Electrical upgrade needed?

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Blind in Texas

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We have a site where the existing chiller failed. The electrical is still functioning perfectly. The existing installation was in 1984 or so. The existing chiller had an MCA of 697A and the replacement chiller has an MCA of 635A. The existing feed is (6) 500kcmil conductors in a single 4" C. The derate for more than 3 conductors in a single conduit brings the allowable ampacity to 608A. Since this is a one for one replacement of the chiller (repair of an existing mechanical system) and no electrical changes are taking place are we required to replace the feeders?
 
Blind in Texas said:
We have a site where the existing chiller failed. The electrical is still functioning perfectly. The existing installation was in 1984 or so. The existing chiller had an MCA of 697A and the replacement chiller has an MCA of 635A. The existing feed is (6) 500kcmil conductors in a single 4" C. The derate for more than 3 conductors in a single conduit brings the allowable ampacity to 608A. Since this is a one for one replacement of the chiller (repair of an existing mechanical system) and no electrical changes are taking place are we required to replace the feeders?
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Unfortunately I say yes, you must replace the feeder. If the original install was not wire properly then the new install must comply with the nec.
 
I am curious how you got 608-- I get 688amps on 6 - 500kcm conductors, assuming 90C conductors. Are the conductors 75C?
 
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