4. A 480-volt, 3-phase, 4-wire, 225-ainp, main lugs only (MLO) (no main breaker)

Question

4. A 480-volt, 3-phase, 4-wire, 225-ainp, main lugs only (MLO) (no main breaker) lighting panelboard will be fed from a spare fused disconnect switch in the building's service entrance switchgear. The fused disconnect switch protecting the panelboard and feeder conductors will be fused with a 150-amp fuse. Both the fused disconnect and panelboard have 75 degree C conductor terminations. The lighting panel serves 277-volt metal halide lighting load exclusively. (a) Assuming RHW compact aluminum conductors and a dry location, what would be the minimum phase conductor size that could be used for this 150-amp feeder? (b) What is the minimum RHW compact aluminum equipment grounding conductor (EGC) that can be used for this ISO-amp feeder? (c) What is the minimum electrical metallic tubing (EMT) trade size that can be used for this ISO-amp feeder?

Explanation / Answer

Consider the given 480-V, 3-phase, 4-wire, 225-A MLO lighting panel board.

(a)

The National Electrical Code

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