Consider a 3-phase chiller driven by the 4160V secondary of a utility transformer through a 600A load disconnect switch and a 400A fuse. The conductors are #4/0 MV-90. The normal chiller load is 178A.
The conductor ampacity (from Table 310.77) is 235A. Thus, the conductors are suitably rated for the load. However, the 400A fuse rating seems to be too high to protect the conductors. Nonetheless, NEC 240.101(A) allows a fuse rating up to 3 times the conductor ampacity (for overcurrent protection for circuit voltages > 600V).
This just doesn't feel right to me. Am I missing something? Is there something else in the code that would limit the size of the overcurrent protector? Or is this installation in conformance with the NEC? And if it is, why are overloads that significantly can exceed ampacity ratings allowed in this case?
Would much appreciate your usual expert comments.
Heinz R.
The conductor ampacity (from Table 310.77) is 235A. Thus, the conductors are suitably rated for the load. However, the 400A fuse rating seems to be too high to protect the conductors. Nonetheless, NEC 240.101(A) allows a fuse rating up to 3 times the conductor ampacity (for overcurrent protection for circuit voltages > 600V).
This just doesn't feel right to me. Am I missing something? Is there something else in the code that would limit the size of the overcurrent protector? Or is this installation in conformance with the NEC? And if it is, why are overloads that significantly can exceed ampacity ratings allowed in this case?
Would much appreciate your usual expert comments.
Heinz R.