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Old 08-10-2012, 07:40 PM   #1
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A/C Conumdrum - Shore power vs. generator

We have two a/c's in our 35' Class A. They both run fine on generator but only the front a/c runs on 50 amp shore power. We can hear a click in the rear one when operating the thermostat so it's getting 12 v. Could it be our 15 year old shore power connection cord which looks fine? I have nothing to be able to test it. All else in the coach that requires electricity is working. We definitely need help from someone with good electrician skills. Camping right now at Stone Mountain, GA....and buddy, it's hot here.

Thanks for your help !!!
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Old 08-10-2012, 07:59 PM   #2
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the generator has a breaker[I believe] for an ac unit there may be an inhouse breaker for the 2nd breaker, both units need breakers regardless of yhe source of power. I would check the house breakers.
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Old 08-10-2012, 09:13 PM   #3
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You definitly have an issue. Check your connections for your shore power and your panel. I have 2 separate breakers in the panel.
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Old 08-10-2012, 09:44 PM   #4
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A 50 amp system has two legs where each AC is on a separate leg. You generator is a single leg where the two legs are tied together as in a 30amp system. I suspect it is your hook up cable or plug. Most likely the plug or the socket on the post. Get a neighbor with an ohm meter and as ask him to check it out. Or go buy a meter for about ten bucks and learn to use it yourself.
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Old 08-10-2012, 09:53 PM   #5
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DarrelC

Need more information such as make, model, year of rig. Do you need to plug in your shore power cable to use you generator or do you have a transfer switch? When on shore power and you start the generator does the rig switch over to the generator automatically? JM2...
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Old 08-10-2012, 10:04 PM   #6
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Quote:
Originally Posted by targaboat View Post
A 50 amp system has two legs where each AC is on a separate leg. You generator is a single leg where the two legs are tied together as in a 30amp system.
I'm not entirely sure that's correct. If I understand right, most 4000-6000 watt generators have two windings. The legs are not tied together, otherwise you'd see 5000-6000 watt generators with 50A breakers. Technically they have 2 legs, generally 20-30A each, which is why you have two breakers on the genset.

It's like having 2 x 2500 watt generators (in the case of a 5000 wat generator).

What I don't know is if the two windings are out of phase and can be used for 240v... (Like most 4000-6000 watt non-RV gensets)

In regard to the rear AC:
Will it work on shore power w/o the front one going?
If not, I'd start by checking for 120V power to the rear AC. If you don't have 120V at the rear, I'm thinking that I'm then looking at the breaker and then the transfer switch..
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Old 08-10-2012, 10:14 PM   #7
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My Onan has two windings 30a & 20a with the neutrals and grounds bonded and in phase with each other. JM2...
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Old 08-11-2012, 06:33 AM   #8
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I know on my diesel pusher I have one 30 amp circuit breaker on the genny and it will run two ACs. I also know that 50 amp service has two legs which are out of phase with each other and measure 240 volts across the two legs. I also know that as a result each leg is treated as a separate circuit in the RV and that there is an AC on each leg. I also know that if one of the prongs on the plug is not making contact, only one leg will be hot there by one AC will not work. And if both ACs are function while genny, circuit breaker are probably ok. SO GET A METER AND FIND OUT WHERE YOU DO NOT HAVE VOLTAGE WHERE ARE SUPPOSED TO HAVE VOLTAGE WHILE ON SHORE POWER.
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Old 08-11-2012, 08:37 AM   #9
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targa,
I wonder if your generator has 240v 30A and (on yours), which would make sense. I have a back-up generator now (General garage type) and it's the same way. We run stuff off of the 240v 30A circuit.

I've just never seen an Onan setup this way...
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Old 08-11-2012, 11:18 AM   #10
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Unless you have confirmed with the service manual I'd not be so sure your Onan has two windings.. It may have just one 50 amp winding.

Many motor homes have an Energy Management system, some of these also have a man ual "Front/Rear" switch which needs to be properly set to let the EMS control the AC units,

If a generator has both a 30 and a 20 amp outlet the 20 is often a "pass through" breaker in line with the rear air conditioner. Bit of a rube goldburg wireing set up but that is how INtelletec shows it on some of their Auto-Transfer diagrams.

My ONAN has a pair of 30 amp breakers, it is capable of just shy of 50 amps total, Of course that is NOT what the salesman said. but what does he know (Happens to be what I trained for,, Electronics, not sales, though I've had expierence there too).
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Old 08-12-2012, 07:19 AM   #11
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As I remember the OP does not have a problem with his generator for both ACs work fine. The problem occurs when plugged into shore power. So why are we talking about generators?
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Old 08-13-2012, 02:16 PM   #12
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I was able to check out the power cord when we got home. One leg of the Marinco connector was bad....giving me only one leg to shore power and the front a/c. The rear a/c does not have a dedicated central breaker. I've ordered a replacement cord and will let you know what the results were the next time I have access to a 50 amp service. Thanks, all, for your input.
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