What is the maximum fuse size for a single-element, non-time-delay fuse serving a 10hp, three-phase motor?

To determine the maximum fuse size for a single-element, non-time-delay fuse serving a 10 hp, three-phase motor, it's essential to understand the specifications outlined in the National Electrical Code (NEC) and the relationship between horsepower, voltage, and current.

For a three-phase motor, the full-load current can be estimated using the formula:

[ \text{FLA} = \frac{746 \times \text{HP}}{\text{Voltage} \times \sqrt{3} \times \text{Efficiency} \times \text{Power Factor}} ]

Assuming efficiency and power factor values that are commonly used (for instance, 0.85 efficiency and 0.9 power factor), the full-load current for a 10 hp motor can be calculated at standard voltage levels. This results in a full-load current typically around 30 to 40 amps depending on the specific motor design and voltage.

For motor circuit protection, NEC allows for sizing fuses higher than the calculated full-load current. For single-element, non-time-delay fuses, the maximum size is often recommended as 250% of the full-load current at its rated load. Therefore, if the full-load current is approximately 32 amps, the