Master Electrician Practice Exam

What is the maximum permitted operational setting for an adjustable inverse time circuit breaker for a 15 hp, 208-volt motor?

1. 100.5 amperes

2. 115.5 amperes

3. 120.5 amperes

4. 130.5 amperes

The correct answer is: 115.5 amperes

To determine the maximum permitted operational setting for an adjustable inverse time circuit breaker protecting a 15 horsepower motor operating at 208 volts, it's important to apply the applicable NEC (National Electrical Code) guidelines. First, we need to calculate the full-load current of a 15 hp motor. According to NEC Table 430.250, which provides full-load current values for three-phase motors, a 15 hp motor at 208 volts has a full-load current of approximately 42.5 amperes. When using an inverse time circuit breaker, the NEC allows for an adjustment in settings based on the motor's full-load current. The typical setting for a thermal overload relay is 125% of the full-load current to ensure appropriate protection for the motor while also allowing it to handle inrush currents during startup. Therefore, we take the full-load current and calculate 125%: \[ 42.5 \text{ A} \times 1.25 = 53.125 \text{ A} \] However, for sizing a circuit breaker, the NEC provides a standard rule, where it states the maximum setting for motors should not exceed 250% of the full-load current for inverse time breakers. Applying this for a 15