Chiller Motor - FLA, MCA and Circuit Breaker Size
Chiller Motor - FLA, MCA and Circuit Breaker Size
(OP)
Good Morning everyone,
I hope everyone had a nice weekend. I haven't had to do a motor cable specification in quite some time, so just looking to make sure I am not to rusty.
I have a Chiller motor with an FLA of 1045 amps and a MCA of 1306.
Because the MCA is 1306 amps, I will have to provide wire for at least 1306 amps. So in this particular case, 3 sets of 600s would not be acceptable and we would have to go to 4 sets of 400s minimum.
My question is what size breaker can be utilized on this equipment? Is it allowable to use a 1200 amp breaker with 1200 amp class L fuses (or other appropriate fuse class for the equipment we are feeding?) The chiller is VSD driven so inrush current is limited to the maximum FLA, so inrush is not a concern. The 1200 amp fuse curves are also fine for continued operation at the 1045 amp design power consumption.
More generally stated: "Is there a requirement that the breaker size is larger than the MCA determined cable size by any certain margin?" I don't believe so, but just want to double-check that I am not missing anything from a code compliance perspective.
This is all 60hz, 460V power.
Thanks
Rob
I hope everyone had a nice weekend. I haven't had to do a motor cable specification in quite some time, so just looking to make sure I am not to rusty.
I have a Chiller motor with an FLA of 1045 amps and a MCA of 1306.
Because the MCA is 1306 amps, I will have to provide wire for at least 1306 amps. So in this particular case, 3 sets of 600s would not be acceptable and we would have to go to 4 sets of 400s minimum.
My question is what size breaker can be utilized on this equipment? Is it allowable to use a 1200 amp breaker with 1200 amp class L fuses (or other appropriate fuse class for the equipment we are feeding?) The chiller is VSD driven so inrush current is limited to the maximum FLA, so inrush is not a concern. The 1200 amp fuse curves are also fine for continued operation at the 1045 amp design power consumption.
More generally stated: "Is there a requirement that the breaker size is larger than the MCA determined cable size by any certain margin?" I don't believe so, but just want to double-check that I am not missing anything from a code compliance perspective.
This is all 60hz, 460V power.
Thanks
Rob
RE: Chiller Motor - FLA, MCA and Circuit Breaker Size
" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden
RE: Chiller Motor - FLA, MCA and Circuit Breaker Size
I believe the minimum requires short circuit withstand rating of a 1000A+ motor controller is 85kA. I'm not sure if that applies to VFD's as well but it certainly applies to soft-starters and contactors.
RE: Chiller Motor - FLA, MCA and Circuit Breaker Size
Mike
RE: Chiller Motor - FLA, MCA and Circuit Breaker Size
I should have provided some additional information.
The VFD manufacturer did provide a maximum OCP size, which is typical. The SCCR rating of the drive is much greater than the available fault current. The drive is at the top of a building and the service entrance is at the bottom.
To restate my quesiton.
- I have a motor load with an FLA of 1050 amps.
- The MCA is thus 1312 because it is a chiller
My traditional interpretation of this would be that I cannot use a disconnect switch anywhere in the run with a frame rating of less than 1312. The fuse can be selected for any amp level as described above. However, given the requirement to provide MCA of at least 1312, and knowing that you are prohibited from putting wiring rated for anything less than 1312 anywhere in the run, I would assume you need to have a switchgear bus rating of at least the MCA.
I just want to confirm, I am not crazy.
Rob
RE: Chiller Motor - FLA, MCA and Circuit Breaker Size
RE: Chiller Motor - FLA, MCA and Circuit Breaker Size
" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden