HVAC and 125%

T

TealHeals

Member
Location
Oregon
Occupation
journeyman electrician & licensed signing supervisor
I'm having a circular conversation with an electrical engineer (EE) right now. Can you help me understand what I'm overlooking? This is a bare-bones commercial space, about 2,880 square feet. Our largest motor is from the HVAC unit - the compressor.
- 220.14 (C) directs us to us 430.22, 430.24 and 440.6 for load calculations
- 440.6 specifically covers air-conditioning and refrigerating equipment
- 430.1 and 440.1 seem pretty clear on the scope of what's covered by the respective article
We have sized the compressor motor OCPD at 175% of the rated load current from the face plate.
Why on earth is my EE wanting us to add another 25% for our feeder calculations? I don't see that article 430 applies to our HVAC unit.
 
hbiss said:
Did you see this thread in the NEC forum?

i need help understanding a correction

this i a correction i am dealing with . we have a heat pump with a MCA 21 max OCP 25 . I ran #12 THHN and put it on a 25 amp breaker. The following is what he stated when i questioned the correction. if im wrong could somebody please educate me here. While Table...
forums.mikeholt.com

-Hal
Click to expand...
I was just reading it :)
Academically speaking, we had the 125% pounded into our heads. However, I cannot find the NEC citation as to why this EE wants us to add another 25% to our HVAC compressor motor. We're not in a dwelling or multi-dwelling situation, or any of the specific occupancies called out in Article 220. There's no standard/optional calcs for this office building.
 
TealHeals said:
I'm having a circular conversation with an electrical engineer (EE) right now. Can you help me understand what I'm overlooking? This is a bare-bones commercial space, about 2,880 square feet. Our largest motor is from the HVAC unit - the compressor.
- 220.14 (C) directs us to us 430.22, 430.24 and 440.6 for load calculations
- 440.6 specifically covers air-conditioning and refrigerating equipment
- 430.1 and 440.1 seem pretty clear on the scope of what's covered by the respective article
We have sized the compressor motor OCPD at 175% of the rated load current from the face plate.
Why on earth is my EE wanting us to add another 25% for our feeder calculations? I don't see that article 430 applies to our HVAC unit.
Click to expand...
Is the HVAC the smaller of two noncoincident loads? Is the compressor the largest motor supplied by the service?
If so, see 220.60. Note, this section has been changing over the past couple of code cycles, so the answer will depend on what code cycle applies to your project.
 
wwhitney said:
Article 430 applies to all motor installations, Article 440 applies in addition for motor compressors.

So the feeder load calculation will include an extra 25% for the largest motor on the feeder.

Cheers, Wayne
Click to expand...
Aha! So we ARE using both 430 and 440. Ok. Thanks
 
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