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Author Topic: AC Amps  (Read 605 times)

Marshall212

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AC Amps
« on: May 30, 2018, 07:24:00 AM »
Good Morning,

First, I know nothing about electricity. We are parked in a 30 amp state park site. My coach is a 50 amp with 2 AC units. Here is my question: do AC units pull more amps in reducing the interior temp by 10 degrees than when reducing the interior temp by 5 degrees? We came back to the unit yesterday after visiting a national park fairly early in the morning. In the afternoon when we returned the ambient temp was 86 and the internal coach temp (by thermostat). was 85. I turned on both AC units and about 30 minutes later the pedestal  breaker (30 amp) tripped. I checked the amps used via the magnum remote after I reset the breaker and 1 ac unit was pulling 26 amps (with associated draws from fridge and other parasitic loads). We have run both AC units on 30 amps before but not when it was this warm. Hence my question.

Thanks         
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Ernie n Tara

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Re: AC Amps
« Reply #1 on: May 30, 2018, 07:51:00 AM »
No! There may be slight differences due to high temperatures but basically they draw the same. However, you have been fortunate to run them both on 30A. If only the air is on sometimes that works out until they both try to start at once as they draw much more when starting.

Ernie
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Back2PA

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Re: AC Amps
« Reply #2 on: May 30, 2018, 07:53:26 AM »
30 amps is barely enough to run 2 ACs. It will depend on what other loads you’re running and how old the breaker on the post is as to whether you can do it successfully
Scott
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Re: AC Amps
« Reply #3 on: May 30, 2018, 07:53:39 AM »
In a nutshell, yes.  As the temperature outside goes up during the day, the condenser coil has a harder time dissipating the heat.  Because the refrigerant is now at a higher pressure (pressure/temperature relationship law), that causes the compressor to work harder to continue to pump refrigerant through the system, thus causing an increase in current draw for the compressor
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NY_Dutch

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Re: AC Amps
« Reply #4 on: May 30, 2018, 08:07:47 AM »
Besides the extra current draw, the extra heat would make the two A/C's cycle more frequently, increasing the odds of both compressors starting at once and overloading the breaker. Something else to keep in mind is that most standard breakers don't do well with sustained loads more than 80% of their rating, and breakers that have been frequently tripped will typically begin to trip at lower load levels at some point.
Dutch
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William52

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Re: AC Amps
« Reply #5 on: May 30, 2018, 09:34:14 AM »
Same problem when we pulled into a park in Texas hotter than Hades could not run both A/C's kept sheding off power had to settle for just the front. Some shade would have helped. Looked into the cheater box? IDTS.  30-Amp, 20-Amp Cheater Boxes
Posted by RV Doctor
I was recently given a setup which has a 30-amp plug on one side and a 20-amp plug on the other side. The output is a 50-amp plug. I was told that if you plug in one side to a 30-amp source and the other to a 20-amp source, you will end up with 50-amp. Do these things actually work? No brand name on it, so I assume they are homemade. (Jerry M.)



Jerry, what you have is called a “cheater box” and though some are indeed homemade, there are a couple of suppliers that have them available online (as pictured). I do not recommend cheater boxes for a couple of reasons. First and foremost, they are not “approved” for RV use. Safety is paramount and I would never recommend something not approved for RV use. In most cases, the 20-amp receptacle at a campground will be GFCI-protected (it is a code requirement). Cheater boxes will automatically trip that GFCI breaker and you’ll only be provided power from the 30-amp side.


Also, if the 20-amp receptacle and the 30-amp receptacle are connected to the same electrical phase, the cheater box will not function. To my knowledge, most 20-amp and 30-amp receptacles in the same pedestal at the same site are indeed wired to the same phase. If each receptacle is connected 180-degrees to an opposite phase, it might work. However, if there is an EMS (electrical management system), installed in the coach, or a sophisticated inverter/charger in the system, the cheater box will likely not function, or possibly add confusion to the EMS.

Furthermore, you cannot use a cheater box with surge protection. They simply will not allow the voltage to pass through to the RV. Another reason is the sizing of the conductors. The neutral wire in the cheater box feeding its 50-amp receptacle must have the capacity to carry 50-amps as provided by two, opposite-phased sources of 120-volt hot wires.

And finally, they’re called “cheater boxes” for a reason. If the campground is not aware of you using a cheater box, you’d simply be using more power than what it is expected of you (and charged accordingly by the campground owner), in those rare instances when the cheater box might actually work. Indeed, when all the right circumstances are correct, including permission from the campground owner, they have been known to work. A lot depends on how that particular campground was electrically constructed.

The bottom line is that I consider it risky to use a cheater box. It’s much better to connect a 50-amp coach to a 50-amp receptacle. I’m not even fond of those 30-50 adapter plugs because it eliminates one of the safety factors built into the RV. FYI, the new 2017 electrical code for campgrounds calls for more 50-amp sites in campgrounds, so hopefully the park owners can keep up with the demand. The smart campground operators are already upgrading their parks to accommodate more 50-amp sites since more and more RVs demand the higher ampacity than ever before.

rvt#814



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Gary RV_Wizard

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Re: AC Amps
« Reply #6 on: May 30, 2018, 10:59:22 AM »
First of all, the a/c doesn't know or care how many degrees you want to lower the temp.  It runs when the thermostat calls for cooling, and stops when no more cooling is needed. It runs exactly the same whether you set the thermostat one degree below the room temperature or 20 degrees below it.  So the simple answer is: the amps used are exactly the same.   More amp-hours get used because the a/c runs longer, but at any instant the number of amps flowing through the wire stays the same.

