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

To determine the maximum size dual-element fuse for a 40 hp, 3-phase, 460V motor with a Design Letter D, first, we need to understand the relationship between motor horsepower and the permissible fuse rating.

For a Design Letter D motor, the standard practice is to use a rating that corresponds to the full-load current (FLC) multiplied by a factor outlined in the National Electrical Code (NEC). For a 3-phase motor specifically, it’s common to refer to the American National Standards Institute (ANSI) or the NEC tables to find the full-load current for a 40 hp motor operating at 460V.

Typically, for a 40 hp motor, the FLC is approximately 48 amps when calculated. For dual-element fuses, which provide both overload and short-circuit protection, the maximum size is often determined by applying a multiplier to the full-load current based on the motor's design letter, which accounts for the starting current.

In the case of a Design Letter D, this suggests that the motor has a higher starting current, and as such, it may require a fuse that can tolerate this initial surge without blowing. According to the NEC guidelines, the recommended maximum size for dual-element fuses is usually