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Old 08-16-2012, 07:02 PM   #1
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All three A/C units on one one phase (QBH Phaeton)

I've been running the generator to power all three rooftop a/c units over the last several days. I don't have shore power where it's currently parked, and I've been working on the rig intermittently for a few hours each day.

Today, after getting a late start, the coach was already 98F when I cranked the genny and started the air. About 30 min later, the breaker on the genny tripped. I went out and reset it, and maybe 10 min later it did it again.

Since it hadn't done it on any day prior today, I thought perhaps the breaker was getting old and tired. I broke out my ammeter and checked the 50 amp main breaker on the circuit panel in the coach (closet). One leg was pulling 44 amps, the other 20. After looking closely at the breaker lineup, it is clear ALL THREE A/C UNITS WERE ON THE SAME LEG!

That certainly seemed odd to me. So, I popped A/C#3 off of the A phase, and moved it the B bus (moved the breaker over one slot).

Now one leg reads 38A and the other 28A. The main breaker on the genny is rated for 44A per phase. All three rooftop airs stayed running for the rest of the day.

I don't know what other detrimental affects having such an unbalanced load may have on other components in the rig (i.e. generator windings, relay contacts, breakers, contactor contacts, etc.), but wonder if this was just a one-off mistake, or if other rigs were wired this way.

I guess it's possible that the previous (first) owner may have been in there, but the wiring in the panel looked pristine and undisturbed.

Just thought I'd give a heads up to anyone else who wants to check theirs.
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Old 08-17-2012, 05:47 AM   #2
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Good thing you found that it was right up to the limit and over causing the trip most likely when the unit cycled those numbers sound better could been heating the main wire up a bit too
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Old 08-17-2012, 07:54 AM   #3
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IT IS TRUE THAT ALL 3 OF YOUR A/C UNITS MAY HAVE BEEN ON THE SAME PHASE, HOWEVER YOU MUST TURN ON EVERYTHING FOR A TRUE LOAD BALLANCE OF THE SYSTEM. YOU MAY FIND IN THE FUTURE THAT IF YOUR A/C IS ON AND YOU RUN THE MICROWAVE, YOU WILL AGAIN TRIP THE MAIN GENNY BREAKER.

BREAKERS ARE RATED TO TRIP AT 80% OF THE RATING OF THE BREAKER. SO THE 50 AMP BREAKER YOU HAVE ON YOUR GENERATOR SHOULD TRIP AT 40 AMPS IF IT IS WORKING ACCURATELY.

IT IS ALSO POSSIBLE THAT YOU MAY BE ABLE TO CALCULATE THE PEAK CAPACITY OF THE GENERQATOR AND THE MAX LOAD OF THE UNIT AND CHANGE THE MAIN BREAKEWR TO A 60 AMP. THIS WOULD ALLOW 48 AMPS PEAK BEFORE TRIPPING. i WOULD HOWEVER CHECK WITH THE MANUFACTURER BEFORE THIS CHANGE DUE TO OVERLOAD OR WARRANTY ISSUES. THE #6 WIRE WHICH IS RAN IS RATED FOR 60 AMP MAIN BREAKER... ITS JUST THE PEAK CAPACITY OF THE GENERATOR WHICH WILL BE THE STOPPER.

I HAVE A 35 AMP 2 POLE MAIN BREAKER ON THE GENNY AND I HAVE BLOWN IT ONCE AS I GUESS IT IS NOT CAPABLE OF RUNNING 2 A/C AND THE FRIDGE WHICH WAS ON "AUTO".

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Old 08-17-2012, 09:44 PM   #4
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KEVEN,

Thanks for the input. I had turned every breaker off, except for the individual A/C units. Then, as I added each one, I watched the current on the first leg (phase A) ramp up to 48A. The magnetic/thermal breaker will allow slight over current, but as it heats up, it'll trip. I'm sure I'm not telling you anything you don't already know. But, it's why these types of breakers tend to trip faster with each reset (they're already heated up).

