What is the maximum size dual-element fuse for a 40hp, 3Ø, 460V motor with a Design Letter D?

• A. 80A
• B. 90A
• C. 100A
• D. 110A

Answer: (C)

To determine the maximum size dual-element fuse for a 40-horsepower (hp), three-phase (3Ø), 460V motor with a Design Letter D, it is necessary to first calculate the full-load current (FLC) of the motor.

The full-load current for three-phase motors can be calculated using the formula:

FLC = (Horsepower × 746) / (Voltage × √3 × Efficiency × Power Factor)

For a 40hp motor at 460V, if we assume typical efficiency and power factor values, the FLC comes out to around 50-54 amps. However, for the purposes of selecting fuses, the National Electrical Code (NEC) provides guidelines for the maximum fuse size based on the motor's Design Letter.

A Design Letter D indicates that the motor has a high starting current, and the NEC suggests using a fuse rating of 250% of the full-load current. Therefore, if we accept an FLC of about 50 amps, we would multiply this by 2.5 (which corresponds to 250%):

50A x 2.5 = 125A.

However, dual-element fuses are designed to protect against both overloads