What is the ampacity for each of 8 No.2 THHN copper conductors in a single conduit?

What is the ampacity for each of 8 No.2 THHN copper conductors in a single conduit?

• A. 75 A
• B. 90 A
• C. 80.5 A
• D. 85 A

Answer: (C)

Ampacity is the maximum current a conductor can carry without overheating its insulation in a given situation. When multiple current-carrying conductors share the same conduit, you must apply a derating factor because heat from neighboring conductors reduces the amount of current each one can safely carry. For No.2 copper THHN, the published ampacity at the 75°C rating is 115 A per conductor. With eight current-carrying conductors in the same conduit, the NEC derating table uses a factor of 0.70 for 7–9 conductors. Multiply 115 A by 0.70 to get 80.5 A. So the ampacity per conductor in this setup is 80.5 A. If you were using 90°C-rated terminations, you could apply a different base ampacity, but with typical terminations the 75°C-based calculation yields 80.5 A.

Ampacity is the maximum current a conductor can carry without overheating its insulation in a given situation. When multiple current-carrying conductors share the same conduit, you must apply a derating factor because heat from neighboring conductors reduces the amount of current each one can safely carry.

For No.2 copper THHN, the published ampacity at the 75°C rating is 115 A per conductor. With eight current-carrying conductors in the same conduit, the NEC derating table uses a factor of 0.70 for 7–9 conductors. Multiply 115 A by 0.70 to get 80.5 A.

So the ampacity per conductor in this setup is 80.5 A. If you were using 90°C-rated terminations, you could apply a different base ampacity, but with typical terminations the 75°C-based calculation yields 80.5 A.