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Old 10-05-2016, 12:09 PM   #31
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Originally Posted by 17 Oaks View Post
On my rig with its 1 AC unit I could not get it to run on anything except RV power (30 amp), drove me nuts for years.

Discovery: Found out the electrical company I paid BIG BUCKS to run from my shop out to my barn. Yes they charged me for 30 amps pulled from my 100 amp box but it seems they forgot the 30 amp wire and figured I would never know the diff. They were right and they only gave me 20 amps.

That said:

A single AC Coleman11k BUT never eats all 20 amp according to specs so I being an engineer (NOT a electrical engineer) I agonized over this...for YEARS. I also think I bought every Hard Start setup out there and not a single one worked!

Finally I was able to contact and speak with engineering at the AC manuf and I got an answer: 'as the AC unit ages the amps to start the compressor gradually go up and up due to wear and tear on the unit, you are past the point that Hard Start works for you, either wait for fail or replace compressor now and should solve your problem.

I decided to wait for fail, but I still had the issue of no AC unless I was on a 30 amp circuit.

Then one sleepless night (engineers have LOTS of those) at about 3 am "I GOT IT". I headed to the barn and disconnected everything and prepared to run off battery power. With nothing but a flashlight in hand I turn on the fan, then turned to AC...drum roll AC came one without hesitation and blew Ice Cold.

Phase II: Battery's started it (they should with dual Odyssey Grp 31's hooked up.! I then fired up my 2.5KW genset which had never started the AC. AC on running off battery power, then turn off battery power with genset only running....YEP still running, still blowing cold.

Phase III: Plug in barn power 20 amp and turn off genset. It runs and blows cold. So it works and that is how I did it.


BUT wait the OP has a near new rig, surely the compressor is an ez start. Should be, but my $12,000 all new Trane HVAC system and a 'frozen compressor' coming off the truck! Crap happens.

That rig was an '08 model and the AC is still chugging along putting out ice cold air here in S Tx and in Phoenix AZ in the summer time, just a bit hard to start, but once you get it going off the battery power or a 30 am circuit its great.
Which wire size did they use, and how far is the barn run from the 100 amp panel? They may have used wire size that's fine at 20 or 30 feet, but if it's 60-100 feet, it would likely need a larger wire gauge to actually deliver the 30 amps without a huge voltage drop. That voltage drop is likely the reason your AC won't start, because the compressor motor typically draws way more current at startup than during run. You may need 8 ga or even 6 ga, depending upon distance. The root of the issue is that you want a very low voltage drop at the rated current to start your AC. Otherwise, it either won't start, or it will operate at low voltage and very high current, both of which are bad for the motor.
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Old 10-05-2016, 12:44 PM   #32
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Which wire size did they use, and how far is the barn run from the 100 amp panel? They may have used wire size that's fine at 20 or 30 feet, but if it's 60-100 feet, it would likely need a larger wire gauge to actually deliver the 30 amps without a huge voltage drop. That voltage drop is likely the reason your AC won't start, because the compressor motor typically draws way more current at startup than during run. You may need 8 ga or even 6 ga, depending upon distance. The root of the issue is that you want a very low voltage drop at the rated current to start your AC. Otherwise, it either won't start, or it will operate at low voltage and very high current, both of which are bad for the motor.
Not sure i put it all underground. shop is around 135 Ft +/- from barn, its a 270 ft run from the house when I pulled water from the house to the barn, so maybe a bit more than half for electric from shop to barn.

I have since got a different contractor and it was he that told me it was wire size was not large enough to support 30 amps at the distance, 20 amps OK. That guy who did the work first is not out of business, SOP.

Got no one to blame but me, could-a, should-a done it myself, but was in the middle of a studs out home remodel and just lacked the cycles to mess with it.
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Old 10-05-2016, 12:57 PM   #33
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If the conduit will fit the larger wire size, you might still get it done. If not, you're hosed.
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Old 10-05-2016, 01:06 PM   #34
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If the conduit will fit the larger wire size, you might still get it done. If not, you're hosed.
It prob will but at this stage of the game and after this many years of not having and not knowing and I rarely do any projects out there so lights and keeping my John Deere, Jayco etc charged up I have plenty.

