HOLT Electricians Practice Exam

If using an inverse-time breaker, what size protection is required for a set of phase motors with specific horsepower ratings?

1. 150 amperes

2. 175 amperes

3. 200 amperes

4. 180 amperes

The correct answer is: 175 amperes

To determine the proper size of protection required for a set of phase motors using an inverse-time breaker, several factors must be considered, specifically the motor horsepower and the applicable NEC (National Electrical Code) guidelines. Inverse-time breakers are designed to handle temporary overloads caused by motor startup conditions, which often draw significantly more current than normal operating parameters. Typically, for three-phase motors, the NEC suggests using a protection scheme based on the horsepower rating and efficiency of the motor. For a specified horsepower rating, the typical calculation involves taking the full-load current of the motor. This value is influenced by the motor’s efficiency and voltage, with ampacity tables available for specific horsepower ratings. The motor protection must allow for a reasonable margin during startup without tripping unnecessarily while still protecting the equipment and conductors from sustained overloads. In this case, selecting 175 amperes as the protective device size aligns with the standard practice of rounding up to the nearest size that would provide adequate protection under the anticipated operational conditions of the motors. This sizing allows for adequate headroom during startup while also being sufficient to protect against sustained overload scenarios. This understanding stems from the need for protectiveness yet flexibility in design for motor applications, and thus, positioning the breaker at 175 amper