So, to see a 44A breaker (which is the rated capacity of the breaker on teh generator itself) hold a 48A load for 30 minutes doesn't surprise me - especially since it was only one leg of the breaker that marginally exceeded the rating.

However, what is disconcerting is that the load of all three A/C unit (on their own) push the current above the breaker rating. No matter how you slice it, that's too much for the breaker. Adding in additional load from the inverter/charger, reefer, etc will only compound that problem. Taking one unit off phase A and putting it on phase B - which, with everything turned on had only a 20A load, keeps both phases under 40A.

I would call RB, but I've called them so many times this week already, I'm afraid they'll stop answering the phone
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Old 08-20-2012, 01:51 PM   #5
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BREAKERS ARE RATED TO TRIP AT 80% OF THE RATING OF THE BREAKER. SO THE 50 AMP BREAKER YOU HAVE ON YOUR GENERATOR SHOULD TRIP AT 40 AMPS IF IT IS WORKING ACCURATELY.


KEVEN ( Master Electrician)[/QUOTE]

Hmm...the information I read says a curcuit should only be loaded to 80% of rated capacity but it is designed to carry the rated load of the breaker for 3 hours plus. If a 15 amp breaker was designed to release at 80% or 12 amps I would never be able to make toast with our 12.5 amp toaster or boil water with our 12.5 amp kettle.

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Old 08-20-2012, 03:52 PM   #6
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At last check a 50 amp breaker trips when the load applied to it exceeds 50 amps...

Quote:
Originally Posted by 72driver View Post
BREAKERS ARE RATED TO TRIP AT 80% OF THE RATING OF THE BREAKER. SO THE 50 AMP BREAKER YOU HAVE ON YOUR GENERATOR SHOULD TRIP AT 40 AMPS IF IT IS WORKING ACCURATELY.


KEVEN ( Master Electrician)
Hmm...the information I read says a curcuit should only be loaded to 80% of rated capacity but it is designed to carry the rated load of the breaker for 3 hours plus. If a 15 amp breaker was designed to release at 80% or 12 amps I would never be able to make toast with our 12.5 amp toaster or boil water with our 12.5 amp kettle.

Don[/QUOTE]
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Old 08-20-2012, 04:10 PM   #7
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Couple of questions? Do you have a 240v coach (never seen one)..most are single phase 110v. ( you speak of phases in your post). What is your genny size? From what you say you have a 50amp breaker at the generator, if single phase, that would indicate a 5500 watt generator.

My point here is with 3 A/C's your power system is way under rated. Did you install the 3rd unit or did it come from factory this way? I would not think the factory would install 3 A/C's with your current power setup.
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Old 08-20-2012, 04:59 PM   #8
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All modern 50 amp coaches are powered by a 220VAC two phase circuit. If done correctly, each phase supplies power to half the coach. (Thus the OP's original problem of an unbalanced load.) Most do not use the 220VAC potential between the two phases. (I won't say none as I'm no expert by any means.) The coaches have 220 VAC available, but use only each phase to neutral which provides for two independent 50 amp circuits for the coach.

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Couple of questions? Do you have a 240v coach (never seen one)..most are single phase 110v. ( you speak of phases in your post). What is your genny size? From what you say you have a 50amp breaker at the generator, if single phase, that would indicate a 5500 watt generator.

My point here is with 3 A/C's your power system is way under rated. Did you install the 3rd unit or did it come from factory this way? I would not think the factory would install 3 A/C's with your current power setup.
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Old 08-20-2012, 05:11 PM   #9
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Quote:
Originally Posted by awwall View Post
Couple of questions? Do you have a 240v coach (never seen one)..most are single phase 110v. ( you speak of phases in your post). What is your genny size? From what you say you have a 50amp breaker at the generator, if single phase, that would indicate a 5500 watt generator.

