OCPD for two ACs

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MNSparky

MNSparky

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Location
Minneapolis, MN
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Electrical Contractor - 2023 NEC
Need some clarification/opinions on how I'm figuring this please. 2017 NEC.

We have been asked to wire a mini-split (split ductless) AC for an added porch on a dwelling townhome. The panel is nearly impossible to get to without cutting up the finished ceiling in the basement so running a new circuit is undesirable. There is an existing central AC unit sitting next to where the mini-split condenser will go. I want to add the mini-split to the central AC circuit. The circuit is run in 10-2 NM, the central AC is FLA 18.7a, max OCPD 25a, min OCPD 20a. The mini split is FLA 8.2a, max/min OCPD is 15a.

The way I read and calculate 440.22(B)(1) is I can breaker the 10-2 NM at 40a if I have fuses at each piece of equipment to limit the current to them. Here's my math:

18.7ax175%=32.7a. 32.7a+8.2a=40.9a

As far as I understand, I'm also good for 440.33 at 40amps because #10cu is rated at 40a in the 90 degree column. This is in violation of 334.80 however, but doesn't 440.22 "overrule" 334 in this case?

Does anyone see any issues with this that I am missing? Thanks!
 
I don't think you can protect the #10 at its 90-degree rating of 40a because it's NM.

I also wonder if a 40a breaker would start the central AC if the other unit was running.
 
440.22 is for the BCGF protection whereas Art 334 would apply to the conductor amapcity so both have to be kept in mind.
IMO, you would be Code ok considering the MCA of the units together does not exceeded the 30 amp conductor ampacity.
I agree with the other post in that the inrush current of the two units might pose a problem.
 
Another thought: with OCPDs at the units, the #10 becomes a feeder; would the HVAC allowances apply?
 
augie47 said:
440.22 is for the BCGF protection whereas Art 334 would apply to the conductor amapcity so both have to be kept in mind.
IMO, you would be Code ok considering the MCA of the units together does not exceeded the 30 amp conductor ampacity.
I agree with the other post in that the inrush current of the two units might pose a problem.
Click to expand...

Yes, that's what it was. I must have had myself all confused there for a bit with the 90 degree column thing.

I thought about the inrush as well, but I am guessing it'll be ok. I'm not sure how I'd calculate it to prove that or not, guessing it will be one of those thing we just have to test live.
 
MNSparky said:
The way I read and calculate 440.22(B)(1) is I can breaker the 10-2 NM at 40a if I have fuses at each piece of equipment to limit the current to them. Here's my math:

18.7ax175%=32.7a. 32.7a+8.2a=40.9a
Click to expand...

440.22(B)(1) is the wrong section for sizing the feeder OCPD. You need to look at 430.62.

You would also need to know the rating for each motor/other load, not just the rating of each piece of equipment as a whole.
 
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