By what percentage must the allowable ampacity for eleven No. 12 THHN copper conductors be adjusted?

• A. 25%
• B. 40%
• C. 50%
• D. 60%

Answer: (C)

The allowable ampacity for conductors must be adjusted based on the number of conductors grouped together, as specified in applicable electrical codes, such as the National Electrical Code (NEC). When multiple conductors are installed in the same raceway, additional heat can build up due to the closeness of the cables, which can lead to overheating.

For eleven No. 12 THHN copper conductors, the NEC indicates that there is a requirement to adjust the ampacity due to this phenomenon. Specifically, if there are more than three conductors in a raceway or cable, the allowable ampacity must be derated.

In this case, for eleven conductors, the ampacity adjustment factor is 50%. According to the NEC Table 310.15(B)(3)(a), it specifies that if you have more than nine conductors in a raceway, you should reduce the allowable ampacity to maintain safety and effective operation. Therefore, the correct adjustment to the allowable ampacity for eleven No. 12 THHN copper conductors is 50%.