Using two roof a/c units on 30 amps

Frank Hurst said:

After many tripped breakers and some burned 30 amp cords, I go with one AC and a fan.

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+1.  2nd unit is no help if it's constantly causing problems. I'd either go with one and a fan, or when you cut in the 2nd one wire it separately so you can power it off another shore plug. Much like us camping where there's only 20amp. It'll run the AC, but you need to dial it off when using the microwave. Doable, but  not ideal.

Can it be done, maybe depending on where you are getting your shore power from. Not every place has another source to run the second AC on. My rig has a 50amp connection but my seasonal campsite only has a 30 amp service. How much current is drawn is dependent on the voltage of the service. If everything is on the up and up and you get a full 120V that doesn't sag you are looking at 12.5 Amps for a 12.5K BTU and 13 for a 15K BTU. Say you have one of each that's 25.5 running amps and does not include the inrush when starting. Electrical systems by design should not be loaded past 80% of their rated value so a 30 service should never be run past 24 amps which 2 ACs running will exceed with nothing else running. Now if you have a service thats a little on the high side a 125V you only exceed by about 1/2 an amp with nothing else running.

As an electrical engineer my advice would be not to do it. things heat up when run to full capacity which is 24 amps for a 30 amp service. Get a loose connection and you will be calling your insurance company to report a total loss by fire.

Best practice is how you use it. Once you get it cool do not turn it off. Its best to get it cranking early in the morning to pull the coach down then reduce it back to a comfortable level and leave it be. Do not turn it off during the day when you are out and about. Once the temperature rises you have to first do whats called latent cooling which is moisture removal before the tempt even starts to drop. Then you have to cool everything in the coach back down, furniture, carpets, clothing, bedding etc which takes a long time. If at all possible have a shaded campsite.  People make the same mistakes with residential cooling by turning it off during the day. It actually costs more to turn it off! If you leave it be the AC will cycle on and off to maintain and the amount of running time doing that is pretty much, with very few exceptions, less time than if you trun it on when its hot at 5pm and its still running full bore at midnight and not cool yet. Keep window shades closed and chain your rugrats to the bed post or a tree outside to keep the revolving door action to a minimum. I run one AC in a 38 ft 5er on a shaded campsite and using good practices its always comfortable in the coach even when its 95 on the exterior.

I kind of get a chuckle for A/C threads. Where i live in Idaho I see 100-105*F every summer right here at my stick and bricks. Do I have A/C here? No. As for my RV I do have A/C on it and have used it a few time when I had full hook ups. What I found out is JustinThyme is right you need to get the A/C on early and leave it ON. It will maintain the RV temp rather well. My last time using mine in Southern Idaho with temps over 100*F with a single A/C it was a nice cool 73-75*F inside my 31 foot Jayco with a single unit. As for me typically as summer heat comes on I typically go up in altitude and the weather get cooler. As the weather gets cooler I go lower in altitude. But as for my stick and brick I'm working from solar/hydro power on a 4,000w inverter (30A service) so this what makes me chuckle is I've got a full 2 story house without A/C and with 30A service and summer heat up to 100-105*F... Hmmm... Just depends on how you deal with the summer heat. 

Gary RV_Wizard said:

You can make a manual transfer switch for the rear a/c using a basic double pole, double throw switch. Just switch rear a/c from the genset to the new secondary shore power line when needed.

Your best bet is to use the 50A source and feed the 30A off one side and the 15A secondary off the other. That way you are balancing the load somewhat. In many power pedestals, both the 30A and the 15 A outlets are pulled off the same side (leg) of the 50A supply. That probably means you would make your own Y adapter, with a 50A male and 30A & 15A females on the legs of the Y.

Of course, the campground is gonna charge you for 50A service if you do that, whereas you might escape notice using the just 30 & the 15 outlets.

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Sorry for raising this old topic but I'm finally getting around to doing this on my 1999 Itasca.

So  do  I add the extra extension cord to the second line for the genset or the second line for the original power cord? I assume it's the genset since I'm pretty sure this is the feed that's specifically for the second a/c.

I'm trying to attach a picture. I temporarily pulled out the L2 line from the generator. This photo is from the junction box where the power cord comes into the camper, right next to the generator.

If you are trying to use both the 20 and 30 amp park outlets

USUALLY one of hte A/C's connects more or less directly to the 20 amp breaker on yoru Genset. You have to identify that line  Either at the TRANSFER switch or at the Generator, and this is where you "Break" teh circuit by installing an outlet an and olug.. the plug of course goes to the A/C and the outlet to whatever is "HOT when you are plugged in.  Make sure you get the right leads.

Then it's easy from there on  Plug in use a 12 ga cord though none of the wimpy stuff.

John From Detroit said:

If you are trying to use both the 20 and 30 amp park outlets

USUALLY one of hte A/C's connects more or less directly to the 20 amp breaker on yoru Genset. You have to identify that line  Either at the TRANSFER switch or at the Generator, and this is where you "Break" teh circuit by installing an outlet an and olug.. the plug of course goes to the A/C and the outlet to whatever is "HOT when you are plugged in.  Make sure you get the right leads.

Then it's easy from there on  Plug in use a 12 ga cord though none of the wimpy stuff.

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I appreciate your response.  I'm looking at the transfer switch, the photo I attached is a portion of it. I'm trying to make sure I get the right lead.  Not 100% sure if I should choose the genset lead or the power cord lead. I believe it's the genset lead in the photo.

I believe the L1 line from the genset runs the entire coach except for the second AC. I believe the L2 line runs the second AC. Actually I'm just going to disconnect the L2, put a wire nut on the end of the wire, fire up the AC and find out.

OK I have answered my own question through trial and error, both of the L2 inputs in the photo do nothing but energize the second AC. I'm going to leave the genset input as is and add my extension cord input on the power cord L2.

