Air Conditioner Min Circ. Calculation Question

[KWH]

The minimum circuit ampacity has the 1.25 already figured in from the factory correct. The customer is wanting to add several units which will require a sub-panel and loads from the mca's are around 180-185 amps total. I can go with a 200amp panel, this panel will be dedicated for the new units.

 

[Dennis Alwon]

Yes the mca on an a/c unit has the 125% figured in

 

[Dennis Alwon]

You don't have to figure 125% for the feeder so the a/c loads get calculated at the FLA.

 

[kwired]

You don't have to figure 125% for the feeder so the a/c loads get calculated at the FLA.

Fairly certain you need to include 125% of the largest motor plus 100% of others in the feeder calculation.

 

[augie47]

It does not make sense to me and I may be mis-reading, but it appears (to me) that 440.33 would require you to add 25% to the largest MCA

 

[david luchini]

It does not make sense to me and I may be mis-reading, but it appears (to me) that 440.33 would require you to add 25% to the largest MCA

I think you are misreading it. 440.33 requires conductor ampacity to be not less than the sum of the motor-compressor rated load currents plus the other motor full load currents, plus 25% of the largest motor or motor-compressor (in other words, it tells you the MCA.)

 

[don_resqcapt19]

David,
It appears to me that you need to add an additional 25% to the largest compressor unit. It requires the sum of the largest of the rated load current or the branch circuit selection current, which will be shown on the nameplate as the MCA, plus 25% of the largest unit.

 

[augie47]

I think you are misreading it. 440.33 requires conductor ampacity to be not less than the sum of the motor-compressor rated load currents plus the other motor full load currents, plus 25% of the largest motor or motor-compressor (in other words, it tells you the MCA.)

IMO, that's the way I should be, but as don notes above:

440.33 Motor-Compressor(s) With or Without Additional Motor Loads.
Conductors supplying one or more motor-compressor(s) with or without an additional load(s) shall have an ampacity not less than the sum of the rated-load or branch-circuit selection current ratings, whichever is larger, of all the motor-compressors plus the full-load currents of the other motors, plus 25 percent of the highest motor or motor-compressor rating in the group.

 

[david luchini]

David,
It appears to me that you need to add an additional 25% to the largest compressor unit. It requires the sum of the largest of the rated load current or the branch circuit selection current, which will be shown on the nameplate as the MCA, plus 25% of the largest unit.

This is not correct. The "branch circuit selection current" is not the MCA.

The MCA is determined in 440.32 for a Single Motor-Compressor, and in 440.33 for Motor-Compressor(s) With or Without Additional Motor Loads.

For example, 440.32 says

Branch-circuit conductors supplying a single motor-compressor shall of an ampacity (i.e., minimum circuit ampacity) not less than 125 percent of either the motor-compressor rated load current or the branch-circuit selection current...

That is to say, MCA= 1.25 * Branch-Circuit Selection Current (not MCA = Branch-Circuit Selection Current)

 

[kwired]

I think you are misreading it. 440.33 requires conductor ampacity to be not less than the sum of the motor-compressor rated load currents plus the other motor full load currents, plus 25% of the largest motor or motor-compressor (in other words, it tells you the MCA.)

David,
It appears to me that you need to add an additional 25% to the largest compressor unit. It requires the sum of the largest of the rated load current or the branch circuit selection current, which will be shown on the nameplate as the MCA, plus 25% of the largest unit.

Aren't you both disagreeing, yet each of you is saying the same thing in different words?:?

Besides the question is about sizing a feeder with multiple air conditioners, not an individual branch circuit.

 

[curt swartz]

I have always added 25% to the largest motor then added the full load current of all the motors. An easy way to do this is take the MCA of the largest A/C unit then add the individual motors of the remaining units.

 

[JoeStillman]

Article 440.33 is for sizing multi-motor branch circuit conductors. It doesn't apply to the panel feeder. For that, you use 430.24 as referenced by 220.50.

If you take the sum of the manufacturer's MCA's for all the air conditioners, you will get a larger feeder size than necessary. The minimum feeder size is the largest MCA plus the sum of all the other FLA's and RLA's.