Chiller Feeder Calculation

[mull982]

I am trying to confirm a feeder calculation for a new 480V feeder which will feed a new panelboard which will in turn feed (3) new identical chillers.

All (3) chillers have MCA=321A and MOP=450A.

I know that branch circuits to each chiller in new panelboard will require 450A breaker with 350MCM feeder.

For the new feeder circuit to this panel I know that the feeder calculation will require 125% of largest motor plus 100% of all other motors. Since the MCA value for each chiller already inclues the 125% of largest motor for each chiller would that not be overkill to use MCA for each chiller in feeder calculation since you'd in essence be including the 125% of largest motor (3) times?

I would think that the correct calculation for feeder would be to use the RLA of (2) of the chillers (100% motors) and then use the MCA of (1) of the chillers in order to account for the 125% of largest motor as well as 100% motors in 3rd chiller?

If the RLA of the chillers is not given would a conservative approach be to just use the MCA of each chiller in feeder calc? So new feeder would require 321A x 3= 936A. New Feeder circuit would require 950A with (2) sets of 750MCM (or (3) sets of 500 MCM)

 

[kwired]

I am trying to confirm a feeder calculation for a new 480V feeder which will feed a new panelboard which will in turn feed (3) new identical chillers.

All (3) chillers have MCA=321A and MOP=450A.

I know that branch circuits to each chiller in new panelboard will require 450A breaker with 350MCM feeder.

For the new feeder circuit to this panel I know that the feeder calculation will require 125% of largest motor plus 100% of all other motors. Since the MCA value for each chiller already inclues the 125% of largest motor for each chiller would that not be overkill to use MCA for each chiller in feeder calculation since you'd in essence be including the 125% of largest motor (3) times?

I would think that the correct calculation for feeder would be to use the RLA of (2) of the chillers (100% motors) and then use the MCA of (1) of the chillers in order to account for the 125% of largest motor as well as 100% motors in 3rd chiller?

If the RLA of the chillers is not given would a conservative approach be to just use the MCA of each chiller in feeder calc? So new feeder would require 321A x 3= 936A. New Feeder circuit would require 950A with (2) sets of 750MCM (or (3) sets of 500 MCM)

Yes to what I bolded above. Not really a code violation to do that, but good chance you can run smaller feeder and still be code compliant.

Also good chance your units have multiple compressors within them, you only need to factor 125% of the largest one of those and FLA/RLA of all other loads. Data sheets/nameplates should give you rating of each compressor/motor/any other loads that are present.

Might be good chance you can get all this on a 800 amp feeder, which can mean no GFP, next size up overcurrent protection, 800 amp breakers/fuses are a different price range than over 800...

 

[Sahib]

Good chance your units have multiple compressors within them, you only need to factor 125% of the largest one of those and FLA/RLA of all other loads. Data sheets/nameplates should give you rating of each compressor/motor/any other loads that are present.

Might be good chance you can get all this on a 800 amp feeder, which can mean no GFP, next size up overcurrent protection, 800 amp breakers/fuses are a different price range than over 800...

Component loads with current draw of 1 A or less may not be taken into account and it is a pity that effect of having VFD and electronic speed control in MCA reduction is not allowed.

 

[kwired]

Component loads with current draw of 1 A or less may not be taken into account and it is a pity that effect of having VFD and electronic speed control in MCA reduction is not allowed.

IMO those less than 1 amp loads are negligible, unless there is enough of them present that they begin to add up.

The VFD's, if the unit is allowed to run at full speed and full load, don't you need to account for it? If programmed to never allow over certain load, then I could see taking that into consideration.

 

[steve66]

Might be good chance you can get all this on a 800 amp feeder, which can mean no GFP, next size up overcurrent protection, 800 amp breakers/fuses are a different price range than over 800...

Best case is that each chiller has one motor load rated at 80% of 321 amps, or about 257 amps.

So 257+257+321 is still 835 amps.

If the chillers have multiple compressors, the rated load amps for each unit will go up.

So its going to be somewhere between 835 amps and 963 amps.

 

[kwired]

Best case is that each chiller has one motor load rated at 80% of 321 amps, or about 257 amps.

So 257+257+321 is still 835 amps.

If the chillers have multiple compressors, the rated load amps for each unit will go up.

So its going to be somewhere between 835 amps and 963 amps.

If each unit has multiple compressors, only one of them is the largest and needs 125% factored into this.

If MCA is 321 and it only has one load - then 80% of that is 321. But chances are there is at least two loads one being a blower motor.

