Running 2ac's on 30amp

[Bob Denman]

Originally Posted by cavie

PED, Autoformer, EMS.RV.

got it; Thanks!

[Chance]

Originally Posted by Beau388

The Hughes auto transformer is made to plug in directly to the pedestal. It comes with a security cable. Most used an extension cable and permanently mount it in a basement compartment and hard wire it to the transfer switch.

Watts is equal to amps times volts. The cut-over point for the extra coils in the transformer is 113 volts. Less say your A/C is drawing 1500 watts @ 120 volts (12.5 amps) and the campground voltage drops to 110 volts so the A/C needs 13.6 amps to keep running. Heat in the motor varies logarithmically with amperage. The transformer kicks in and the voltage to the coach increases by 10 volts back to 120 volts and the A/C now draws 12.5 amps but primary stage of the auto-transformer is still drawing 13.6 amps @ 110 volts from the pedestal.

How low is too low a voltage that will cause actual damage to air conditioners, etc.? Is there agreement or is it one of those things that if voltage is less than 115V it must be corrected?

[cavie]

Originally Posted by Chance

How low is too low a voltage that will cause actual damage to air conditioners, etc.? Is there agreement or is it one of those things that if voltage is less than 115V it must be corrected?

109 volts

[Pete'sMH]

Originally Posted by Bob Denman

Now it sounds as if I'd be replacing the current onboard system...

I'd consider that.

It works great and I’m glad I have it onboard. Be advised that there is some controversy around Autoformers. See post 36. Using the device will draw an amp or two more from the pedestal as the increased voltage isn’t magic. Some folks think it’s “stealing” more power from other sites or overtaxing the parks system. But from the campgrounds perspective it’s no different from switching your fridge from gas to electric or adding an accessory that draws an amp or two. You can’t draw more power than the pedestal can provide or more than you paid for or the pedestal breaker will trip.

I’m no expert but I believe ACs and other devices start to struggle if the voltage drops below about 107 or so and they work best between 115 and 125.

[Chance]

The reason I asked is that some or many air conditioners are rated under severe conditions expected in the desert, and at least for Coleman A/Cs, the severe cooling is combined with low voltage of 103.5 Volts. That happens to be 90% of 115 Volts, their standard rating.

No doubt lower voltage will cause higher current, but so do other conditions. The question for me is whether high temperature will occur often enough and at an elevated level that will shorten the A/Cs life significantly?

I’ve never used one, and also never replaced an A/C, so I’m not sure if the added “insurance” premium is worth it.

[Bob Denman]

Pete,
Just one more device to include in our power management exercises...
Thanks!

[Beau388]

Originally Posted by Chance

The reason I asked is that some or many air conditioners are rated under severe conditions expected in the desert, and at least for Coleman A/Cs, the severe cooling is combined with low voltage of 103.5 Volts. That happens to be 90% of 115 Volts, their standard rating.

No doubt lower voltage will cause higher current, but so do other conditions. The question for me is whether high temperature will occur often enough and at an elevated level that will shorten the A/Cs life significantly?

I’ve never used one, and also never replaced an A/C, so I’m not sure if the added “insurance” premium is worth it.

My Progressive Industries EMS shuts down at 103 volts. It will come back on if the voltage rises to 104 for 40 seconds.
As for a motor's temperature, it is power is equal to amperage squared times resistance. In electric motors the resistance is not fixed but varies with temperature because copper wire more resistant the hotter it gets until it melts. The compressor is usually oil cooled, sealed unit with an internal heat cutoff at about 150 degrees. So the 1500 watt A/C is using 12.5 amps@ 120 volts and has a resistance 9.6 ohms. Drop the voltage to 104 volts and the amperage draw is 14.4 amps or 33% more heat. Now the resistance increase and the compressor struggles to maintain pressure causing more heat. What happens next depends on the outside temperature and your A/Cs design parameters.

[Chance]

Originally Posted by Beau388

My Progressive Industries EMS shuts down at 103 volts. It will come back on if the voltage rises to 104 for 40 seconds.
.....cut.....

That is consistent with Coleman-Mach range of 103.5 to 126.5 Volts, which is 115 Volts +/- 10%.

.

[Chance]

The best news is how much air conditioners have improved in efficiency, and that there’s still theoretical room for even more improvement.

A few years ago Coleman advertised how their 13,500 BTUH A/C reduced current from 15.3 to 11.2 Amps, and now they have specs showing 10 Amps. That 10 Amps is 1,075 Watts, which represents an EER of 12.5. There’s definitely room to make that EER much higher and thus reduce electrical requirements below 10 Amps.

Even at 10 Amps, two units for a total of 20 Amps is quite below 30A service.


P.S. — I’m a little skeptical about 10A rating and would want to see/measure myself, but even if close, it’s pretty good.
.

