Electrical engineers: Carefully use the Neher-McGrath formula

摘要

The history and mystery of the Neher-McGrath formula must be carefully reviewed and used, for electrical engineers, even though it appears straightforward. A while back, a colleague brought up a problem that had occurred on a $100 million industrial project. A few months after plant turnover, several feeder breakers started tripping. There are few things that will cost an electrical engineer more sleep than random breaker operations. After considerable consternation, the under-slab feeders were pulled out and found to have heat damaged insulation. The cables had been sized per National Electrical Code (NEC) Article 310 and there wasn't any overloading, so what was the problem?