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Old 07-29-2021, 07:59 AM   #1
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Electrical- A/C question

Good morning all

I have a question- If I want to run one A/C unit using only a standard 120volt connection, can I connect the coach to the power, start the generator (for the power spike), turn on the A/C, and then turn off the generator using only the house to run the A/C?
I know I will not be able to run much else.

Best

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Old 07-29-2021, 08:16 AM   #2
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No - assuming your coach has a transfer switch - it’s hard on the transfer switch contacts to switch under load.

If you’ve flown on an airplane, you may recall while sitting at the gate that sometimes you’ll hear the air vents do a quick “blip” in providing air.
Same thing will happen in your coach.

Best solution if you really need to run the AC - get the largest gauge and shortest extension cord that you can find to power the coach.

Check/monitor the 120VAC while the AC is running to be sure you always have at least 108Vac (except when the AC compressor is trying to start - you can probably get away with a drop to 90Vac momentarily at that time).

And yes, you won’t be able to run much else except for a few lights.

And remember, the convertor is trying to charge the coach batteries when the coach is plugged into shore power. If your coach batteries are low, then there is going to be some consumption there.
Make sure your coach batteries are fully charged before you try to run the AC off the extension cord.
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Old 07-29-2021, 08:16 AM   #3
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I just tried running mine off 120v and after a few hours, the house breaker popped and the 30 amp to 120v adapter was very hot and somewhat melted.

Your thought about the generator, I would assume is about taking the extra amps during compressor start up. That may work initially, but the compressor cycles on and off throughout its operation

So I guess my opinion is no, it is probably not a good idea.
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Old 07-29-2021, 08:31 AM   #4
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Depending on the circuit ampacity and distance to the panel, and the size of the A/C unit, it may be possible to start and run it without issue. More likely to run on a 20A circuit of course. But attempting to switch from generator power to shore power under load isn't likely to work and will be hard on the contacts even if it does work. If the controls sense a momentary loss of power, the compressor may shut down. Or stall and trip the breaker attempting to restart.

Obvious first choice would be to install a proper shore power circuit, but next best would be to shut off all breakers except the A/C unit's, including and especially the water heater and converter/charger. Shortest cord possible, and use a 20A circuit instead of 15A if one is close.

You can also install a soft start unit, but that'll cost as much as adding a shore receptacle IF the panel is close by. The cost of a new shore power circuit will depend on lots of variables, though.
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Old 07-29-2021, 09:08 AM   #5
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Lots of good advice above!

I agree, don't deliberately start on generator and switch to shore. This would be hard on your ATS and not help your situation.

Connect available shore power and start the A/C. Listen to see if it starts properly. Disconnect if it does not.

I second the post that says, measure the line voltage inside the coach while the A/C compressor is running. It is the definitive method. A/C usually requires 108 volts to maintain low running temperature. It is running temperature that will kill the A/C motor.

I use 110 volts while running to determine if it is OK. Line voltage can vary some. 110 leaves a small margin for change.

We can guess what setup will work, but it will always be a guess. Voltage inside to coach is what matters.

Use the coach 50 amp cord as much as possible. Use only one adapter if possible. Make sure batteries are nearly fully charged. Make sure all other AC appliances are "off".

If you need additional extension, first choose a 50 amp extension cord. Next, a 10 gauge as short as possible. If the package at the hardware store does not tell what the gauge is, get something else. Unadvertised gauge probably means too small. Lowest priced cords will be "cheap" in every sense of the word.

I wish you good luck and happy trails ahead!
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Old 07-29-2021, 09:59 AM   #6
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Well, that is why I came here first.
A good dose of information that I had not though of.

When out, I always use the 50amp supply and when she is put away in the garage I have the 15 amp hooked up just for the batteries. Everything except the CO detector and map lights is turned off.

The reason why I am asking is we will be at families homes for about a week and all we will have is the generator and a 15 amp supply.

Thanks again

Dr. Mike
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Old 07-29-2021, 10:14 AM   #7
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Almost all houses have some 20 amp circuits in them, you need to find one.
See if they will check the circuit breakers for them.

My home, built in 1977, had 20 amp circuits in the kitchen, dining room and washing machine. Of course, they will need to not use that circuit with your AC running.
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Old 07-29-2021, 10:35 AM   #8
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Quote:
Originally Posted by Dr. Mike View Post
The reason why I am asking is we will be at families homes for about a week and all we will have is the generator and a 15 amp supply.
Depending on the age of the house, it's possible or even likely that an outdoor receptacle is 20A. It was common years ago to wire the required outdoor receptacle to one of the kitchen counter receptacle circuits (Small Appliance Branch Circuits), which were and still are required to be 20A. That's no longer allowed, and the requirements for outdoor receptacles (both front and back) and garage receptacle have never specified ampacity.

