Breaker size for Chiller replacement

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john2016

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I'm currently working on a Chiller replacement project. The existing chiller is fed by an existing 3P1200A breaker. On the chiller manufacturer's cut sheet it states the following for the electrical data.

Compressor Electrical Data
Power Input: 187.9 kw
Nominal Running Current: 265A
Rated Load Current: 333A
Locked Rotor Current: 366A
There are a total of 4 compressors

Chiller Electrical Data
Electrical: 460V, 3Ph, 60Hz
Power Input: 751.6 kw
Nominal Running Current: 1060A
MOP: 1391A
MCA: 1126A

The existing chiller is currently being fed by a 3P1200A breaker. Being that the MCA is at 1126A and the MOP says 1391A, am I able to reuse the existing 3P1200A breaker or do I need to upsize it to the next larger size?
If I calculate it using 175% it would equate out to 265A x 175% + 265 * 3 (additional motors) = 1258.75A

Thank you.
 
Since 1200 is greater than the MCA of 1126A the NEC is perfectly happy with you using a 1200A breaker.
But there is a chance that at that value you may be seeing nuisance trips.
In that case a larger breaker may be useful. But since the MOP is 1391, you could not just use a 1400A breaker. I don't think the "next size up" rule applies to MOP, and in any case you are over 800A.
I am not sure whether the code will allow you to use a larger breaker with and adjustable rather than fixed trip unit to get close the 1391 figure.

You may be lucky and not have nuisance trips with 1200, but you need to recognize that you may be forced to change it once it has been tested.
 
Thank you for your response. I'll need to look into this further. I believe NEC also allows you to use 225% factor fo,r start up current. Anyone else have an opinion? Thanks.
 
john2016 said:
Thank you for your response. I'll need to look into this further. I believe NEC also allows you to use 225% factor fo,r start up current. Anyone else have an opinion? Thanks.
Click to expand...

You're required to use the nameplate values.
 
john2016 said:
Thank you for your response. I'll need to look into this further. I believe NEC also allows you to use 225% factor fo,r start up current. Anyone else have an opinion? Thanks.
Click to expand...
This NEC allowance does not override the manufacturer's MOP, which in theory already includes any necessary allowance for starting.

One rationale is that machines that give a nameplate MOP typically contain more than just one motor, and so someone who has access to the full package for testing can determine whether the MOP given really does allow for motor starting. And that that value of OCP does in fact properly protect all of the components of the machine from short circuits and ground faults.
 
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