How Well Does Your AC work?

[Dav5942]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by afrank1971:
I did notice on my 8k Onan that the breaker in the unit is only 35amp, perhaps a larger breaker could be installed. Anyone have any experience with that? </div></BLOCKQUOTE>

You should have two 35A breakers. Ours is wired one AC on one breaker and the other two ACs on the 2nd breaker(done thru the house wiring of course).

[afrank1971]

Ahh, that makes more sense than what I was looking at, I thought the 35 amp breaker didn't make much sense, but two 35's does. Would seem 70 amps would be plenty to run three 13.5m units, but will have to look into that further...

[afrank1971]

For those following this thread, Cummins response to my questioning whether the 8k would run three 13.5's...

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">This unit is recommended to run two A/C units along with the other coach loads. When we go to 3 A/C we move up to a 10 and 12 KW. </div></BLOCKQUOTE>

As I have an 8k and upgrading to a 10 or 12 really isn't a reasonable option, I followed up with if running only the 3 units and nothing else, would it do it, and received the predictable...

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">We have not done any testing on this unit with 3 A/C. But this is not something we would recommend. </div></BLOCKQUOTE>

[afrank1971]

And the plot thickens. Onan says the don't recommend it, Dometic says even a 7kw genset will run three 13.5's just fine.

From Dometic:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">The size of the generator for one 13,500 BTU unit would be 3.5 KW. For 3 units a 7 KW would be needed. </div></BLOCKQUOTE>

[RVDude]

The problem sounds like load balancing between the two legs.

Each AC draws around 15amps (running). Having 3 AC units means 2 are one 35amp leg. If both on that leg were to start at the same time (even just the compressors cycling on/off at the same time) that starting load would be right at the 35amp leg limit even w/o any additional loading on that leg. There should be enough time delay in the 35amp breaker to allow the start, BUT, if there is the slightest additional loading on that leg it'll likely pop the breaker at every near-simultaneous compressor start.

Clearly, an 8kw is right at the limit for 3 ACs due to leg limits. Hence, it's always going to be a balancing act to drive it. That's why Onan says 10kw+ for 3 ACs. Afrank, Dometic is playing the theoretical odds with that last statement. That type of pushing-the-limit philosophy seems to fit with their recent engineering recalls too. The math (all 120VAC) 7kw = 58amps. 2 legs = 29amps/leg. Hmmm. 13-15amp run load per AC times 2 ACs is more than stretching that and we haven't even done a start load. Bad Dometic, very bad Dometic. This is so close to theoretically impossible that the reality of doing it is off the charts. Kinda like trying to electrically restart the Apollo 13 capsule on its flight back to Earth, it can be done, but the procedure to avoid tripping the breaker is darn near impossible.

Check what else is loaded on the leg that has the 2 ACs on it. Try to move those loads to the other leg - or experiment by making sure those loads are off when the ACs are on. Also, that pesky inverter/charger can draw a nasty load when charging and the charger might be on the 2 AC leg. I don't know if they are available, but you might consider a time-delay (slow-blow) 35amp breaker as well.

[afrank1971]

I knew there would be somebody technical enough to lend credence to either Onan or Dometic, thanks RV Dude! Sounds like it could be done, but just barely, and would need to essentially rewire your panel to isolate the two AC's on one leg and put the third and everything else on the other. Perhaps not worth the effort, or the possible future headaches if everything doesn't go exactly as planned. Is there ever any end to the learning?

[Jetsmania]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Clearly, an 8kw is right at the limit for 3 ACs due to leg limits </div></BLOCKQUOTE>
Hi! Great explanation. I do not know in Newmar made the change for '09, but they were talking about putting the 10kw genset in the coaches that had 3 AC units. The smaller gensets had trouble not popping its breaker with 3 units running. Paul

[Jetsmania]

Hi! As a follow up, I just checked the Newmar website. On the Dutch Aire, if you order the 43' with the third AC unit you get the 10kw genset with it. Paul

[Dav5942]

In our case, even though we spend a lot of time dry camping with the 8kw jenny, we would never have ordered our unit without the 3 ACs which when running off 50amp service do not trip any breakers(even if the 3ACs did not work well with the 8kw jenny).

As we said have not had a problem with the 8kw Power Tech jenny running the 3 ACs-but we do have to watch what else is running on which breaker just like we would with 2 ACs(i.e. bat chgr, hair dryers, microwave, etc. etc.). By the way when dry camping we rarely camp in a hot locale where all three ACs are needed-but when not dry camping(sitting here in Florida) it sure is nice to have 3 ACs!

[clnhse]

I have a 08 43 foot Dutchstar with 3 a/c's It has a 10 k Powertec generator. I called Powertec to get filter numbers and the rep told me the only difference between the 10 and 12 is the breaker. He said if I install a 60 amp instead of a 50 it will be a 12k. Think about it. Why would a company build two different units that close in size.....

[footdoc]

Jerry....there is still a problem somewhere if you must do that...I and probably everyone else dont have that particular issue...if you do, now is the time to chime in here....

[Camelot Camper]

I added a third 13.5 Dometic to an 8kW Onan and you can find the electrical results here.

CC