HVAC Load Calculation

L

Leonan

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Location
Massachusetts
Occupation
Electrician
Hey guys, I have a job with 12 condensers with this information. Doubting myself on the type of subpanel to support this. Any tips and suggestions
 

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If you are running a sub panel for just those units I believe you will need to use a 300 amp overcurrent protective device

Not sure what you mean by what type of sub panel
 
I get:

12 units supplied L-L, 4 on each of the 3 pairs of the 3 phase supply. First unit at MCA = 25A. Second through fourth units at nameplate rating, 20.5A. Total is 25 + 20.5*3 = 86.5A on each of the 3 pairs. Load on each line conductor is sqrt(3) * 86.5A = 150A.

Cheers, Wayne
 
wwhitney said:
I get:

12 units supplied L-L, 4 on each of the 3 pairs of the 3 phase supply. First unit at MCA = 25A. Second through fourth units at nameplate rating, 20.5A. Total is 25 + 20.5*3 = 86.5A on each of the 3 pairs. Load on each line conductor is sqrt(3) * 86.5A = 150A.

Cheers, Wayne
Click to expand...

wwhitney said:
I get:

12 units supplied L-L, 4 on each of the 3 pairs of the 3 phase supply. First unit at MCA = 25A. Second through fourth units at nameplate rating, 20.5A. Total is 25 + 20.5*3 = 86.5A on each of the 3 pairs. Load on each line conductor is sqrt(3) * 86.5A = 150A.

Cheers, Wayne
Click to expand...
If I decided to use a single phase sub panel what would I need?
 
Jaybone812 said:
You’d need to provide a 300 amp ocpd to protect a single phase sub panel feeding those 12 units. 12 units- MCA 25 amps - 300 amp ocpd
Click to expand...
That's more than is required--MCA includes a 125% factor for the largest motor (or equivalent when you have power conversion equipment, I assume), and the feeder only requires the 125% factor once, not 12 times.

So 25+11*20.5 = 250.5, I'd say a 250A supply would suffice. [Yes, I'm dropping the 0.5.]

Cheers, Wayne
 
wwhitney said:
That's more than is required--MCA includes a 125% factor for the largest motor (or equivalent when you have power conversion equipment, I assume), and the feeder only requires the 125% factor once, not 12 times.

So 25+11*20.5 = 250.5, I'd say a 250A supply would suffice. [Yes, I'm dropping the 0.5.]

Cheers, Wayne
Click to expand...

You’re 100% correct . I had a stupid moment and you caught it and I appreciate that lol. We can size this feeder in accordance with 430.24 . So like you said the load for these 12 units is calculated at 250.5 amps .
And like you said a 250 amp supply would definitely suffice but a 300 amp ocpd would be compliant for this 250.5 amp load if need be because of that .5 amps lol


Sent from my iPhone using Tapatalk
 
wwhitney said:
Eu entendo:

12 unidades fornecidas LL, 4 em cada um dos 3 pares do fornecimento trifásico. Primeira unidade em MCA = 25A. Segunda a quarta unidades na classificação da placa de identificação, 20,5A. O total é 25 + 20,5*3 = 86,5A em cada um dos 3 pares. A carga em cada condutor de linha é sqrt(3) * 86,5A = 150A.

Saudações, Wayne
Click to expand...

wwhitney said:
Eu entendo:

12 unidades fornecidas LL, 4 em cada um dos 3 pares do fornecimento trifásico. Primeira unidade em MCA = 25A. Segunda a quarta unidades na classificação da placa de identificação, 20,5A. O total é 25 + 20,5*3 = 86,5A em cada um dos 3 pares. A carga em cada condutor de linha é sqrt(3) * 86,5A = 150A.

Saudações, Wayne
Click to expand...

wwhitney said:
I get:

12 units supplied L-L, 4 on each of the 3 pairs of the 3 phase supply. First unit at MCA = 25A. Second through fourth units at nameplate rating, 20.5A. Total is 25 + 20.5*3 = 86.5A on each of the 3 pairs. Load on each line conductor is sqrt(3) * 86.5A = 150A.

