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Chiller Motor - FLA, MCA and Circuit Breaker Size

Chiller Motor - FLA, MCA and Circuit Breaker Size

Chiller Motor - FLA, MCA and Circuit Breaker Size

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.


RE: Chiller Motor - FLA, MCA and Circuit Breaker Size

Generally, the VFD manufacturer will provide you with not only the maximum OCPD size, but also the type that it has been tested and listed with. 460V 60Hz implies you are in North America, where the Short Circuit Current Rating (SCCR) of the drive will be required to be greater than or equal to the available fault current in your system. Without series listings, usually with fuses, the “courtesy” SCCR will only be no more than 10kA and fir a circuit capable of 1300A, I can guarantee that your fault current will be more than 10kA. So the size and type of OCPD that the drive is listed with is something you must get from the specific mfr of that drive, anything else is a SWAG.

" 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

Protection is typically given as a maximum rating only both in codes and by manufacturers to co-ordinate. You could install 1A fuses and pass the inspection even though it would never work.

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

My opinion (was an electrician in another life)...anything over 500's is a pain to train and work with. Granted, sometimes you have no choice.


RE: Chiller Motor - FLA, MCA and Circuit Breaker Size

Good Morning and thank you for the replies.

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.


RE: Chiller Motor - FLA, MCA and Circuit Breaker Size

And I completely agree. We try as hard as possible, not to use anything bigger than 500s. 600s are much more than 20% harder to work with than 500s.

RE: Chiller Motor - FLA, MCA and Circuit Breaker Size

MCA is Minimum CIRCUIT Ampacity, so yes, everything in that circuit must be rated for 1306A or greater. But also, 430.122 requires that the conductors feeding the VFD are rated for a minimum of 125% of the VFD INPUT current rating. In most cases the VFD input current is going to be lower than the output rating (because of PF improvement), but not always, because of foreign suppliers basing their sizing on standard IEC motor kW ratings, not NEMA HP. So that's something specific that only you can determine.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

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