What is the maximum size inverse time circuit breaker permitted for a single-phase, 7.5 hp motor operating at 208 volts?

To determine the maximum size inverse time circuit breaker allowed for a single-phase, 7.5 hp motor operating at 208 volts, it's important to understand the relationship between motor horsepower, voltage, and the required protection.

For a single-phase motor, the full-load current can be calculated using a standard formula or reference table. For a 7.5 hp motor at 208 volts, the full-load current is typically around 36 amps. According to the National Electrical Code (NEC), when selecting a circuit breaker for motor protection, you can size the inverse time circuit breaker at 250% of the motor's full-load current.

Calculating 250% of the 36 amps gives approximately 90 amps. However, the NEC requires that for inverse time breakers, you must also consider the breaker size limitations specifically for motors. The NEC limits the size of the breaker to 125% of the full-load current to ensure that the circuit breaker provides adequate protection while allowing for temporary inrush currents when the motor starts.

Calculating 125% of 36 amps results in 45 amps. Since conventional circuit breaker sizes are typically found in standard values, the next standard breaker size available that accommodates this requirement while staying below 125% is