My application is that I have an existing application installed in the 70's where a 1000A cable is feeding an 800A Busway and I would like to keep the existing 1000A fuses for overcurrent protection at the primary of the 1000A cable. In Busway - 368.17(B), the exception is that you don’t have to provide overcurrent protection for the reduced amperage busway if the total length of the reduced ampacity bus is less than 50 ft, the reduced ampacity is at least 1/3 the size of the overcurrent protection for the larger bus, etc.
My problem is that the NEC does not specifically address my situation one way or the other since it is not covered in section 240 either. If I had a 1000A busway feeding a 800A busway, I could claim this exception and fuse it at 1000A. However, since I have a 1000A cable feeding a 800A busway, my application is not specifically covered even though the principle is the same as is covered in the bus-bus exception. My question is how do I determine my minimum requirement? Note that I am in an industrial plant that does not have local inspectors come in. Thanks!