[Bill Johnson]

Originally Posted by Chance

Always 30-Amp at campground pedestals all over the country. We’ve also successfully powered the microwave and A/C simultaneously from Onan 4,000-Watt gasoline generators on various motorhomes. The generator has a much easier time if you start the A/C first, let it run a few minutes, then start using microwave as needed. Start-up current can be a lot of load on generator, but a breaker doesn’t have the same mechanical limitations of a generator.

The key here is to consider that all air conditioners and cooling conditions are not the same and will load a 30-Amp breaker differently. The newest air conditioners can have higher Energy Efficiency Ratio (EER) which means they will use less power and therefore current to deliver the same 13,500 BTU/hour of cooling. Also, air conditioners require more power and current when it's warmer inside and or outside. Cooling in 90 F weather requires much less power and current than in 105 F weather. Likewise, it takes more power when it’s 90 F inside motorhome than when it’s 80 F inside.

I’m not saying it’s not possible or even likely to trip a 30-Amp breaker under right conditions when trying to power “two” A/Cs, but just because some can’t, it doesn’t mean others will have similar problems.

The Devil is in the details. And let’s not forget that various manufacturers are installing dual 11,000 BTU/hr Power Saver air conditioners in mid-size motorhomes with 30-Amp and 4,000-Watt generators. Also, the difference between 11,000 and 13,500 BTU/hr Power Savers is not that great.

I was just thinking an AC unit doesn't know 110 degrees from 60 degrees, the AC unit will pull the same amps period, it will just run longer to satisfy the thermostat setting. It's kind of like when I ran the maintenance for many buildings for a large company people would turn the thermostat down to 60 to make it cool quicker, it doesn't.

[Bill Johnson]

Originally Posted by Chance

The best news is how much air conditioners have improved in efficiency, and that there’s still theoretical room for even more improvement.

A few years ago Coleman advertised how their 13,500 BTUH A/C reduced current from 15.3 to 11.2 Amps, and now they have specs showing 10 Amps. That 10 Amps is 1,075 Watts, which represents an EER of 12.5. There’s definitely room to make that EER much higher and thus reduce electrical requirements below 10 Amps.

Even at 10 Amps, two units for a total of 20 Amps is quite below 30A service.


P.S. — I’m a little skeptical about 10A rating and would want to see/measure myself, but even if close, it’s pretty good.
.

10 amps is running amps not start up amps. Start up amps are close to 24 amps starting

[Bob Denman]

Originally Posted by Bill Johnson

10 amps is running amps not start up amps. Start up amps are close to 24 amps starting

That's when those Soft-Start devices will make a big difference.

[cavie]

Originally Posted by bob denman

that's when those soft-start devices will make a big difference.

[Bill Johnson]

That's right I'm going to put them on my motorhome

[16ACE27]

Originally Posted by Bill Johnson

I was just thinking an AC unit doesn't know 110 degrees from 60 degrees, the AC unit will pull the same amps period, it will just run longer to satisfy the thermostat setting. It's kind of like when I ran the maintenance for many buildings for a large company people would turn the thermostat down to 60 to make it cool quicker, it doesn't.

But you would be wrong. The pressure in the condenser depends on the temperature of the ambient air going through it. The higher the pressure, the harder the compressor has to work.

In fact, the correct high and low side pressures on an A/C unit depends on the ambient temperature as illustrated by the following chart:

[Bob Denman]

But hotter air is less dense... Lower pressure.

[Chance]

Originally Posted by Bill Johnson

I was just thinking an AC unit doesn't know 110 degrees from 60 degrees, the AC unit will pull the same amps period, it will just run longer to satisfy the thermostat setting. It's kind of like when I ran the maintenance for many buildings for a large company people would turn the thermostat down to 60 to make it cool quicker, it doesn't.

Both inside and outside temperatures make a huge difference. See Coleman data below that “approximates” effect outside temperature makes. There’s other data for inside temperature. This isn’t that accurate so don’t take it as an exact correction.

Note also on 13,500 BTU/hour Power Saver data above that standard running Watts is 1,075 but that desert conditions is 1,320 Watts. Assuming same efficiency, power factor, and that voltage is reduced from 115 to 103.5 VAC per desert rating conditions, current would go from 10 Amps to over 13 Amps.
.

[Rcstroll]

Hi! Have 50 amp as well. Was able to use 2 a/c units on it and thought that i was doing great! I was... after 3 days of such use, broke camp and ...wow! The cables were super hot and the 50 to 30 cable burning hot!! Just pray I didn't do any damage. Never again... even if it works....

[Bob Denman]

Glad to know that you didn't start melting the cables.

[Bill Johnson]

Originally Posted by 16ACE27

But you would be wrong. The pressure in the condenser depends on the temperature of the ambient air going through it. The higher the pressure, the harder the compressor has to work.

In fact, the correct high and low side pressures on an A/C unit depends on the ambient temperature as illustrated by the following chart:

Thanks I'll have to put my amp probe on it to see what the difference is when its 80 and when it's 95