Gary RV_Wizard said:

You can make a manual transfer switch for the rear a/c using a basic double pole, double throw switch. Just switch rear a/c from the genset to the new secondary shore power line when needed.

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Gary could you please direct me to some more information on this use of the dpdt switch?  I have one, not exactly sure how to wire it.  Would like to have it as "original configuration" when the switch it thrown one way and have it go to the extension cord socket when thrown the other way.

So it's two different inputs and one output, basically.

Not sure how to do the one output without running two separate wires from the switch to the AC, maybe it requires two separate wires, not sure.

Thanks for all the help.

Curt in KY

I can do it. but it is easier than that and I'll do that too

You can get a double pole double on/off/on switch at an electrical supply. I recommend a 30 amp transfer switch (they are expensive 50-100 dollars) unless they have a 20 (Should be cheaper) make sure you get the proper size box and faceplate

Now. The switch has SIX terminals.. You find the wires that go from the breaker to the Air Conditioner and remove them from teh breakr box. Connect teh black and white to the CENTER terminals on teh switch.

A new 12ga wire from teh breaker box to the BOTTOM (or top. really does not matter) terminals.. The Neutral wires connect together on this these wires

Run new 12ga to an INLET, with a circuit breaker (you can get that at the same electrical supply) I suggest a 20 amp inlet but a common 15 will do. This connects to the remaining terminals.  I'd leave the ground not connected on this one. but you can connect it with the others too.

Use 12ga extension  cord to the park box.


HOW I DID IT

I added not a transfer switch but a junction box.. I ran 12ga wire from the existing wire to an outside compartment, wher I had both a socket and a plug.. The plug goes to the lead up to the roof, the socket to the breaker.. I plug into the socket on 50 amp sited or the afore mentioned 12ga Extension cord on 30 amp sites... Like the one I'm on now.

I knew I was making it hard on myself with the DPDT switch. I had this elaborate plan to use a 10 gauge RV extension cord, also. I guess I never quite believed that I could run an AC unit off a 12 gauge wire.

However, now I am convinced.

Will do the same thing you did: the pigtail with a plug goes to the AC unit.  Plugging into the socket is basically the same as the original configuration.

Plugging into the extension cord gives a separate power supply to the second AC.

Of course being paranoid I will use heavy gauge components.

As an aside, if I plug my regular 30 amp RV extension cord into a 50 amp adapter, then plug into a 50 amp service both air conditioners will run.

But after a while the thing melts as below!! So I'm not doing that anymore.

But I will use the splitter that splits the 50 amp into two legs as in the second photo. One leg going to the regular rv extension cord, the other going to the second AC.

Mosagra said:

But I will use the splitter that splits the 50 amp into two legs as in the second photo. One leg going to the regular rv extension cord, the other going to the second AC.

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Then you have a potential problem as there's nothing between the 50 amp breaker on the pole and the air conditioner on the roof to limit the current to a safe value if the A/C has a problem.  You'll be able to draw up to 50 amps before the pedestal breaker trips if the a/c develops a short, through wiring rated to handle 20 amps.  That's sure way to start a fire.

The original idea of using a 20 amp rated extension cord and plugged into the 20 amp outlet on the park box means the park's breaker will blow at anything above 20 amps, keeping the wiring safe.  You're missing this protection if the only breaker in the circuit is the 50 amp pedestal breaker.

Lou Schneider said:

Then you have a potential problem as there's nothing between the 50 amp breaker on the pole and the air conditioner on the roof to limit the current to a safe value if the A/C has a problem.  You'll be able to draw up to 50 amps before the pedestal breaker trips if the a/c develops a short, through wiring rated to handle 20 amps.  That's sure way to start a fire.

The original idea of using a 20 amp rated extension cord and plugged into the 20 amp outlet on the park box means the park's breaker will blow at anything above 20 amps, keeping the wiring safe.  You're missing this protection if the only breaker in the circuit is the 50 amp pedestal breaker.

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OK, I understand, will follow your advice. I was just thinking that bigger wires are safer than little wires.

I believe the circuit is protected by the existing 20 amp breaker on the main panel in the RV.

The hot wire from the transfer switch goes directly to the 20 amp breaker before going on to the AC unit.

Our 30A fiver was woefully under cooled with a single 13,500 BTU A/C, and it came to the point where the wife would not go to places in the southwest in the summer. Unfortunately we were a bit ignorant of how much cooling power it takes to cool a high ceiling fiver.


I'm not much of an electrician, and did quite a bit of research before coming up with this relatively simple way to add a portable heat pump to our rig for additional cooling, and has the added benefit of being to use park power for heating instead of consuming propane.


I posted the story on this forum some time ago, complete with pics if anyone is interested. http://www.rvforum.net/SMF_forum/index.php/topic,107061.msg963489.html#msg963489


http://www.rvforum.net/SMF_forum/index.php/topic,107061.msg963489.html#msg963489

Lou Schneider said:

Then you have a potential problem as there's nothing between the 50 amp breaker on the pole and the air conditioner on the roof to limit the current to a safe value if the A/C has a problem.  You'll be able to draw up to 50 amps before the pedestal breaker trips if the a/c develops a short, through wiring rated to handle 20 amps.  That's sure way to start a fire.

The original idea of using a 20 amp rated extension cord and plugged into the 20 amp outlet on the park box means the park's breaker will blow at anything above 20 amps, keeping the wiring safe.  You're missing this protection if the only breaker in the circuit is the 50 amp pedestal breaker.

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OK, so just to be clear you all use regular 12 gauge extension cords.  The blades on the male plug are parallel, like most plugs (These are rated for 15 amp). The plugs rated  for 20 amps have the blades at 90 degrees to each other.