Chiller conductor & OCPD sizing

[SLentions]

hi all,

i'm a bit confused on NEC chiller cable sizing. my chiller has
RLA = 160A
MCA = 360A
MOP = 500A

my understanding is that the cable is sized per MCA, so a 500 MCM cable would be appropriate here. the OCPD is sized to MOP? or sized to protect the cable within 125%, so somewhere between 360A and 450A (which ever is the standard size).

i always thought that MCA = RLA * 1.25, but the math doesn't work out in this case. is this because RLA is just the motor and not the "system" (the chiller is made up of all sorts of internal components, motors, compressors, etc.)

if i were to run this chiller flat out at max, then put a current meter on it, what would it read, the RLA number? MCA ?

thanks for the clarification!

 

[Dennis Alwon]

I also thought the FLA times 125% would be the MCA in this case it is 225%. ????

 

[Dennis Alwon]

Does a chiller fall under art. 440 or art. 430? I think art. 440

 

[david luchini]

I'd be willing to bet that the RLA is for the compressor only, and there is another 160A of additional load, or there are two 160A compressor motors.

MCA would be 160x1.25 + 160 = 360

MOCP would be 160x2.25 = 360, 350A is standard OCP size.
350+160=510. 500A would be the standard size (not to exceed 510A). Per section 440.22.

 

[SLentions]

yes, i think there are additional loads within the chiller unit. the datasheet shows all sorts of motors and stuff.

so you would say that the cable is sized to MCA and the breaker is sized to whatever the standard value is between MCA and MOP? that sounds right to me.

I'm pretty confident now after reading more about chillers and your responses, that if I were to run this chiller at it's max, I would read with a clamp on meter a current somewhere around the MCA value.

 

[SLentions]

now, what if the cable size were a paultry 3/0 size (200A) and the breaker was also set at 200A. so these numbers are below MCA .. but at least the breaker will protect the cable.

seems to me the breaker will simply trip of the chiller is run at roughly 60% of it's capability or more...

is this a violation of NEC?

 

[david luchini]

the breaker is sized to whatever the standard value is between MCA and MOP?

I don't see any reason to size the breaker smaller than the MOP.

I'm pretty confident now after reading more about chillers and your responses, that if I were to run this chiller at it's max, I would read with a clamp on meter a current somewhere around the MCA value.

At full load, it should run somewhere around the MCA-25% of the largest motor. (320A in your example.)

now, what if the cable size were a paultry 3/0 size (200A) and the breaker was also set at 200A. so these numbers are below MCA .. but at least the breaker will protect the cable.

seems to me the breaker will simply trip of the chiller is run at roughly 60% of it's capability or more...

is this a violation of NEC?

Yes, that would be a violation of NEC. Per 440.33, the conductors must have an ampacity NOT LESS THAN the sum of the motor full load currents plus 25% of the largest motor. Conductors with an ampacity smaller than 360 would violate 440.33.

Regarding the c/b, the breaker can't be larger than the MOP, but also must be large enough to carry the starting current of the equipment (440.22(B).) So a 200A c/b MAY be in violation also.