2 rooftop A/C's on 30-amp?

[scottydl]

My neighbor two houses down just brought home a BEAUTIFUL 35-foot bunkhouse 5th wheel the other evening.  We had to go check it out.  The rig has 2 rooftop A/C's, but the one over the master bed was an aftermarket addition installed by the dealer.  My neighbor said the unit was plugged in (30A service), and both of the A/C's were running when they went to pick up the trailer.  I was always under the impression that only 1 A/C could be run on 30A, absent a generator supplying extra power?  Is this a wiring screw-up on the dealer's part, or is there a way to run both roof units on 30?

 

[Gary RV_Wizard]

It's nearly impossible to get two a/c's running at the same time on 30A. Note I said "nearly", though. Some of the newest (and smaller) a/cs are fairly conservative on amps, down around 9-10 amps with the compressor running. Coleman has a 7100 & a 9000 btu model and Dometic an 11,000 btu, each with proportionately less amp draw than their larger brethren. The 7100 btu Coleman Mini Mach draws a mere 7.6 amps at full load, so would make a nice second air for the bedroom in a 30A powered RV.

And with the use of an energy management system, it may appear that both a/cs are running when in fact only one has its compressor engaged. The other, at any given moment, would only be running its fans. The EMS prevents the compressor on cycle from beginning on one until the other has cycled its compressor off, thus keeping the total amp draw under 30. The fans on an a/c use only an amp or two - its the compressor that sucks the big power.

 

[John From Detroit]

I have managed to get both of my AC's to run on 30 amps. However

1: NOTHING else was on
2: Batteries were full up

Also, some AC's take just a little bit less power than others,  in theory 30 amps will support two AC's. JUST, some 30 amp breakers will actually pass a bit more (Up to around 33 amps) and trust me it is that close.

Frankly. I would not count on being able to do it (As I said, I've done it... ONE time never managed to do it again)

 

[PancakeBill]

Like RVRoamer says.  If it has EMS system it will allow both to be running, but not both 'working'.  It will switch out between whichever circuits need power.  One compressor at a time, or the microwave etc.  AAlso, with smaller capacity A/C's it is possible. 

I think a smart mfg might spec out a unit with 2 10k units and keep it under 30 amp draw.  If the second one had been added, I would doubt the primary would have been a 10.  Most likely a 13,500.  Possibly upgraded to a 15,000.

 

[Old Soldier 1]

I have run both our a/c's on 30amp.  I start the living area a/c first, allow it to run for awhile, then turn on the b/r unit.  Never had a problem.  Of course, there's always that campground that may have used lighter gauge wiring, and then it ain't gonna work.

 

[Jim Dick]

We used to own a 96 Bounder that had an Intellitect system that would allow two A/C units to operate at the same time on 30 amps. It would control the start up of the compressor to prevent tripping of the breaker. It would also shed loads as needed if you turned on a microwave or other power hog. My guess is you might have something along this line.

Now my wife has managed to operate both of our A/C units in our current coach without tripping the breaker. My guess is she started one and then the other so both compressors did not come on line at the same time. Somewhere down the line they would try to come on line together and that would trip the breaker. It's the starting current of the compressor that draws the most.

 

[scottydl]

My guess is you might have something along this line.

It's my neighbor's new 5er actually, I *wish* I had that feature.    My coach as a Shore<->Generator selector switch, and will only allow the rear A/C to be operated in Generator mode.  I don't think that switch does anything else, because the coach will receive power from either source (shore power or genset) no matter where the switch is at.  I suppose I could cheat the system and leave it in Gen mode when plugged into shore power, and see if both of my A/C's will run.  I'm guessing I don't have one of those "smart" systems that will keep the breaker from tripping though.

 

[Gary RV_Wizard]

The Dometic Brisk Air High Efficiency model draws only 10A when running, though its "locked rotor" amps are about the same as the regular models. A pair of those leave a fair amount of slack in a 30A system and usually they can cycle on OK as long as both do not try to start at exactly the same time.

Two older 15k units, on the other hand, will consume about 33A simply running with both compressors on. No way one of them could cycle back on again while the other is working - the start up load is simply too great.

"Locked rotor" amps (the maximum start up draw) on this type of a/c unit is typically in the vicinity of 53 amps. That peak lasts for less than a second and is back down near the normal running range (13-16 amps) within  3-4  seconds. An old unit whose compressor is "tired" may take several seconds longer to settle down.

 

[Jim Dick]

Scotty,

I've seen quite a few rigs with the switch you mentioned. I have a feeling if you left it in the Gen position you would not have power to the second A/C unit unless the genset was running.

 

[scottydl]

Hmm, could be.  I'd try it now, but all I have is household current and even if it worked I'd probably burn up my extension cord.