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Monday, February 14, 2022
Question:
Hello CQD,
I'm hoping you can settle a kerfuffle my co-worker and I are having about the code. We have a chiller that draws 306 FLA, with 400A fuses in a disconnect next to it, which is fed from a distribution panel where the feeder breaker is 500A. All the wiring is two parallel sets of #3/0 THHW, which together are good to 400A according to table 310.15(B((16). He claims that having a 500A breaker being the only protection device upstream of the 400A of wire is okay (this seems wrong to me). He points to the wire having overcurrent protection by the downstream devices of the fuses and motor overload, and short circuits being picked up by the breaker. I argue that because Figure 430.1 ends with the motor feeder wire (the breaker in the distribution panel is not shown) that the sizing of that breaker is not included in section 430. Therefore, either the breaker should be 400A to protect the feeder wire, or the wire should be larger.
Thanks for your opinion.
Curtis Syswerda
A
Answer:
Hi Curtis,
The simple response to your question is don't expect the conductors to be protected at their rated ampacity from Table 310.15(B)(16). This is a motor load and the rules in Article 430 apply. Allowances have to be provided for motor-locked rotor (inrush) current during starting. The total overcurrent protection for the motor circuit is the combination of the branch circuit short-circuit and ground-fault protective device and the overload protection for the motor. Section 430.6 and Example D8 in Table 9 do a good job of describing the Code requirements for motors. Section 430.52 provides information about sizing the branch circuit short-circuit and ground-fault protective device for a motor, depending on the type of OCPD selected for the installation. According to Table 430.52, a breaker could be sized at up to 800 percent of the motor FLA (for an instantaneous trip) and up to 250 percent of the motor FLA (for inverse time). For a one-time fuse, it could be sized at 300 percent of the motor FLA and for a dual-element, time-delay fuse it could be sized up to 175 percent of the motor FLA. The branch circuit conductors for the motor circuit should be sized at 125% of the FLA of the motor, based on the motor FLA in the applicable tables in Article 430. Check Section 430.6(A) and Example D8 in NEC Chapter 9.
Hope this helps.