What is the maximum standard size rating of the motor branch-circuit short-circuit and ground-fault protective device for a 5-horsepower, 208-volt, 3-phase motor using time-delay fuses when starting current is not an issue?

What is the maximum standard size rating of the motor branch-circuit short-circuit and ground-fault protective device for a 5-horsepower, 208-volt, 3-phase motor using time-delay fuses when starting current is not an issue?

• A. 20 A
• B. 25 A
• C. 30 A
• D. 40 A

Answer: (C)

The main idea is how to size motor branch-circuit protection when using time-delay fuses and the starting surge isn’t a concern. For motors, the protection is tied to the full-load current, with NEC guidance allowing the protective device rating to be higher than the motor’s running current to accommodate inrush, but still within a defined upper limit. First estimate the motor’s full-load current. A 5-horsepower, 208-volt three-phase motor has a full-load current around 14 A (rough calculation: I_FL ≈ HP × 746 / (√3 × V × η × PF); using η ≈ 0.9 and PF ≈ 0.8 gives about 14 A). With time-delay fuses, you can size the protection up to about 2.5 times the full-load current when starting current isn’t an issue. That gives an upper limit of roughly 2.5 × 14 A ≈ 35 A. Standard fuse sizes available are typically 15, 20, 25, 30, 35, 40 A, etc. The largest standard size that stays within the 35 A limit is 30 A (in many standard sets, 35 A may or may not be used, but the common practical choice here is 30 A). So, the maximum standard size rating you’d use for this motor with time-delay fuses and no starting-current constraint is about 30 A.

The main idea is how to size motor branch-circuit protection when using time-delay fuses and the starting surge isn’t a concern. For motors, the protection is tied to the full-load current, with NEC guidance allowing the protective device rating to be higher than the motor’s running current to accommodate inrush, but still within a defined upper limit.

First estimate the motor’s full-load current. A 5-horsepower, 208-volt three-phase motor has a full-load current around 14 A (rough calculation: I_FL ≈ HP × 746 / (√3 × V × η × PF); using η ≈ 0.9 and PF ≈ 0.8 gives about 14 A).

With time-delay fuses, you can size the protection up to about 2.5 times the full-load current when starting current isn’t an issue. That gives an upper limit of roughly 2.5 × 14 A ≈ 35 A.

Standard fuse sizes available are typically 15, 20, 25, 30, 35, 40 A, etc. The largest standard size that stays within the 35 A limit is 30 A (in many standard sets, 35 A may or may not be used, but the common practical choice here is 30 A).

So, the maximum standard size rating you’d use for this motor with time-delay fuses and no starting-current constraint is about 30 A.