AC motor 208 vs 240 volts

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frofro19

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Master Electrician
If an AC compressor is rated for 208/230 volts , what is the calculation to determine how much current it uses with a 208 system as opposed to a 240 volt system?
Example:
208/230 volts
Minimum current amps 25
Maxium ckt breaker 35
RLA 15
 
infinity said:
Is this for a feeder calculation?
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Yes, but just in general. I know with resistance loads you calculate by finding the resistance but wasn't sure when it comes to motors and especially ac compressors.
 
Motors tend to be constant-power loads, so will draw more current on a lower voltage.

Your compressor label seems to cover both voltages with its current numbers.
 
ActionDave said:
If there's not a corresponding amp load for 208V then it's whatever the nameplate says for 208/230V.
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I personally haven't seen any ac units that have a corresponding amp load for 208. Most only say 208/230 volts. My question is if lower voltages use more current, is it possible that what the nameplate is based on is the 208 system and that possibly the 240 volt system uses less current? My last few projects were using a 208 volt system and was wondering how to calculate the actual current on the that system as far as a load calculation goes as compared to a 240 volt system.
 
Most everything for some time now has been made to run on 208/240. It's not that much of a difference in actual amp load. I wouldn't expect to see more than an amp or two different in actual load.

I'm working on some exhaust fans today rated 13A at 208/240. The actual voltage is right around 204 or 205 and the motors are pulling 10.78 to 11A. A couple of the same fans are running 480V, nameplate calls 6.5A and they are running at 5A.
 
It's my guess that the MCA is the worst case scenario meaning the lowest rated voltage and the highest current. If the voltage is 240 the current is just lower.
 
LarryFine said:
I meant within the motor's designed voltage range, of course.
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The range is only 5% either way. Motor voltage is a little under so distribution voltage is 208 or 240 but motor voltage is 200 or 230. That’s a 15% span or 20% if we target minimum utilization to maximum nominal distribution voltage (200-240) and NEMA requires 10% above and below the stated name and plate range so we are really pushing things to stay within NEMA MG-1. Still the chart shows how torque, efficiency, and current all change with voltage. It should be no surprise that with DOL motors it is preferable to run a little higher voltage than a little lower. Torque is proportional to the square of voltage. Second it gives you an idea that name plate ratings have some safety margin. The reality is that motors are designed on a computer these days though. There is no extra safety margin.
 
kwired said:
I think rule of thumb is that a motor driving a specific load will draw 10% higher current on 208 than it will on 240.
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So, would it be safe to say that an AC compressor that had a nameplate of 25 mca would pull 10% less on a 240 volt system as opposed to a 208 system? And for calculation purpose, would you figure 25 amp x 208= 5200w or 25 amps x 240=6000w?
 
frofro19 said:
So, would it be safe to say that an AC compressor that had a nameplate of 25 mca would pull 10% less on a 240 volt system as opposed to a 208 system? And for calculation purpose, would you figure 25 amp x 208= 5200w or 25 amps x 240=6000w?
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Probably so, and remember that if nameplate says 208-240 then that 25MCA is likely worst case loading with 208 input.

Other than in worst case conditions many those compressors don't draw rated amps maybe only 1/2 to 3/4 rated amps most the time. This is a machine that essentially moves heat from evaporator to condenser, the more heat it is moving the more it is loaded. Heat pumps in cold ambient temp aren't working that hard and may only draw 1/4 of nameplate amps or even less than that sometimes.
 
frofro19 said:
So, would it be safe to say that an AC compressor that had a nameplate of 25 mca would pull 10% less on a 240 volt system as opposed to a 208 system? And for calculation purpose, would you figure 25 amp x 208= 5200w or 25 amps x 240=6000w?
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MCA is ALWAYS based on the "worst case scenario" because they are giving you the MINIMUM circuit ampacity requirements, meaning the device can draw LESS than that but will not draw more (on a continuous basis). MCA is based on the rules for the LARGEST motor nameplate FLA x 1.25 + all other loads at FLC. In this case, the LARGEST motor nameplate value would be the comressor motor when fed with 208V, because 208V is LISTED on the name plate. The fact that it will pull less than that if fed with 240V is imaterial, you STILL must size everything per the MCA if that's what they provide to you.
 
frofro19 said:
So, would it be safe to say that an AC compressor that had a nameplate of 25 mca would pull 10% less on a 240 volt system as opposed to a 208 system? And for calculation purpose, would you figure 25 amp x 208= 5200w or 25 amps x 240=6000w?
Click to expand...

As stated previously, there is no need to calculate anything. You simply run a circuit capable of 25A (minimum) and protect it at 35A (maximum) and you’re done!
 
Compressors typically run into the service factor when loaded assuming they will modulate (load and unload). There is your worst case.
 
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