Help with finding Air Conditioning load for service sizing

[strawburner]

I have 2 - 20 ton units, 1 - 10 ton unit, 1 - 7.5 ton unit, and 1 - 5 ton unit of air conditioning load. Can somebody give some direction on how to calculate AC load for service sizing when there is this many different size units. Are there some demand factors that can be used? If I add all MCA's together that would be a 125% calculation on every unit and would not be correct? The service will by 120/208 3 phase.

 

[Chamuit]

What are your nameplates? What have you calculated, and how?

 

[strawburner]

I attached the file for the 20 ton unit which has MCA of 89 amps there will be 2 this size. The 10 ton has a MCA of 52 amp, 7.5 ton has 42 MCA, and the 5 ton has 29 MCA. I think I may just add all the MCA's together which gives me a total of 301 amps.

If I am correct this will be overkill since I am figuring 125% for each unit but I did some calculations with the taking a 125% of the FLA on the biggest unit and just adding the FLA on the rest of the units and that total is not much less than adding all the MCA's.

If somebody could give a code reference in either article 440 or 220 I would be happy to learn.

 

[infinity]

Overkill would be using the MCA for each unit because that amp rating already included the 125%.

 

[Chamuit]

@infinity is correct. Motor loads = largest @125% and all the rest @100%. I come up with 258.6A.

 

[Mgraw]

Overkill would be using the MCA for each unit because that amp rating already included the 125%.

I don't see it. Per the submittal Cooling FLA = 83.9 amps MCA = 89 amps. Where is the 1.25%?

 

[infinity]

I don't see it. Per the submittal Cooling FLA = 83.9 amps MCA = 89 amps. Where is the 1.25%?

I didn't do any math (too early and no coffee) but I'm guessing that since the data sheet states that there are 4 compressors only one of them will have the 125% factor added in to find the MCA.

 

[Mgraw]

I didn't do any math (too early and no coffee) but I'm guessing that since the data sheet states that there are 4 compressors only one of them will have the 125% factor added in to find the MCA.

That would make the supply fan the largest motor and that would work.

 

[Mgraw]

@infinity is correct. Motor loads = largest @125% and all the rest @100%. I come up with 258.6A.

I think it would be more like 285 amps.

 

[infinity]

That would make the supply fan the largest motor and that would work.

I think that's it.

 

[Chamuit]

@Mgraw
Largest motor load - 89 (MCA)
Then rest, 89+52+42+29 = 212
212/1.25 = 169.6 + 89 = 258.6

 

[electrofelon]

@Mgraw
Largest motor load - 89 (MCA)
Then rest, 89+52+42+29 = 212
212/1.25 = 169.6 + 89 = 258.6

No, you treat all the motors as a group and do 125% of the largest one, NOT the largest unit.

 

[infinity]

The largest motor is the fan. 125% of that plus the other motors at 100%.

 

[kwired]

I have 2 - 20 ton units, 1 - 10 ton unit, 1 - 7.5 ton unit, and 1 - 5 ton unit of air conditioning load. Can somebody give some direction on how to calculate AC load for service sizing when there is this many different size units. Are there some demand factors that can be used? If I add all MCA's together that would be a 125% calculation on every unit and would not be correct? The service will by 120/208 3 phase.

No demand factor, at times when the unit is pushed to it's design limits everything can be running, and can be near full rated load.

MCA of an individual unit has 1.25 factor added to the largest motor though as mentioned. For your feeder/service calculation you only need 125% of largest motor of all 5 units plus all other load that could run simultaneously.