There may be differences in the amp draw when the outside ambient is higher or lower, and there surely is some difference if the supply voltage gets higher or lower (anywhere from about 105v to 132v is acceptable), but you don't need to worry about it because its all within the bounds of "normal".
Gary
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Re: AC Amps
« Reply #7 on: May 30, 2018, 11:14:47 AM »
Gary is correct in his first paragraph.  The A/ C doesn't care where you set the thermostat....1 degree cooler or 10 degrees cooler.....same current drawn by the compressor motor.  My answer was alluding to the last part of the OP's question about running both of them in hotter ambient air temperature areas.....and yes, the current will increase then, and that is the most likely reason the 30 amp breaker tripped; combined with the other loads that were on the circuit.
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John From Detroit

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Re: AC Amps
« Reply #8 on: May 30, 2018, 11:54:09 AM »
First. My A/C's draw the same amps no matter how much they are reducing temp (By measurement) however amps will climb a bit as OUTSIDE temps climb or as dirt builds up on the condenser coils (outside) I have one that needs cleaning now.

Second The "Cheater Box" If you have an OLD campsite.  One that does NOT have a GFCI outlet, it might work and it might also fry the park side wiring (happneed not far from where I'm parked now (As in the other side of the drive).

IF the site has a GFCI. they will not work

I have a "Trick" that does work however

I disconnected one of my A/C's from the RV's power panel and made it into a  (Plug in) the plug (And the outlket that replaced it on the RV's pwoer panel) are outrside the RV in a comparment.

Results.. One of my A/C's is fully independent of the RV, so it does NOT trip the GFCI.
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youracman

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Re: AC Amps
« Reply #9 on: May 30, 2018, 09:27:46 PM »
Gary is correct in his first paragraph.  The A/ C doesn't care where you set the thermostat....1 degree cooler or 10 degrees cooler.....same current drawn by the compressor motor.  My answer was alluding to the last part of the OP's question about running both of them in hotter ambient air temperature areas.....and yes, the current will increase then, and that is the most likely reason the 30 amp breaker tripped; combined with the other loads that were on the circuit.

^^^^^ Yup ^^^^^

FWIW: Just for grins, I  looked up some data on my Coleman Mach 15 on the mfr's site.  They had a data chart that showed the compressor current increase with ambient (Outdoor Dry Bulb) temperatures.  The chart showed that for each 5 degree increase (above their 95 degree reference point) the compressor RLA increased 1 amp (at a constant 115 vac supply.)  Their chart went to 115 F (which they refer to as desert conditions) and the current increase was 4 amps.  That squares pretty well with my data from Arizona last year.  At 95F, my unit draws just over 16 amps with good (115vac or so) power.  At those 110+ AZ temps it was about 20 amps.   I have a load shedding device; apparently it unloads the water heater elec element cuz I only tripped the 30A breaker when I slipped up and tried to use the microwave simultaneously with the A/C.  Of course the oft-observed power droop at RV park pedestals occurs when it is the hottest.

The 1 amp increase for each 5 degrees over 95 ambient is probably typical for rooftop units, I'm thinkin'

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A Traveler

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Re: AC Amps
« Reply #10 on: May 31, 2018, 10:49:14 PM »
Besides the extra current draw, the extra heat would make the two A/C's cycle more frequently, increasing the odds of both compressors starting at once and overloading the breaker...

If the AC units are running constantly in a high heat situation, there is no cycling going on. The compressors are running constantly until the interior temperature gets down to the thermostat setting.

NY_Dutch

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Re: AC Amps
« Reply #11 on: June 01, 2018, 07:10:29 AM »
If the AC units are running constantly in a high heat situation, there is no cycling going on. The compressors are running constantly until the interior temperature gets down to the thermostat setting.

Obviously... At the 86 deg.F ambient temp the OP reported my A/C's certainly cycle. I seem to recall they ran constantly at about 98 deg.F ambient though.
Dutch
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Gary RV_Wizard

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Re: AC Amps
« Reply #12 on: June 01, 2018, 11:25:10 AM »
Thanks for the amp data, youracman.  It's probably a factor above 95 ambient.

The question asked was relative to the thermostat setting, i.e. 5 degrees vs 10 below the interior temperature (not external ambient).  There is no cycling until the thermostat set temperature is reached, so cycling is not a factor here.  Even once the set temperature is reached and the a/c cycles off, the frequency of cycling will depend on the rate of heat gain in the RV, i.e. insulation and shade, more so than anything else. Obviously the heat gain increases with ambient, but whether cycling ceases above 80, 85 or 90 is a matter of the RV construction and sun-loading on the roof and walls.

My last coach had excellent insulation but a gorgeous black & gold paint. If it was in full sun, the a/c units ran almost non-stop at any temperature warm enough to be using the a/c (that's around 84 for us). If shaded, though, it cycled as expected right up to 90 or so.
« Last Edit: June 01, 2018, 11:29:55 AM by Gary RV_Wizard »
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