OCPD for multiple AC units

Grouch

Grouch

Senior Member
Location
New York, NY
I went through this thread from awhile back:

Subpanel size for 5 HVAC units

Hello, i want to make sure i got this correct. We are bidding job for hvac and electrical. The hvac will need a subpanel installed. The name plate says min circuit ampacity is 40 amps and the max breaker size is 60 amps. I am thinking we need a 300 amp sub panel 60*5=300 or a 320 amp sub panel...
forums.mikeholt.com

My question is regarding Post #16 (in bold here):

430.62(A): The maximum permitted OCPD for the 24.4A compressor is 60A...plus the sum of the full load currents of the other motors.

Edit: I forgot the other motors in the first unit however:
60+1.7+6.9+4(33)=200.6. A 200A breaker with an MCA of 171.1.

I follow everything except for the value of 60 amps. 430.62(A) states that for compressors, you refer to 440.22(A)... 440.22(A) states not to exceed a max of 225% for single compressors. 24.4 amps, which the RLA for one compressor, x 2.25 = 54.9. Then you round down to a 50 amp breaker, since you can't exceed 54.9 amps.

So shouldn't the math be 50+1.7+6.9+4(33) = 190.6? Hence the breaker size for the feeder serving all the AC units is 175 amps.
 
Grouch said:
So shouldn't the math be 50+1.7+6.9+4(33) = 190.6? Hence the breaker size for the feeder serving all the AC units is 175 amps.
Click to expand...
I agree with your conclusion but not the 190.6A calculation.

For reference, the data sheet for the product in the thread above shows a compressor RLA of 24.4, and additional fan FLAs of 1.7A, 6.9A, and 0.24A. Apparently the data plate is ignoring the 0.24A, as it lists the MCA as 39.1, which equals 125% * 24.4 + 1.7 + 6.9. So we'll ignore the 0.24A as well.

Then the question was the largest OCPD for a feeder supplying 5 of these units, when the conductors are sized to the minimum ampacity (which would be 39.1 + 4 * 33 = 171.1A. Note that 33 is just the sum of 24.4A RLA, 1.7A FLA, and 6.9A FLA). That computation would 2.25*24.4 + 1.7 + 6.9 + 4*33 = 195.5. So if we want to choose a standard size OCPD, 175A would be the largest allowed, as 200A is too large. We could still use a 200A OCPD if we provide conductors of ampacity at least 176A, using 240.4(B) as usual for any circuit.

I think the error in the post quoted in the OP comes from using the maximum OCPD value (of a single unit) of 60A and then further adding the 1.7A FLA and 6.9A FLA for the first unit, even though the maximum OCPD value already includes those. So the calculation ended up with an extra 8.6A (along with unnecessarily reducing to a standard size OCPD in the middle of the calculation, rather than only at the end if desired).

Cheers, Wayne
 
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