Now if it had say 4 compressors rated at 70 amps each, one is taken at 125% = 87.5, add the other three at 100% = and you have 297.5, maybe you have another 23.5 amps of blower motors or other loads.

Now say you have three of those units: 87.5 for the largest compressor, 11 more at 70 amps = 770, and additional 23.5 amps x 3 = 70.5. Looks like 800 amp feeder is still going to be a little short, but getting close.

Different combinations of loads, or even three compressors instead of four per unit could possibly still get down to 800 amps. Some units may not have all compressors of equal size, or all blower motors of equal size.

 

[steve66]

If each unit has multiple compressors, only one of them is the largest and needs 125% factored into this.

If MCA is 321 and it only has one load - then 80% of that is 321. But chances are there is at least two loads one being a blower motor.

Now if it had say 4 compressors rated at 70 amps each, one is taken at 125% = 87.5, add the other three at 100% = and you have 297.5, maybe you have another 23.5 amps of blower motors or other loads.

Now say you have three of those units: 87.5 for the largest compressor, 11 more at 70 amps = 770, and additional 23.5 amps x 3 = 70.5. Looks like 800 amp feeder is still going to be a little short, but getting close.

Different combinations of loads, or even three compressors instead of four per unit could possibly still get down to 800 amps. Some units may not have all compressors of equal size, or all blower motors of equal size.

I'm not following you.

Say each condenser has 2 equal size compressors. Then they would be rated at 142 amps each. (142+142x 1.25 = 321 amps). And the RLA of each condenser goes up to 142x2 = 284 amps.

Given an MCA, the lowest Rated Load Amps a piece of equipment could possible have is always going to be for a single motor or compressor.

If the equipment has more than one motor or compressor, the RLA will be higher than if it has a single motor or compressor.

 

[kwired]

I'm not following you.

Say each condenser has 2 equal size compressors. Then they would be rated at 142 amps each. (142+142x 1.25 = 321 amps). And the RLA of each condenser goes up to 142x2 = 284 amps.

Given an MCA, the lowest Rated Load Amps a piece of equipment could possible have is always going to be for a single motor or compressor.

If the equipment has more than one motor or compressor, the RLA will be higher than if it has a single motor or compressor.

MCA marked on nameplates is usually same thing you would get if you took each individual load and used 440.33 to come up with a result.

Conductors supplying one or more motor-compressor(s) with or without an additional load(s) shall have an ampacity not less than the sum of the rated-load or branch-circuit selection current ratings, whichever is larger, of all the motor-compressors plus the full-load currents of the other motors, plus 25 percent of the highest motor or motor-compressor rating in the group.

If you had two equal sized compressors one would be taken at 125%, and all other motors or other loads would be taken at 100%

If MCA is 321 and you have two equal compressors they are probably less than 142 amps unless that is all you have for load is two compressors, you probably have some blower motors at very least to factor into the calculation as well (at 100% FLA)

If you had say four compressors that 125% gets applied to a lower figure than if there is two compressors leaving some more room for loads at 100% in the overall 321. If they are not all equal sized compressors, the extra room can work either way depending on size of the largest in the group.

For supplying a common feed to all three chillers, only one of those mentioned compressors is taken at 125%. The smaller that largest compressor is, the more room you have for your loads calculated at 100%.

 

[steve66]

The smaller that largest compressor is, the more room you have for your loads calculated at 100%.

You have it backwards. The larger the compressor is, the larger the additional 25% spare capacity is.

For 3 chillers each with an MCA of 321 amps, the smallest the actual rated load ampacity could ever be is 834 amps. That's (321*.8) + (321*.8) + 321 = 834 amps.

If you have more smaller motors, the rated load amps will be higher, and get closer to 963 amps (3 x 321). That's because the 0.8 factor applies to a smaller motor.

 

[kwired]

You have it backwards. The larger the compressor is, the larger the additional 25% spare capacity is.

For 3 chillers each with an MCA of 321 amps, the smallest the actual rated load ampacity could ever be is 834 amps. That's (321*.8) + (321*.8) + 321 = 834 amps.

If you have more smaller motors, the rated load amps will be higher, and get closer to 963 amps (3 x 321). That's because the 0.8 factor applies to a smaller motor.

Three chillers with 4 compressors each (assume all same size) only needs one of the twelve compressors to apply the 1.25% to it for a common feeder calculation for a feeder for the whole shebang.

 

[steve66]

Three chillers with 4 compressors each (assume all same size) only needs one of the twelve compressors to apply the 1.25% to it for a common feeder calculation for a feeder for the whole shebang.

