What is the maximum standard fuse size for a 3-phase, 75-horsepower, 240-volt motor with a full-load current of 192 A, using time-delay dual-element fuses (next higher standard rating allowed)?

What is the maximum standard fuse size for a 3-phase, 75-horsepower, 240-volt motor with a full-load current of 192 A, using time-delay dual-element fuses (next higher standard rating allowed)?

• A. 300 A
• B. 350 A
• C. 360 A
• D. 400 A

Answer: (B)

Sizing motor fuse protection uses a multiplier of the full-load current to accommodate startup surges, and for time-delay (dual-element) fuses used on motors the fuse rating must be not less than 175% of the motor’s full-load current. Multiply 192 A by 1.75 to get 336 A. Since fuses come in standard ratings, you choose the smallest standard size at least 336 A. The next standard rating above 336 A is 350 A, so the appropriate fuse size is 350 A. This is why the correct choice is 350 A: it meets the minimum 175% of FLC and uses the next available standard rating. Larger options like 360 A or 400 A would also protect the circuit but aren’t the smallest standard rating that meets the requirement, and 300 A would be below the minimum.

Sizing motor fuse protection uses a multiplier of the full-load current to accommodate startup surges, and for time-delay (dual-element) fuses used on motors the fuse rating must be not less than 175% of the motor’s full-load current. Multiply 192 A by 1.75 to get 336 A. Since fuses come in standard ratings, you choose the smallest standard size at least 336 A. The next standard rating above 336 A is 350 A, so the appropriate fuse size is 350 A. This is why the correct choice is 350 A: it meets the minimum 175% of FLC and uses the next available standard rating. Larger options like 360 A or 400 A would also protect the circuit but aren’t the smallest standard rating that meets the requirement, and 300 A would be below the minimum.