But back a few decades it was common practice to tap either kitchen or bathroom 20A circuits, probably because the SABC and bathroom receptacle circuits were GFCI protected already.

So you may get lucky and find that the garage or outdoor receptacle is actually 20A. But if so, just don't let anyone use the toaster while the coach A/C is running.
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Old 07-30-2021, 07:52 AM   #9
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Quote:
Originally Posted by KanzKran View Post
Depending on the age of the house, it's possible or even likely that an outdoor receptacle is 20A. It was common years ago to wire the required outdoor receptacle to one of the kitchen counter receptacle circuits (Small Appliance Branch Circuits), which were and still are required to be 20A. That's no longer allowed, and the requirements for outdoor receptacles (both front and back) and garage receptacle have never specified ampacity.

But back a few decades it was common practice to tap either kitchen or bathroom 20A circuits, probably because the SABC and bathroom receptacle circuits were GFCI protected already.

So you may get lucky and find that the garage or outdoor receptacle is actually 20A. But if so, just don't let anyone use the toaster while the coach A/C is running.
There is a receptacle, but my theory (and question) were more related to the power spike on start-up.
How about a follow-up question: What are the group's thoughts about this Soft Start device?
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Old 07-30-2021, 08:08 AM   #10
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Quote:
Originally Posted by Dr. Mike View Post
There is a receptacle, but my theory (and question) were more related to the power spike on start-up.
How about a follow-up question: What are the group's thoughts about this Soft Start device?
Circuit breakers used in residences are thermal-magnetic type. For small overloads, the time to trip depends on the current above rated. Small overcurrent can take a long time to trip the breaker, up to hours. Large overloads can trip it in seconds, with time-at-overload values everywhere in between. The magnetic trigger is likely to be somewhere in the 6 to 10 times rated current region, and is usually high enough that hard starting loads like air conditioners and other induction motor loads can start without tripping it. I posted a typical trip curve on another recent thread on this.

But a big roof A/C on a 15A circuit may not work. Starting current may be too high, which is where soft-starters come in, as they ramp the startup and limit the current to a much smaller peak. But that's only half the issue, with the other one being running current being too high for a small circuit. You don't want to run a continuous or motor load above 80% of a circuit's ampacity, or 12A for a 15A circuit.
Voltage drop in the supply wiring makes that worse, as A/C motors draw more current as voltage sags, being constant-power loads (P = V x A, in a general sense, so less V means more A).

I run my roof air on my son's 20A garage circuit, but it's only 11,000 Btuh, not some jumbo 15k unit like bigger RVs would have. It will run on a 15A, though, as that's what I had in my garage until I added a proper TT-30 receptacle. But again, it's only 11kBTUh.
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Old 07-30-2021, 08:52 AM   #11
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The breaker in the RV is 20 amps, A home 20 amp breaker will handle the same.

Its not the breaker you need to worry about, its the gauge of the wire you use to connect up to the 20 amp outlet.

If your RV cord reaches the outlet then a good dogbone adaptor is all you need.

If not, using a 12 gauge extensions cord will work, but keep it 25 feet or so.

I used that kind of setup at my brothers house many times. The extension cord went in the bathroom window, under the door and to the 20 amp outlet, near the breaker panel.
Used the AC, charger and fridge in summer.
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Old 07-30-2021, 09:10 AM   #12
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I am just seconding what others have said. I have done this with a 20 amp circuit and a 50 foot 12 gauge extension. (Should have been shorter, but it was all I had). Once tried it with a 15 amp circuit, and no way it would work. As others have said, low batteries draw a LOT of current, so make sure they are at “float”
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Old 07-30-2021, 09:14 AM   #13
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Some owners have been able to run one AC on Honda 2000 watt genny with soft start. Never hear any bad reports on the soft starts. Cord ends and outlets can get old and loose adding to heating at near max amps. But as pointed out not full issue starting amps.
My old TT AC on cheaper 3000watt inv. genny, I ran genny on economy setting with AC maxed out so it never cycled. It was easier to start it early before temps were top mid day heat as well.
Quote:
Originally Posted by Dr. Mike View Post
There is a receptacle, but my theory (and question) were more related to the power spike on start-up.
How about a follow-up question: What are the group's thoughts about this Soft Start device?
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Old 07-30-2021, 09:45 AM   #14
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Quote:
Originally Posted by Dr. Mike View Post
There is a receptacle, but my theory (and question) were more related to the power spike on start-up.
How about a follow-up question: What are the group's thoughts about this Soft Start device?
This might help you -

https://www.irv2.com/forums/f103/not...me-491457.html
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