minimum circuit ampacity

[arnettda]

I have a cut sheet for a three phase refrigeration compressor. 208-230 3 phase RLA is 21.5 LRA is 156. There is 1 fan 1/3HP FLA of 3.5. They give me a minimum circuit ampacity of of 44 amps.
How do they get to 44 amps? the evaporator fan they say is 12 amps is this included in the 44?
Thank You

 

[kwired]

I have a cut sheet for a three phase refrigeration compressor. 208-230 3 phase RLA is 21.5 LRA is 156. There is 1 fan 1/3HP FLA of 3.5. They give me a minimum circuit ampacity of of 44 amps.
How do they get to 44 amps? the evaporator fan they say is 12 amps is this included in the 44?
Thank You

Is evaporator fan part of same unit? If so then that does put you close to 44.

MCA should be 1.25% of largest motor plus all other loads, I come up with 42.375 they must have rounded some things I didn't? Even rounding up to whole numbers in each step I come up with 43 though.

 

[petersonra]

Is evaporator fan part of same unit? If so then that does put you close to 44.

MCA should be 1.25% of largest motor plus all other loads, I come up with 42.375 they must have rounded some things I didn't? Even rounding up to whole numbers in each step I come up with 43 though.

They can put any number they want on for MCA. It does not have to be the number that is calculated according to NEC. Just has to be equal or higher.

There might also be a small control transformer that needs to be accounted for.

 

[arnettda]

Is evaporator fan part of same unit? If so then that does put you close to 44.

MCA should be 1.25% of largest motor plus all other loads, I come up with 42.375 they must have rounded some things I didn't? Even rounding up to whole numbers in each step I come up with 43 though.

Will you show me your exact formula? I only come up with 30.375

 

[Ingenieur]

Will you show me your exact formula? I only come up with 30.375

add the 12 A

yep, some control power, contactors, etc

 

[arnettda]

add the 12 A

yep, some control power, contactors, etc

So the 12 amps would be control power and the such?
I would then use the minimum circuit ampacity of all the 4 condensors to get my overall load?

 

[Ingenieur]

So the 12 amps would be control power and the such?
I would then use the minimum circuit ampacity of all the 4 condensors to get my overall load?

no, the 12 A is the evaporator fan
we are assuming the difference between the 44 and 42.375 is control power

you are going to have 1 feeder to the 4 units?

each unit also has a mocp max cb or fuse size

 

[arnettda]

no, the 12 A is the evaporator fan
we are assuming the difference between the 44 and 42.375 is control power

you are going to have 1 feeder to the 4 units?

each unit also has a mocp max cb or fuse size

Each unit will have its own feed and breaker, Looking for overall load to make sure my added load calculation for the building is correct.

 

[Ingenieur]

load = 4 x (comp + fan + evap + assumed controls)
= 4 x (21.5 + 3.5 + 12 + 1.6) = 155 A rounded up

assumed controls ~ 44 - 42.375 ~ 1.6

 

[kwired]

They can put any number they want on for MCA. It does not have to be the number that is calculated according to NEC. Just has to be equal or higher.

There might also be a small control transformer that needs to be accounted for.

Nearly every time I have run components against a marked MCA you get right on or very close to marked MCA just taking 125% largest motor plus all other loads.

Control transformers often can pretty much be ignored. ~ 50 VA @ 240 volts is only 0.20 Amps. Many of them are only 40 VA or less.

Will you show me your exact formula? I only come up with 30.375

I added 12 amps for the evaporator fan - not sure if it actually is part of the unit or not, but you were trying to justify MCA of 44 - adding this fan gets you close.

 

[GoldDigger]

What the question did not specify is whether the evaporator fan is powered on the same circuit as the compressor (common for single cabinet refrigeration units and mini-split A/Cs) or on its own circuit (common for full house A/Cs, at least separate wiring even if derived from same branch breaker.) Most common for residential forced air heat and A/C combo is that evaporator fan is part of "furnace" circuit and compressor and condenser fan are one separate "A/C" circuit.)

 

[arnettda]

What the question did not specify is whether the evaporator fan is powered on the same circuit as the compressor (common for single cabinet refrigeration units and mini-split A/Cs) or on its own circuit (common for full house A/Cs, at least separate wiring even if derived from same branch breaker.) Most common for residential forced air heat and A/C combo is that evaporator fan is part of "furnace" circuit and compressor and condenser fan are one separate "A/C" circuit.)

Evaporator will be powered from a separate circuit. They are not three phase and will be piped differently than the runs going out to the Compressors.
I have cut sheets for the evaporators they are only a couple amps. They should only be fan loads.
Does the Minimum circuit ampacity usually include the evaporator?

 

[arnettda]

Nearly every time I have run components against a marked MCA you get right on or very close to marked MCA just taking 125% largest motor plus all other loads.

Control transformers often can pretty much be ignored. ~ 50 VA @ 240 volts is only 0.20 Amps. Many of them are only 40 VA or less.

I added 12 amps for the evaporator fan - not sure if it actually is part of the unit or not, but you were trying to justify MCA of 44 - adding this fan gets you close.

12 amps seams large for a evaporator. The are only fan loads? The defrost heater is separate. I have separate cut sheets for evaporators and they are only 2 amps.

 

[Ingenieur]

post the cut sheets or mfg/model and full pn

 

[arnettda]

post the cut sheets or mfg/model and full pn

I tried but can not get them small enough

 

[kwired]

12 amps seams large for a evaporator. The are only fan loads? The defrost heater is separate. I have separate cut sheets for evaporators and they are only 2 amps.

Other possible items often on same circuit is lights or door frame heaters. Defrost heaters often are separate circuit.