I think you have an issue with your generator. I think getting an AC ammeter as others suggested is a good idea. Possibly your generator breaker is not functioning correctly.
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Old 08-05-2015, 09:01 PM   #10
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With a 4KW genset, and trying to operate all of the mentioned equipment at one time, you are overloading the genset.

A 5.5 will be at the edge with the load you are putting on your 4.0. The only thing you can do is cut back on the load by selecting only the items that can be run without overloading your genset.

If you are not planning to change units in the near future, you could consider upgrading your genset to at least a 5.5kw model. The cabinetry should be the same size but that something you'd have to verify that.

By the way, like mentioned above, you should be tripping a breaker not bogging down the genset when you do overload it. This could be a sign of a failed or bypassed breaker, not good at all.
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Old 08-06-2015, 06:52 AM   #11
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AC 13-15 amps depending on make, model, how long since you cleaned the condenser, other things.
Microwave about 11 amps
Water heater 10-12
Fridge measured yesterday 2-3
15+11+11+3=40 amps
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Old 08-06-2015, 10:33 AM   #12
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Thanks all! I will check on the condenser, then find out what the power output of the generator is doing. If voltage is low that will also pull more amps, right?
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Old 08-06-2015, 02:09 PM   #13
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You have a problem with the contoll board of the generator, shuts generator off with load under 30 amps. As board progressively gets worse, it will shut generator off when A/C cycles on the pump. Vapor lock is not keeping you from restarting the generator, it is the controller board over heating. One thing you can do now is turn A/C off and leave fan running, then run microwave. With my controll board, sometimes turning off A/C would shut generator off. I amp measured every possible combination of running different things, usually it took three major things to go over 30 amps. A/C, microwave, electric hot water, fridge and a few lights measured to 28 amps. If you push over 30, the breaker should trip before genny shuts down. You could try checking trouble codes, but that usually a wild goose chase.
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Old 08-09-2015, 08:48 AM   #14
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Ok, so got some readings from the generator yesterday. Ran it with no loads and it's output was 121.1vac, the frequency was around 180hz. Turned on the A/C, voltage dropped to 119.5, but was steady. Frequency varied between 60 and 62hz, and was fairly steady.

I made sure the refrigerator was off and switched off the converter at the breaker panel. MH doesn't have electric water heater. So only major sources of power consumption are the a/c and the microwave.

While the a/c was running, I turned on the microwave, generator became erratic, surging up and down, voltage began to drop 115, 112, 109, 104, 102 then the engine stalled. Frequency was all around 120hz and very erratic.

After restarting the generator, ran the MW by itself, voltage was steady around 114vac, but the frequency was wildly fluctuating between 142 and 155 hz. Checked what the MW was drawing with a WattsUp unit, peaked at 2.4kw, then held steady at 1.3kw.

Now the meter I'm using is not a Fluke, but has been reliable and is pretty good quality. Thinking something might be wrong with it, I took it inside to check what it would do with shorepower. It showed a steady 119.9 vac and 59.997 hz. No hunting, perfectly steady.

Today, while out on a day trip to the beach, we ran the generator for about 1/2 hour to heat up a few things in the MW. Longest the MW was on was 4 minutes at one time, on one of the shorter warm ups, the generator started surging. I shut off the microwave, then turned it back on, and the generator was fine.

It seems like the generator and microwave don't get along too well and when you throw in the A/C, all hell breaks loose. Yeah, it would be easy enough to only run one thing at a time, but it certainly seems something is wrong. Any ideas?

I have an extended warranty, so I will probably contact a local Cummins service facility this week, and would like an idea what to say a where to point them.

Thanks to everyone for their input!


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