Our customers often disagree about how 3 phase branch circuits should be protected when they serve control transformers. We interpret the electrical code(s) as required, but I am interested to see what others think about this relatively simple issue.
Specifically, imagine a simple enclosure with three motor starters (IEC combos, each with line disconnect/circuit breaker, the contactor, and output overload), all branched from a single main panel disconnect/circuit breaker. The 3 phase wiring from the main panel disconnect is daisy chained from starter to starter, using the same AWG wire, rated according to the main circuit breaker. An additional branch, connected identially as with the starters, serves a control transformer, which has the normal fuse on it's secondary. The issue that keeps coming up is: does the transformer PRIMARY need a fuse/circuit breaker? Customer opinion comes to about 50/50 on this, and all can support opinion with pages from the NEC. I am interested to hear what others, expert or otherwise, do in this (common!) situation.
Specifically, imagine a simple enclosure with three motor starters (IEC combos, each with line disconnect/circuit breaker, the contactor, and output overload), all branched from a single main panel disconnect/circuit breaker. The 3 phase wiring from the main panel disconnect is daisy chained from starter to starter, using the same AWG wire, rated according to the main circuit breaker. An additional branch, connected identially as with the starters, serves a control transformer, which has the normal fuse on it's secondary. The issue that keeps coming up is: does the transformer PRIMARY need a fuse/circuit breaker? Customer opinion comes to about 50/50 on this, and all can support opinion with pages from the NEC. I am interested to hear what others, expert or otherwise, do in this (common!) situation.