What is the maximum size dual-element time-delay fuse allowed for a 10-horsepower, single-phase motor?

For a 10-horsepower, single-phase motor, the maximum size dual-element time-delay fuse is determined based on the National Electrical Code (NEC) guidelines. When sizing fuses for motor circuits, the NEC recommends that the maximum fuse size should not exceed 250% of the motor's full-load current.

For a 10-horsepower single-phase motor running at 230 volts, the full-load current can typically be calculated to be around 50 amps (though this varies slightly based on specific motor efficiency and power factor ratings). Multiplying this full-load current by 2.5 gives a maximum fuse size of 125 amps. However, the next step involves looking at the practical and commonly available sizes of dual-element fuses that best suit this requirement.

Among the options provided, 90 amps falls within the permissible range and is a practical choice for providing adequate protection for starting currents, which can exceed the full-load current significantly, especially in larger motors like a 10-horsepower unit. It balances the need for protecting the motor against overload and short circuit conditions without leading to nuisance tripping during normal motor startup conditions.

This makes 90 amps the appropriate answer for the maximum size dual-element time-delay fuse allowed for a