The only reason I got involved was because from time to time we do some housecleaning in the RV in the summer and it would have nice to run the AC (which I solved that issue) plus is ate at me that (thinking I had 30 amps) I could not get it to run and even tho long since retired from the engineering world, my brain kept chasing it...
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Old 10-06-2016, 12:58 AM   #35
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I'm missing something. At 150 feet, 20 amp pull, you would have less than a 10 volt drop, which should cause no issue with the AC starting. I run my 15,000 btu AC on 20 amp all the time, along with lights, ref etc. You said you disconnected all power and started it on battery? I think I missed understood what you said. Did they install a 30 amp breaker on 12 ga wire ? Still would draw 30 amps but the wire may over head, depending on how many wires are in the conduit. Even at a 10 volt drop, your AC should run fine.
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Old 10-06-2016, 07:25 AM   #36
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When a motor that is designed with NEMA specs in mind is started and operated below rated voltage, several things happen. Both pull-up torque and breakdown torque are reduced by the square of the voltage change. In other words, a 10% drop in voltage is calculated by the square of the remaining voltage (90% X 90%), which results in a range just above 80% of expected values. Nameplate full load torque is something significantly less than this (the maximum torque the motor is designed to handle on a continuous basis). But when starting, an ac motor has to overcome a much greater torque demand than when it is running. Whenever a motor is asked to start and run at a voltage lower than nominal nameplate voltage, yet still meet the torque and speed requirement, the electrical current demand increases, it runs with more slip, which creates more heat. That heat tends to reduce the life of the motor. I don't know if these motors in RV air conditioners are designed to operate under NEMA specs, but it's likely their design is at least close. However, NEMA does not specify how much active material is in the motors. I think it's safe to say these are designed and built with minimal materials, and that the refrigerant is circulated through the air gap between the rotor and stator, because they are grossly over-rated motors and the refrigerant helps keep them cool enough to survive.

Having said all that, what it amounts to is, these are likely fairly unforgiving motors being operated in conditions well beyond their design limitations. The end result of a 10 degree C increase in operating temp of the motor is a loss of approximately half its service life. When you are operating your air conditioner at almost 10% below voltage, it is capable of producing just a bit over 80% of its rated torque, and is operating much hotter than it is designed to run. You're killing your air conditioner by operating it at that low of a voltage.
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Old 10-06-2016, 08:08 AM   #37
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I'm missing something. At 150 feet, 20 amp pull, you would have less than a 10 volt drop, which should cause no issue with the AC starting. I run my 15,000 btu AC on 20 amp all the time, along with lights, ref etc. You said you disconnected all power and started it on battery? I think I missed understood what you said. Did they install a 30 amp breaker on 12 ga wire ? Still would draw 30 amps but the wire may over head, depending on how many wires are in the conduit. Even at a 10 volt drop, your AC should run fine.
Wags, here is the take away and its the root of my issue: Old system. In talking with the techs who make the AC unit, as it ages the amps to start continue to increase, it becomes harder and harder to start. Thus creating a market for a "hard start" kit. Mine had gone past that point, the AC unit having built in '07 and put on my '08 RV.

As for my circuit, according to my current company the wire is suitable for 20 amps and so is the breaker. They do not recommend a 30 amp breaker.

Why start off battery power. Simple lots of amperage available and since the unit runs on 30 amp RV camp power I am getting my 30 amps to get it up and running and it will continue to run off either my 2.3KW genset or my 20 amp barn power.

Bottom line is the AC is just getting old and coming up on a compressor replace or the whole unit.

Its days across the SW in Phoenix and S Tx were temps can fry eggs take their toll...
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Old 10-06-2016, 08:12 AM   #38
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Correct... I should have been more specific in my 50 AMP description.

Another good reason for a EMS unit.... If you plug a 30 AMP unit and it detects 50 AMP it'll kill the flow.
This confused me. I plug my 30a Progressive Industries EMS into 50a outlets, with an adapter, and all works fine, as long as the outlet is in good shape. Am I misunderstanding your statement?
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Old 10-06-2016, 08:32 AM   #39
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Not if you have the proper adapters.
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Old 10-06-2016, 01:40 PM   #40
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Wags, here is the take away and its the root of my issue: Old system. In talking with the techs who make the AC unit, as it ages the amps to start continue to increase, it becomes harder and harder to start. Thus creating a market for a "hard start" kit. Mine had gone past that point, the AC unit having built in '07 and put on my '08 RV.

As for my circuit, according to my current company the wire is suitable for 20 amps and so is the breaker. They do not recommend a 30 amp breaker.

Why start off battery power. Simple lots of amperage available and since the unit runs on 30 amp RV camp power I am getting my 30 amps to get it up and running and it will continue to run off either my 2.3KW genset or my 20 amp barn power.

Bottom line is the AC is just getting old and coming up on a compressor replace or the whole unit.

Its days across the SW in Phoenix and S Tx were temps can fry eggs take their toll...
WHat you are doing is creating a temporary condition that provides enough starting amperage to get the motor up to speed, then running it on the lower full load amp rating it requires to keep running. The motor doesn't care where you get the current, whether it's from the wall socket, a power post, or the batteries with an inverter. As long as the amount of current is sufficient to start the motor while overcoming the high acceleration loads involved. The only problem I see you running into is if the compressor stops because the AC is able to get the RV down to temperature, then restarts after a minute or so, once again drawing a high current. I am surprised in all this that you are not tripping the breaker in your RV that supplies the AC unit.
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