Cheers, Wayne
Click to expand...
Thank you for your help!
 
wwhitney said:
That's more than is required--MCA includes a 125% factor for the largest motor (or equivalent when you have power conversion equipment, I assume), and the feeder only requires the 125% factor once, not 12 times.

So 25+11*20.5 = 250.5, I'd say a 250A supply would suffice. [Yes, I'm dropping the 0.5.]

Cheers, Wayne
Click to expand...
I appreciate your help!
 
wwhitney said:
I get:

12 units supplied L-L, 4 on each of the 3 pairs of the 3 phase supply. First unit at MCA = 25A. Second through fourth units at nameplate rating, 20.5A. Total is 25 + 20.5*3 = 86.5A on each of the 3 pairs. Load on each line conductor is sqrt(3) * 86.5A = 150A.

Cheers, Wayne
Click to expand...
Can you help me with this other question: if I decide to place this panel near the condensers, would it still be necessary to install the disconnect for each machine too?
 
Leonan said:
Can you help me with this other question: if I decide to place this panel near the condensers, would it still be necessary to install the disconnect for each machine too?
Click to expand...

Read 440.14 for the rules for ac disconnects . If the panel is within sight of all the units , and that means visible from every unit and within 50’ of every unit , then the breaker would be allowed to be the sole disconnecting means for each unit.
In my mind it’s more practical to have disconnects near the units or on the unit if possible simply because unless it’s a really short run i assume you’ll be transitioning from solid conduit to LFMC somewhere before the unit anyways


Sent from my iPhone using Tapatalk
 
wwhitney said:
That's more than is required--MCA includes a 125% factor for the largest motor (or equivalent when you have power conversion equipment, I assume), and the feeder only requires the 125% factor once, not 12 times.

So 25+11*20.5 = 250.5, I'd say a 250A supply would suffice. [Yes, I'm dropping the 0.5.]

Cheers, Wayne
Click to expand...
I don't see how you got there based on 430.62 which is for overcurrent protective device size. I know 430 is for motor but I thought 440 sent us to 430

I understand that you take one motor and take the 20.5 x 250% for one motor plus the sum of the 11 others. Isn't that what it states?
 
Last edited:
Dennis Alwon said:
I understand that you take one motor and take the 20.5 x 250% for one motor plus the sum of the 11 others. Isn't that what it states?
Click to expand...
I was calculating the feeder "MCA". You're correct that the feeder MOCPD would be higher.

430.62 says to take the MOCPD for the largest motor and add the FLCs for the other motors. The nameplate says the MOCPD is 40A, so the calculation would be 40 + 11*20.5 = 265.5. I don't see any provision for rounding up, so the largest standard size is 250A.

I didn't double check all the sections of 430 for this post or the previous one, so I may have overlooked something.

Cheers, Wayne
 
wwhitney said:
I was calculating the feeder "MCA". You're correct that the feeder MOCPD would be higher.

430.62 says to take the MOCPD for the largest motor and add the FLCs for the other motors. The nameplate says the MOCPD is 40A, so the calculation would be 40 + 11*20.5 = 265.5. I don't see any provision for rounding up, so the largest standard size is 250A.

I didn't double check all the sections of 430 for this post or the previous one, so I may have overlooked something.

Cheers, Wayne
Click to expand...
Yep, I did round up and forgot that I shouldn't. That's one of the few places (I think) in the NEC where you don't round up
 
Jaybone812 said:
Read 440.14 for the rules for ac disconnects . If the panel is within sight of all the units , and that means visible from every unit and within 50’ of every unit , then the breaker would be allowed to be the sole disconnecting means for each unit.
In my mind it’s more practical to have disconnects near the units or on the unit if possible simply because unless it’s a really short run i assume you’ll be transitioning from solid conduit to LFMC somewhere before the unit anyways


Sent from my iPhone using Tapatalk
Click to expand...
I understand thank you for the help.
 
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