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
Subpanel size for 5 HVAC units
- Thread starter john100
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- New Jersey
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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.
Welcome to the Forum.
LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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- Electrical Contractor
John, welcome to the forum.
Try again using the actual RLA of the units.
Try again using the actual RLA of the units.
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 tagThe 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.
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electrofelon
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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.
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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LarryFine
Master Electrician Electric Contractor Richmond VA
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- Henrico County, VA
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200a sounds just right to me.
electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
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
Thanks!
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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
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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- Location
- Connecticut
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That would be 60+1.7+6.9+33+33+33+33 = 200.6, or a 200A breaker with an mca of 171.1A.
Not sure how you got the 90, but the 225% is already figured in the 60A MOCP of the unit.
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electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
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.
electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
David, do you mean 39 instead of the 60? I am not seeing where the max OCPD comes into play for multiple motor conductors.
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- Connecticut
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- Engineer
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
430.62(A) permits a feeder OCPD larger than the feeder conductor ampacity.
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- Connecticut
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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.
electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
Oh Ok. I didnt read 430.62(A) carefully enough and though it was for where there was only one motor.
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- Connecticut
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- Engineer
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.
electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
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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