My point here is with 3 A/C's your power system is way under rated. Did you install the 3rd unit or did it come from factory this way? I would not think the factory would install 3 A/C's with your current power setup.
There are a few, think Marathon, Prevost, Newell, etc that have 240 volt stove top or dryer.
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Old 08-20-2012, 05:15 PM   #10
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I figured there were some all electric rigs that might be using the 2 phase power....

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There are a few, think Marathon, Prevost, Newell, etc that have 240 volt stove top or dryer.
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Old 08-20-2012, 06:33 PM   #11
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The OP did not say what he had and by sheer percentage of 110v units, I assumed that is what he had. If I am correct, my statement that he is under powered still stands.
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Old 08-20-2012, 06:47 PM   #12
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I believe the op's info indicates a 42' Phaeton which I would think should have a 7.5 quiet diesel at minimum. I would think this should easily run (3) roof ac's if wired correctly.
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Old 08-21-2012, 07:41 PM   #13
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At last check a 50 amp breaker trips when the load applied to it exceeds 50 amps...



Hmm...the information I read says a curcuit should only be loaded to 80% of rated capacity but it is designed to carry the rated load of the breaker for 3 hours plus. If a 15 amp breaker was designed to release at 80% or 12 amps I would never be able to make toast with our 12.5 amp toaster or boil water with our 12.5 amp kettle.

Don
[/QUOTE]

My statement above is accurate!.. or 4 years of college and 35 years of experience are all wasted ...haha.

and one last comment on the 1 phase or 2 phase bit:

50 amps 120/240 volts is exactly that: 2 pole 50 amp breaker, 2 lines (A & B) at 120 volts per phase to neutral(A to N or B to N) and 240 volts to each other(A to B). if the neutral was not carrying only the "unbalanced load" but was carrying 2 x 40 amps ( 80% of breaker rating), the neutral could in fact be carrying upwards of 80 amps +. all of your coaches would have burned down by now! ( math here is 40 amps on line A and another 40 amps same line A sharing the same neutral neutrral would see 80 amps... (THIS IS JUST NOT THE CASE)

the neutral of the 120/240 volt 50 amp system only carries the unbalanced portion of the current and so could never carry more than 40 amps at any given moment in time. when you plug your coach into a 30 amp 120 volt 1 phase receptacle, both lines are equally fed from the single phase of 120 volts this is true, but as you can now see, the neutral would never see over 24 amps (using the 80% rule for the breaker)as this is simply 1 line on a single pole 30 amp fuse being fed to both sides of the panel.

clear as mud now?

Keven Lefebvre (Master Electrician)
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Old 08-21-2012, 08:23 PM   #14
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I am curious where you get your information in your 35 years...here is a sample trip chart from Allen Bradley. As you can see, below 40 deg C they actually trip at a point higher than nameplate data. Where can I find a chart or data that breakers trip at 80% of their nameplate data?

My statement above is accurate!.. or 4 years of college and 35 years of experience are all wasted ...haha.

and one last comment on the 1 phase or 2 phase bit:

50 amps 120/240 volts is exactly that: 2 pole 50 amp breaker, 2 lines (A & B) at 120 volts per phase to neutral(A to N or B to N) and 240 volts to each other(A to B). if the neutral was not carrying only the "unbalanced load" but was carrying 2 x 40 amps ( 80% of breaker rating), the neutral could in fact be carrying upwards of 80 amps +. all of your coaches would have burned down by now! ( math here is 40 amps on line A and another 40 amps same line A sharing the same neutral neutrral would see 80 amps... (THIS IS JUST NOT THE CASE)

the neutral of the 120/240 volt 50 amp system only carries the unbalanced portion of the current and so could never carry more than 40 amps at any given moment in time. when you plug your coach into a 30 amp 120 volt 1 phase receptacle, both lines are equally fed from the single phase of 120 volts this is true, but as you can now see, the neutral would never see over 24 amps (using the 80% rule for the breaker)as this is simply 1 line on a single pole 30 amp fuse being fed to both sides of the panel.

clear as mud now?

Keven Lefebvre (Master Electrician)[/QUOTE]
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