Air Conditioner Load Calculations

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jerjwillelec

jerjwillelec

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Location
Nevada, IA
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Master Electrician
I keep going back and forth with this and need some assistance. Unfortunately, I don't have to do many calculations on a consistent basis so I get out of practice.

A school wants to add (ten) 2 ton 'mini split' air conditioning units and (two) 3 ton 'mini split' air conditioning units into some of the class rooms. I'm figuring 1 ton = 3.51 kw so the 2 ton = 34 fla and the 3 ton = 51 fla (source voltage 120/208). I'm planning on installing a new feeder(s) and sub panelboard(s) for all of this but am not quite sure how I want to do it yet, based on the age of the existing service and what I have to work with. My question is regarding sizing the feeder(s)...I should calculate all of these new units at 100% shouldn't I?

As I mentioned, I just haven't had to do this in a very long time and have, well, forgotten. Any help would be greatly appreciated!

Thank you!
 
Personally I would not touch it until I had a cut sheet showing actual data.

That said, HVAC feeders have been discussed here previously and as I recall there is some difference of opinions.
440 does not, I believe, address 'feeders' so you need to default to Art 430, however, 440.33 can give you some guidance in requiring the sum of the rated load OR branch circuit selection current, whichever is larger, plus 25% of the largest.

(As I recall the discussion comes from the term 'rated load' )
 
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From your math results, I infer the equipment is powered at 208 volts, single phase. Is that right?

I agree that the right thing to do is to get copies of the cut sheets.
 
augie47 said:
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That said, HVAC feeders have been discussed here previously and as I recall there is some difference of opinions.
440 does not, I believe, address 'feeders' so you need to default to Art 430, however, 440.33 can give you some guidance in requiring the sum of the rated load OR branch circuit selection current, whichever is larger, plus 25% of the largest.
...
Click to expand...
This is the problem I've always had with respect to feeders of multiple AC units. There doesn't seem to be any recourse to 'decrease' the MCA values at feeder level. As you all know, nameplate MCA values already include the [largest] motor at 125%. At the feeder level, summing MCA values includes an extra 25% for each unit, not just an extra 25% for the largest motor of all the connected motors.
 
I have sent in an RFI to get the equipment specifications. I don't think the school district realizes what their getting themselves into. At these values, we'd be adding over 400 amp of load to an already old service... Maybe the nameplates will come back lower...we'll see. Thanks everyone!
 
Smart $ said:
This is the problem I've always had with respect to feeders of multiple AC units. There doesn't seem to be any recourse to 'decrease' the MCA values at feeder level. As you all know, nameplate MCA values already include the [largest] motor at 125%. At the feeder level, summing MCA values includes an extra 25% for each unit, not just an extra 25% for the largest motor of all the connected motors.
Click to expand...

I agree.
As I recall that was the basis for the thinking that for feeders one would default to Art 430 since 'feeders' are not addressed in Art 4440 and 440.3 defers to Art 430 in those situations.
The problem gets a little sticky however since many HVAC motors are not HP rated and Art 430 is based on amp ratings by HP. One might use the exceptions in 430.6 to use the FLA as your base.
I don't see a clear-cut answer. Perhaps someone else does.
 
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