A/C off of 20 amp service?

Hey Doug,

I have been talking the other side of this for years. People who brag about running electric heaters on 30 amp, pulling a constant 28 amps for many hours on end. I have seen so many burnt up plugs, burnt up electrical junction box's. Inverters ruined.

Running overloaded constantly will eventually cause a fire.

Thanks for your as usual good advise. Some folks just will never listen till they experience a problem. They continue to give poor advise.

Pogoil.

My 2 cents.

It depends.
Size of AC 13.5 maybe 15 doubtful and I would not recommend.

Plugged into a circuit at home would require a dedicate circuit or knowledge on what else is on the circuit.

If an extension cord is used size matters. I use 50ft 10 gauge.

At the campground the outlets are usually burnt, pitted and resistive causing higher amp draw and lower voltage.

If you have low voltage the longer the AC is on the hotter the wire the lower the voltage requiring more amps to do the same work increasing heat.

I would not argue with dougtrainer and take his experience over what I have to say I base mine answer on my experience his experience is based or real life RV repair.

Simple answer sure it can be done BUT you really need to monitor the incoming voltage and shut the AC down at 108 VAC.

At 105AC the compressor is in self destruct mode if not today then soon.

Myself and plenty of other folks do it usually when loading or unloading the RV going on or returning from a trip.

Michigander wrote:
Has anyone ever heard of a single a/c unit not being able to start and run on a 20amp shore power circuit? We bought a new coach and the dealer says don't run a/c unless connected to a 30 amp service or connected to the gen. They say it will cook the compressor. We have always run a/c units in trailers or coaches off a 20 amp circuit without a problem when at home so I am perplexed about what I am missing?? Thoughts?

I don't care whether you have 50 amp, 30m amp, or 20 amp service, the same voltage drop circumstances apply to all. Even if your coach is hooked up to 50 amp service, your AC unit is running on it's own 20 amp circuit, with it's own 20 amp circuit breaker. If the voltage is low for the 50 amp service input, the voltage will be low for the AC circuit as well.
A simple $15 meter(Kill-A-Watt) can check your voltage and current draw for the AC unit, while it is being used, to see if you get any voltage drop that may affect the life of your AC unit. Any speculation as to what may happen when plugged in to a 20 amp circuit is just that, a guess, without actually hooking up a meter to find out if it is doable or not.

Michigander wrote:
Has anyone ever heard of a single a/c unit not being able to start and run on a 20amp shore power circuit? We bought a new coach and the dealer says don't run a/c unless connected to a 30 amp service or connected to the gen. They say it will cook the compressor. We have always run a/c units in trailers or coaches off a 20 amp circuit without a problem when at home so I am perplexed about what I am missing?? Thoughts?

Additionally, is it sometimes easier to just say don't, rather than over complicate it for some people who just can't get the if's and maybes in general.
For instance....you can't back up with a toad in tow....and a very good rule to follow, but in actuality, you can do so, if you know what you're doing.

Hi CA Traveler,

No. It was the Magnum Hybrid inverter that let out the magic blue smoke when running from the generator.

The sola was not involved as there is no need to use it with the generator.

CA Traveler wrote:
OK, didn't know you were referring to the Sola unit. Didn't yours let the smoke out?

OK, didn't know you were referring to the Sola unit. Didn't yours let the smoke out?

Spent a week at daughters house on 20amp service using a dog bone adapter,ran 30 amp cords.Used ac but nothing else.Every thing stayed cool,check plugs and adapters for heat.A dirty or loose connection will get warm.

Hi CA Traveler,

I should have qualified my reply by stating I was at a 30 amp pedestal.

There is no "magic" about an autoformer--it can not create "extra" watts and will consume some watts.

I have been at more than one 50 amp location where voltage was an issue when running "heavy" inductive loads.

The "real" math for the Sola Basic autoformer on a 20 amp source at 105 volts would be:

105 x 20 = 2100 watts

2100 X 96% = 2016 watts (4% overhead from SB)

Boost would be to 115 volts on a 105 volt supply

2016 / 115 = 17.53 amps output to the RV from SB

17.53 X 80% =~ 14 amps for safe continuous duty.

Numbers were rounded down.

What I like most about the Sola Basic is how fast it switches in when the start up surge "hits" the supply from the roof air. It comes on in time to help with that--before I hear the sound of the roof air starting.

I think I'm seeing less wattage "creep" (i.e. roof air drawing more and more watts) when using the autoformer, but the cooling season is over now--so I'll have to do a "back yard" experiment in the future.

Autoformers are very useful devices as they can boost low pedestal voltage. However they cannot replace the lost power due to low pedestal voltage. Let’s take an example of 105V at the pedestal which limits the power to 2100W input to the autoformer running 98% efficiency yields autoformer output of 17.2A at 120V. Here’s the math:

105V*20A=2100W*98%/120V=17.2A

So using a autoformer to boost the voltage reduces the watts and amps available for the rig.

In addition the recommended sustained load on a plug is 80% not 100%. And certainly an A/C especially in hot weather represents a sustained load. So now the amps available are down to 13.7A.

105V*20A*80%=1680W*98%/120V=13.7A

Certainly 17.2A or 13.7A is adequate to run some A/Cs. My last rig had a 15K BTU Dometic A/C that drew 14A for the compressor plus 1.5A for the fan or 15.5A. So I would have been hard pressed to run the A/C on a 20A 120V circuit considering that there are other rig loads. However a autoformer is a more useful device on 30A in this situation. In my 12 years of using 50A power I’ve had exactly one CG with low voltage on 50A which limits my justification for a 50A autoformer.

There is no panacea with free power using a autoformer but they are a useful tool.

A sane (and calm) opinion.

Before I had the autoformer, I could sometimes only run the roof air. To do so, I had to use the load support feature on a hybrid inverter forcing it into voltage support by limiting the input amperage to lower than the amps needed for the air.

Post autoformer, I find I can safely run the roof air and the fridge without using the load support feature.

westernrvparkowner wrote:
In my opinion, if you are operating the AC in an RV on a 20amp circuit, you need to monitor it frequently at the least. Not everyone would do that and that is where trouble would begin.