Right, which means the RLA is pretty close to the MCA.

The biggest difference between the two is for a single motor.

 

[kwired]

Right, which means the RLA is pretty close to the MCA.

The biggest difference between the two is for a single motor.

RLA is pretty close to MCA for one unit, especially if it only has one compressor. We need to know what OP's unit's have for compressors. If each of the three units only has one compressor then yes that 125% of largest one becomes bigger factor in the feeder calculation than if there is four compressors in each unit.

simplicity sake lets say there is 3 100 amp compressors, minimum feeder ampacity becomes 100 x 1.25 (125) plus 2x100 = 325 amps (that doesn't even factor in additional loads like blower motors).

compare that to 3 units with four 25 amp compressors each and the feeder minimum ampacity is now 25x1.25 (31.25) plus 25 x 11 = 306.25 amps (again before considering additional loads like blower motors).

 

[steve66]

RLA is pretty close to MCA for one unit, especially if it only has one compressor. We need to know what OP's unit's have for compressors. If each of the three units only has one compressor then yes that 125% of largest one becomes bigger factor in the feeder calculation than if there is four compressors in each unit.

simplicity sake lets say there is 3 100 amp compressors, minimum feeder ampacity becomes 100 x 1.25 (125) plus 2x100 = 325 amps (that doesn't even factor in additional loads like blower motors).

compare that to 3 units with four 25 amp compressors each and the feeder minimum ampacity is now 25x1.25 (31.25) plus 25 x 11 = 306.25 amps (again before considering additional loads like blower motors).

You are changing the MCA for the two examples. 1st case, MCA is 125 amps. 2nd case 106 amps.

 

[kwired]

You are changing the MCA for the two examples. 1st case, MCA is 125 amps. 2nd case 106 amps.

All hypothetical examples and disregarding any non compressor loads as well, but I think I see a little of what you maybe are getting at.

All started when I suggested that OP may get lucky enough to get to use an 800 amp feeder, but looking like may come close but not quite enough. Still depends on details we don't have.

 

[topgone]

I'll try.
Not including the "other loads" in the unit, just the compressor, using the ultimate trip current recommended as described in NEC 430.32(A)(2) which tells me to be 140% (compressor current greater than 20A), I got an FLA = 108A (considering a 115% external TOL setting).

For three units, that would come out with an MCA = 351 A, the maximum OCP should be 500 A. My two cents!

Checking: MCA = (1.25 x 108) + (108 X 2) = 351A; MOP = (2.25 X 108) + (2 X 108) =459A ~ 500A!

 

[Sahib]

IMO those less than 1 amp loads are negligible, unless there is enough of them present that they begin to add up.

The VFD's, if the unit is allowed to run at full speed and full load, don't you need to account for it? If programmed to never allow over certain load, then I could see taking that into consideration.

VFD would have soft start and also it would not allow current increase due to aging. As such, MCA/MOP calculation without taking it into account is overkill.

 

[Sahib]

Still depends on details we don't have.

Chiller may be direct expansion or water cooled type and in the latter case MCA calculation is different.

 

[david luchini]

I'll try.
Not including the "other loads" in the unit, just the compressor, using the ultimate trip current recommended as described in NEC 430.32(A)(2) which tells me to be 140% (compressor current greater than 20A), I got an FLA = 108A (considering a 115% external TOL setting).

For three units, that would come out with an MCA = 351 A, the maximum OCP should be 500 A. My two cents!

Checking: MCA = (1.25 x 108) + (108 X 2) = 351A; MOP = (2.25 X 108) + (2 X 108) =459A ~ 500A!

I'm not quite sure what you are calculating here, but the MOCP for three compressors each with an RLA of 108 would be 450A, not 500A.

 

[mull982]

Thank you for the responses. I can see based on the examples provided by Kwired and others that the feeder calculation can potentially be reduced based on actual FLA/RLA values of compressors and not conservatively using MCA for all (3) compressors (which includes 125% of largest motor in each compressor)

This is currently in a conceptual stage so unfortunately we were only provided with the MCA & MOP values that I indicated in OP. I am going to try to keep digging to see if I'm able to solicit the actual compressor datasheets that may indicate RLA/FLA.

Thanks!

 

[electrofelon]

VFD would have soft start and also it would not allow current increase due to aging. As such, MCA/MOP calculation without taking it into account is overkill.

Chiller may be direct expansion or water cooled type and in the latter case MCA calculation is different.

How about some NEC references for any of that??