Subpanel size for 5 HVAC units

john100

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Location
TN
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Hvac tech
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 60*1.25=75 plus the 4 other units is 315 amps? can i get some input if this is correct
 

infinity

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New Jersey
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Journeyman Electrician
The MCA already has 125% added to the largest load (compressor) so there is no need to add an additional 25%. It would be more accurate if you had the FLA of each compressor and fan and calculated from there.

Welcome to the Forum. :)
 

john100

Member
Location
TN
Occupation
Hvac tech
The MCA already has 125% added to the largest load (compressor) so there is no need to add an additional 25%. It would be more accurate if you had the FLA of each compressor and fan and calculated from there.

Welcome to the Forum. :)
Hello and thank you. FLA is looking like 34amp per unit. So I don't need to size a sub panel off breaker size then? Below is the data tag

 
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The MCA already has 125% added to the largest load (compressor) so there is no need to add an additional 25%. It would be more accurate if you had the FLA of each compressor and fan and calculated from there.

Welcome to the Forum. :)
John, welcome to the forum.

Try again using the actual RLA of the units.
I agree. Current job I had an existing 200A panel and feeder. Adding the MCA's exceeded the 200A, but correcting for the extra 25% built into the MCA of each unit kept it just under 200. If the units are all the same, take 25% of the FLA of the largest compressor on each unit, and subtract that from 4 of the 5 units' MCA. Keep oneo fhte units MCA as is. Add that up and that is what you need. IF you dont have the unit data, you can take 80% of 4 of the 5 units, but that will be likely be a little low because the compressor is not the only load calculated in the MCA.
 

john100

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Location
TN
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Hvac tech
I agree. Current job I had an existing 200A panel and feeder. Adding the MCA's exceeded the 200A, but correcting for the extra 25% built into the MCA of each unit kept it just under 200. If the units are all the same, take 25% of the FLA of the largest compressor on each unit, and subtract that from 4 of the 5 units' MCA. Keep oneo fhte units MCA as is. Add that up and that is what you need. IF you dont have the unit data, you can take 80% of 4 of the 5 units, but that will be likely be a little low because the compressor is not the only load calculated in the MCA.
So take 60+35+35+35+35?
Full load amps compressor + indoor blower + condenser fan motor is 35 amp rounded up. 35*5=175amp. 175*1.25=218. 225amp? This will be fed from a Main distribution panel.

That would be 39.1 amps per unit. 125% of compressor plus other motors which is the name plate min circuit ampacity. Comes out to just under 200amps for all 5
 
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So take 60+35+35+35+35?
Full load amps compressor + indoor blower + condenser fan motor is 35 amp rounded up. 35*5=175amp. 175*1.25=218. 225amp? This will be fed from a Main distribution panel.

That would be 39.1 amps per unit. 125% of compressor plus other motors which is the name plate min circuit ampacity. Comes out to just under 200amps for all 5
No, so they arrive at the MCA by taking 1.25 of the largest motor (the compressor) so (24.4 x 1.25)+1.7+6.9=39.1. So feeder would only need one of the 1.25% not 5. So (4 x 33) + (1 x 39.1) = 171.1 A. OF course in this case not much of a difference from 39.1 X 5 and you would use a 200A panel either way. But as you can see if they were just a little bit bigger, you would be over 200 just adding up the MCA's
 

john100

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Location
TN
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Hvac tech
No, so they arrive at the MCA by taking 1.25 of the largest motor (the compressor) so (24.4 x 1.25)+1.7+6.9=39.1. So feeder would only need one of the 1.25% not 5. So (4 x 33) + (1 x 39.1) = 171.1 A. OF course in this case not much of a difference from 39.1 X 5 and you would use a 200A panel either way. But as you can see if they were just a little bit bigger, you would be over 200 just adding up the MCA's
Gotcha. Idk why I was confusing myself and making it difficult. I think we will probably go for a 225 panel. We will be adding convient outlets and the always potential up sizing a unit in the future
 

Dennis Alwon

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Location
Chapel Hill, NC
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Electrical Contractor
For a feeder I believe you take the largest motor overcurrent protective device and add the FLC for the others motor load.

60+40+40+40+40= 220 amps

However 430.62 says to use 440.22(A) which states that 225% must be used for the largest motor

90 + 4(40) = 250 amps-- Not sure what to use there
 
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david luchini

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Location
Connecticut
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For a feeder I believe you take the largest motor overcurrent protective device and add the FLC for the others motor load.

60+40+40+40+40= 220 amps
That would be 60+1.7+6.9+33+33+33+33 = 200.6, or a 200A breaker with an mca of 171.1A.

However 430.62 says to use 440.22(A) which states that 225% must be used for the largest motor

90 + 4(40) = 250 amps-- Not sure what to use there
Not sure how you got the 90, but the 225% is already figured in the 60A MOCP of the unit.
 
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For a feeder I believe you take the largest motor overcurrent protective device and add the FLC for the others motor load.

60+40+40+40+40= 220 amps

However 430.62 says to use 440.22(A) which states that 225% must be used for the largest motor

90 + 4(40) = 250 amps-- Not sure what to use there
I think we need to clarify if/when we are talking about feeder ampacity vs feeder OCPD. But if there is a motor panelboard supplied by a motor feeder, wouldnt 408.36 prohibit us from using a larger than ampacity feeder OCPD?

Edit: 430.62(B). apparently there is no allowance for a larger than ampacity OCPD for a motor feeder like there is for motor branch circuits.
 

david luchini

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Location
Connecticut
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Engineer
I think we need to clarify if/when we are talking about feeder ampacity vs feeder OCPD. But if there is a motor panelboard supplied by a motor feeder, wouldnt 408.36 prohibit us from using a larger than ampacity feeder OCPD?
Not necessarily. If for instance the feeder mca was 170 and the feeder MOCP was 200A, the feeder could be 2/0 but the panel bus would have to be rated 200a

Edit: 430.62(B). apparently there is no allowance for a larger than ampacity OCPD for a motor feeder like there is for motor branch circuits.
430.62(A) permits a feeder OCPD larger than the feeder conductor ampacity.
 

david luchini

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Connecticut
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Engineer
David, do you mean 39 instead of the 60? I am not seeing where the max OCPD comes into play for multiple motor conductors.
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.
 

john100

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Location
TN
Occupation
Hvac tech
Ok so if 125% is already added to each name plate. Why couldn't you just take the 5 min circuit ampacity times 5? 195.5amp make it a 200 amp panel
 

david luchini

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Connecticut
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Engineer
Ok so if 125% is already added to each name plate. Why couldn't you just take the 5 min circuit ampacity times 5? 195.5amp make it a 200 amp panel
You could, but you don't have to. Five units at 33A each plus 25% of the largest motor = 171.1A. You could run a 2/0 feeder from a 200A feeder c/b to a 200A panel.

I'm not saying I would do it that way...makes more sense to just make the feeder conductors 3/0...but Code allows it.
 
Ok so if 125% is already added to each name plate. Why couldn't you just take the 5 min circuit ampacity times 5? 195.5amp make it a 200 amp panel
What David said, but also look at post #5 for a situation where it makes a big difference. If I was not aware of it and/or not able to justify it to the plan reviewer, I would have had to replace the feeder and panel
 
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