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.