HOLT Electricians Practice Exam

For a branch circuit serving three different motor sizes, what feeder size is required if rated for 75 C?

1. 2/0 THHN

2. 3/0 THHN

3. 4/0 THHN

4. 2 THHN

The correct answer is: 4/0 THHN

When determining the correct feeder size for a branch circuit serving motors, it's essential to consider the total load of the motors and the applicable ampacity of the conductors at the rated temperature. For conductors rated at 75°C, specific guidelines are presented in the National Electrical Code (NEC) for sizing conductors based on the motor ratings. In this scenario, if you select 4/0 THHN as the required feeder size, it indicates that the combined full-load currents of the three motors exceed the ampacity limits of smaller wire sizes. Generally, 4/0 THHN conductors have a higher ampacity, allowing them to handle larger electrical loads safely without overheating. Using the 75°C rating is crucial because it dictates the maximum allowable current for the chosen conductor material at that temperature rating. By selecting the 4/0 size, you are ensuring that the branch circuit can adequately supply power to the motors while adhering to safety standards, and mitigating the risk of conductor overheating. To summarize, the choice of 4/0 THHN aligns with the need for adequate capacity to support motor loads, considering both the ampacity required by the circuit's operating conditions and the associated safety regulations.