A/C on shore power trips breaker after 30 minustes

wpatters1229 wrote:
I have two meters one being a fluke. I also have a degree in electronics. But was not sure of how the RV deals with this A/C problem. I did figure it out and all is well.

So, I guess all is not well?

We're trying to help you, but you haven't responded to the questions we asked to help you diagnose your problem. I'm still leaning toward then 20A to 30A adapter.

Can you tell us what voltages you measure at an wall outlet inside the motorhome when the A/C is running? It would be interesting to compare these readings on a cooler day and then again on a hot (90+) day, and several times, not just when the A/C is first turned on, but after it's been running for a while. I suspect the voltage at the A/C is sagging under the higher temperature conditions enough to trip the 20A breaker.


"I have an adapter for 30 to 20 and a 10-12 ga extension cord." Is the adapter for a 30A plug (male) to 20A receptacle (female), or for a 20A plug (male) to a 30A receptacle (female)? The latter would allow you to use your 30A RV shore power cord on a 20A outlet. The first one would allow you to plug a standard 120VAC extension cord into a 30A RV receptacle.

You say a "10-12 gauge extension cord"? Which is it, and how long is it? How old is it and does it appear to be in good shape? Do the connection ends get hot while it's being used while being used?

I'm not sure what you mean with this: "...and am also taking the unit out of the wooden box in the RV to check the fan." What unit out of what wooden box?

"Does anyone know how to test the Fan since it comes on via the controller board? Is it a 12v fan or a 120v fan?" Are you referring to the fan on the A/C unit? If so, I believe that should be 120VAC.

If you're trying to run your entire RV off a single 20A connection, be aware that other appliances in the RV might be adding to the current draw in addition to the A/C, unless you're specifically turned them off. Things such as electric water heater, refrigerator, battery charger, TVs, etc. With only a 20A supply, you should be able to get the A/C to run properly, but only if ALL other draws on that 120VAC line are shut-off. Again, AC voltage measurements inside the RV would go along way to help diagnose this. A voltage measurement without the A/C running, then one with it running, and then several other with it running at various times during the day and different temperatures.

Well, good luck. Any specific troubleshooting info you can provide (voltage measurements at various operating conditions) could be very helpful.

~Rick

One other thing. When the temp outside is in the 90's the A/C works for a while and then trips the breaker. If it is in the 80's it does not and this is in an RV site with proper voltage. I have one of those attachments that checks the shore voltage etc.. It is an old unit so changing out a possible bad breaker is easier and cost affective than a whole new unit.

I have an adapter for 30 to 20 and a 10-12 ga extension cord. The problem is that I think the breaker (20/30) dual breaker needs replacing. I ordered a new one and am also taking the unit out of the wooden box in the RV to check the fan.

Does anyone know how to test the Fan since it comes on via the controller board? Is it a 12v fan or a 120v fan?

VA-Apraisr wrote:
I think that electrical cord may well have been your problem, but doubtful on cutting the a/c cover opening...

I would tend to agree with your assessment.


wpatters1229, did you ever make voltage measurements at the A/C with the extension cord & adapter combination and compare it to the voltage with the generator running? Cutting the cover probably wouldn't change the voltage getting to the unit.

Anyway, whatever additional information you can provide that helped you to zero in on what you thought was the solution would be helpful should other folks have a similar issue in times to come.

~Rick

What is an acceptable delta T for a roof air conditioner? I.E. temperature difference between inside and the great out doors?

The point of this site is ask questions/get advice/REVEAL the solution. It's helps all of us that may encounter a similar issue. That said, WHAT was the SOLUTION? What did you figure out?? If cutting the vent fins and changing the electrical cord were the solution, great. I think that electrical cord may well have been your problem, but doubtful on cutting the a/c cover opening....but didn't know if you used the voltmeter to verify.

I have two meters one being a fluke. I also have a degree in electronics. But was not sure of how the RV deals with this A/C problem. I did figure it out and all is well.

wpatters1229 wrote:
OK, all electronic theories are out the window. Yesterday I had the shroud off the A/C and it was very hot in our area. I think it exposed the compressor and coils to more heat than it could handle. Today I put everything back and also cut away some of the grills in front of the exhaust radiator fins and coils. I read that it blocks the airflow and can reduce the cooling effect. I fired the unit up today and it has been running for over an hour. It is a bit cooler today but still plenty hot. 83 today and yesterday 99. I am not sure if there is a safety cutoff on this Coleman A/C but it would not surprise me. Thanks for all your help.

Seriously, get a multimeter, so next time you can determine what is reallyhappening.

You didn't throw any electric theories out the window.
- When it's hot, the compressor load can be a bit higher resulting in more amperage.
- When it's hot, it's common for the electric grid voltage to be lower, resulting in more amps.

But without data, you have no idea if it's simply overloading a 20amp circuit or if there is something unexpected happening.

It was the heat of the day and the fact that the unit had the cover off while I cleaned it. Not sure if that was the problem but today after putting everything back it ran fine for a couple of hours until I shut it down. I do have two multi-meters. My fluke does have an amp adapter. I think the combination of heat outside and the cheap extension cord caused the problem. I have a heavy gauge extension cord and it is a bit cooler today. Not having the cover on exposed the compressor and the coils to heat up. Today I also cut away some of the front plastic grill allowing the heat off the coils to push hot air out faster.

wpatters1229 wrote:
One other thing. I have an extension cord going to a 20 amp wall socket that hooks up to my 30 amp cord using an adapter. It has never been a problem.

Are you saying you are running your rig off of a 20Amp circuit? If so, I would say you've been lucky to get it to work at all.

Certainly the higher heat can be a problem. Not knowing how long the extension cord is, but the resistance in the wires increases with temperature. As the wire resistance increases, less voltage gets to the A/C and the unit has to pull more current to try to compensate for the reduced voltage. Also, every connection, including the adapters, adds resistance to the path. And speaking from experience, if your 20A to 30A adapter is one of those "hockey puck" looking things, they're garbage and have a relatively short life span.

When you use your generator, you don't have those losses to contend with.

My guess is that you'll be fine when your rig is connected to a true 30A 120VAC connection, assuming the voltage is proper. That assumption, by the way, is not always a safe one to make. If you don't have a voltmeter, I'd recommend getting one and learning how to use it so you will KNOW if there is a problem with the supplied voltage to your rig.

Good Luck,

~Rick