400 HP Chiller

Status
Not open for further replies.

travis301

Member
Location
Missouri
We are going to install a 400HP chiller package.The name plate on it says "FLA 414 Amps". At first thought I was going to size the wire off of this nameplate at 125% and go on ,then I got to thinking about sizing the wire off of the code book for a 400 HP motor which was how I was taught to do on any other standard motor. Well the code book has the amperage considerably higher (I think 40-50 amps off the top of my head) so Im at a loss right now for which way to go.Any thoughts which way would be code compliant?Travis
 

M. D.

Senior Member
I found this ,.. I hope it helps.

http://ecmweb.com/mag/electric_stumped_code_10/

Q. I designed a chiller plant based on a shop drawing that listed the following: rated-load current (RLC) at 147A, minimum circuit ampere at 184A, and maximum overcurrent protection at 250A. I specified a 250A circuit breaker with 4/0 AWG conductors (184A?1.25=230A). However, a couple engineers in our office say that 3/0 AWG, rated 200A at 75?C, could be used because you don't multiply the minimum circuit ampere rating by 1.25. They also say if I use 3/0 AWG, then the circuit protection device must be sized not greater than 200A. Are they right?


A. Almost. It's true that the minimum circuit conductor size is based on the ?minimum circuit ampacity? as identified on the equipment nameplate, which is 184A. This is because the rated minimum circuit ampacity for the motor-compressor already includes 125% of the motor-compressor RLC (440.32) (147A?1.25=184A). Table 310.16 identifies 3/0 AWG conductor at 75?C as having an ampacity of 200A, which is acceptable for this minimum circuit ampacity.

The branch-circuit conductors, control apparatus, and motors in circuits that supply hermetic refrigerant motor-compressors must be protected against overcurrent due to short circuits and grounds by a protective device with a rating or setting that doesn't exceed 175% of the motor-compressor rated-load amperes (RLA) (440.22) (147A?1.75=257A). So a 250A circuit breaker is fine to protect the 3/0 AWG conductor for hermetic refrigerant motor-compressors.

Sec. 440.22 does permit overcurrent protection to be as large as 225% of the motor-compressor RLA if the 175% rating isn't sufficient to carry the starting current of the motor. The nameplate on the a/c equipment considers this factor.
 
Status
Not open for further replies.
Top