Dear Friends,
I have been thinking about the current load of two 13,500 BTU rooftop AC units running full blast on my genny. @2,900 watts each, and 115 volts, I get 25.2 amps per unit. (For newbies: Volts X amps = watts)
I know the main energy consumer in the A/C is the compressor. How much current does a typical rooftop A/C fan use? What if I separate the fans from the main circuit and send them to the inverter?
Though I do plan on separate house batteries, the bus's factory A/C system's two squierrel-cage blowers ran off two 51-amp 12-VDC motors, and I think my alternator would handle the load, through a decent battery isolator.
And yes: We are also working hard to insulate the ceiling as best we can, to help reduce heat load.
I think your way off on your wattage. My 13,500 units only take around 12 amp each when running. Startup would be higher, but not running. I would think your 2,900 watts would be a close guess for BOTH units combined. My genny is wired for 240, so I run one on each leg. (I never have had to run more than 2, accept for a quick cooldown when coming into a hot bus.)
Quote from: Mex-Busnut on August 04, 2011, 09:43:49 PM
... two 13,500 BTU rooftop AC units running full blast on my genny. @2,900 watts each, and 115 volts, I get 25.2 amps per unit. ...
That's the start-up wattage, when the compressor is drawing LRA. Running, these units draw between 1,500-1,800 watts, depending on heat load and outside temperature.
-Sean
http://OurOdyssey.BlogSpot.com (http://ourodyssey.blogspot.com)
everything is going to have to be perfect for you to run both units off that generator,a 6.5 Onan RV type generator struggles with 2 - 13,500 units and will throw the breakers that is why they went to the 7000w Marquis unit for the RV market wish you well and the weather cools down for you
good luck
O.K., most honorable mister busnuts Thomas and Sean! Good catch!
So yes, my mistake: That would be initial startup wattage, not running wattage. My genny is 6,500 peak, 5,500 average. So if I start one at a time, I should be O. K. Plus I am planning on the fridge being on the 3 KW inverter.
Mine has a 240-volt, 22.9-amp outlet also. Just hook one A/C up to each side?
How are you switching over to campground connections?
The problem would be if runniing one AC, you would have completely unbalanced loads in the Gen and it would run hot, likely shorten its life.
I have never looked, but can any of these roof units be configured for 240 volts? I know most of the fans are straight 120 units, which might be able to be replaced with something that burns 240, but what about the compressors??
The stick and staples I took my AC units from had an electronic "load management?" device.
It's purpose was to run 2 roof airs on 30 amps. It would only allow 1 AC to start at a time. Compressors are starting and stopping when AC is running and will trip a 30A breaker.
I did not use it and know nothing about them other than they exist. Maybe someone else will chime in?
HTH JIm
Quote from: Mex-Busnut on August 04, 2011, 10:16:57 PM
... My genny is 6,500 peak, 5,500 average. So if I start one at a time, I should be o.k.
Yes. Notwithstanding Clifford's experience, the 5kW Onan that was on my S&S rig ran two rooftops with no issues in all kinds of weather. A 5.5k unit should have no trouble, assuming it is in good condition and has no problems.
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Mine has a 240-volt, 22.9-amp outlet also. Just hook one A/C up to each side?
As I have written here before, for generators up to 6k I strongly recommend re-strapping to straight 120. In your case that would be 50 amps and you'd need to re-wire everything from the genny to the main panel with #6 AWG. This will balance the load on the stator windings, as well as give you more headroom for starting the units as well as any other loads.
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How are you swithing over to campground connections?
Again, this has been thoroughly discussed elsewhere on the board, so you may want to hunt around if you need more detail. You can use a manual or automatic transfer switch, or you can just wire an appropriate receptacle to the generator and plug your shore cord into it. For this size genny, you need a 50-amp shore service to use this latter method.
-Sean
http://OurOdyssey.BlogSpot.com (http://ourodyssey.blogspot.com)
Sean, I have a 6.5 onan here in 85 coachman my wife bought new that would never run 2 very long with out throwing the breaker and with it being wired 30amp only could run 1 off a pole.
I done everything to make it work never could lol
What brand was your S&S I only ever saw 1 Rv with 2 roof airs with a 5.5 but the roof tops were 9,000 btu which would not cool her RV and I changed those to 13,000 and the generator to 7,000 kw per the manufacture they paid for most of it was my in daughters Allegro
good luck
Steve: therory here is great and in a perfect world will work. All connections perfect. No power loss from connections etc. keep install clean. insulation is key.
Quote from: luvrbus on August 05, 2011, 07:32:16 AM
Sean, I have a 6.5 onan here in 85 coachman my wife bought new that would never run 2 very long with out throwing the breaker and with it being wired 30amp only could run 1 off a pole.
I done everything to make it work never could lol
What brand was your S&S I only ever saw 1 Rv with 2 roof airs with a 5.5 but the roof tops were 9,000 btu which would not cool her RV and I changed those to 13,000 and the generator to 7,000 kw per the manufacture they paid for most of it was my in daughters Allegro
Clifford, this was on a '95 Fleetwood Flair. It came with two 13,500 Colemans and an Onan 5.0, Emerald I think. It, too, was a 30-amp rig, so you could only run one air on shore power. It had a switch to choose which one. But when the genny was running you could run both airs together, and I never had a problem with it, even in the desert. Of course, every A/C and every genny is different, so YMMV.
Technically, a 5.5 with 6.5 surge capability should have no trouble starting and running two 13.5's -- at sea level, anyway. You do need to derate these engines at altitude.
-Sean
http://OurOdyssey.BlogSpot.com (http://ourodyssey.blogspot.com)