A/C loads in panel schedules

[hmspe]

My office policy has always been to enter the sum of the compressor RLA and the FLA of any fans as the breaker's load in a panel schedule for a branch circuit to a condensing unit. 25% of the largest motor in the panel is added to the column totals as a separate entry.

I just had a plan rejected by a plans reviewer, who said, at least as I understand it, that the appropriate load to show in a panel schedule for each condensing unit is the MCA. He based this on (1999) 220-14. Before I get too bent out of shape, I though I should ask for comments.

As I see it, 220-14 is for feeders, not branch circuits, so on the face it would not apply. That would put us back at 220-3(b)(1) or 220-3(b)(3). The latter references Article 440, where there's lots of information on conductor sizing, but nothing I can find on what should be used in a load calculation. If 220-14 did apply, the references are to 430-24, 430-25, and 430-26, all of which talk about conductor ampacity, not calculated load.

Comments, please?

 

[wolfman56]

Re: A/C loads in panel schedules

I've been watching for a few hours and no one has attempted an opinion. So I guess I'll take the bait. First I must mention, I'm not an engineer, but the one who installs the hardware.
I see it as a matter of perspective. You could both be right based on what the schedule is to be used for. You seem to view it as a format for main feeder calc's. The reviewer sees it as a template for the sizing of the branch circuits. I side with the reviewer for the following reasons.
If you hand me an engineered print, along with a spec book then I could rightfully assume that all calc's have been done, and I must follow the plan. I would see that in the schedule an ampacity is noted for AC units, and could (doesn't mean I do)assume that is the MCA of that unit, I need not, and shouldn't have to, do any further calculations. (That was what the engineering was for)
Simply put, I believe that Feeder Calc's are Feeder Calc's and Panel Schedules are instructions to installers for circuit planning.
I wonder that if you've been doing this for very long, and possibly have not spelled out conductor size for each circuit, if there are undersize conductors out there because the installer didn't factor in that 25%?

 

[hmspe]

Re: A/C loads in panel schedules

Thanks for the reply.

My issue is strictly with the load to be entered for each circuit. Circuit breaker size is always explicitly called out, usually at the MOCP value. As office policy I also explicitly call our wire size based on breaker size, even when the wire could be just the MCA.

As to experience, my office has done over 950 projects in the last 6 years. That's me with a part-time student for most of the time. Most of those jobs have had A/C called out, and this is the first time I've ever had a reviewer want MCA used as the load in the panel.

Martin

 

[stevebrown]

Re: A/C loads in panel schedules

Help me out, it's early. What is MCA?

 

[hmspe]

Re: A/C loads in panel schedules

Minimum Circuit Ampacity. 125% of the load of the largest motor plus the sum iof the remaining motor loads. NEC 440-33.

 

[charlie b]

Re: A/C loads in panel schedules

As I understand your question, the difference boils down to whether you add 25% to the largest motor once per panel, or once for every A/C unit on the panel. I side with you.

Per 220-3(B)(3) (1999), you compute the branch circuit loads per 420-22, 420-24, and 440. I agree that 440 doesn?t help answer the question. Also, 420-22 does not apply, since you have more than one motor. But 420-24 does say that you add 25% of the ?highest rated motor in the group.? I read the word ?group? as meaning the panel, not ?each single A/C system powered by the panel.? Similarly, if you have four panels being fed from the same upstream ?main? board, and if all four have A/C systems, then when you add up the loads on the main board you only have to add 25% to one motor (not four) out of the four A/C systems.

If you have two identical A/C units on the same panel, and if their condensing units constitute the two largest motors on the panel, then 420-24 explicitly says to add all the loads (i.e., both of the condensing units at 100%), and then add 25% of one of them to the total. To me, this is the load that should appear in a panel schedule. As to the order in which the additions are done, that does not really matter (per the ?Commutative Property of Addition?). But I always add the loads first, and then add the 25%, so that the math process follows the sequence given in the code.