Five 10 AWG THHN/THWN copper conductors are installed in a conduit in an area where the summertime temperature is 138ºF. What is the maximum size OCP permitted to protect the conductors in the conduit?

Five 10 AWG THHN/THWN copper conductors are installed in a conduit in an area where the summertime temperature is 138ºF. What is the maximum size OCP permitted to protect the conductors in the conduit?

• A. 20
• B. 25
• C. 30
• D. 35

Answer: (B)

Sizing the protection requires using the conductor’s ampacity after derating for both the number of current‑carrying conductors in the raceway and the actual ambient temperature. For five 10 AWG copper THHN/THWN conductors in a conduit, you start with the conductor’s base ampacity at the termination rating. THHN/THWN copper of 10 AWG is rated at 40 A on the 75°C column (higher than the 60°C column, which would be 30 A). Because there are more than three current‑carrying conductors in the conduit, you apply a derating factor for 4–6 conductors (the standard factor is about 0.80). This gives about 32 A. Next, you account for the summertime ambient temperature of 138°F (roughly 59°C). The NEC provides ambient temperature correction factors; for an ambient around 60°C the correction reduces the ampacity to about 0.78 of its value. Applying that, 32 A × 0.78 ≈ 25 A. Thus the overcurrent protection must be no larger than approximately 25 A to stay within the adjusted ampacity of the conductors in that conduit, making the 25 A option the correct choice. The general idea is: higher conductor rating, derate for multiple conductors, then apply ambient temperature correction; the end result picks a protection size that just fits the adjusted ampacity.

Sizing the protection requires using the conductor’s ampacity after derating for both the number of current‑carrying conductors in the raceway and the actual ambient temperature.

For five 10 AWG copper THHN/THWN conductors in a conduit, you start with the conductor’s base ampacity at the termination rating. THHN/THWN copper of 10 AWG is rated at 40 A on the 75°C column (higher than the 60°C column, which would be 30 A). Because there are more than three current‑carrying conductors in the conduit, you apply a derating factor for 4–6 conductors (the standard factor is about 0.80). This gives about 32 A.

Next, you account for the summertime ambient temperature of 138°F (roughly 59°C). The NEC provides ambient temperature correction factors; for an ambient around 60°C the correction reduces the ampacity to about 0.78 of its value. Applying that, 32 A × 0.78 ≈ 25 A.

Thus the overcurrent protection must be no larger than approximately 25 A to stay within the adjusted ampacity of the conductors in that conduit, making the 25 A option the correct choice. The general idea is: higher conductor rating, derate for multiple conductors, then apply ambient temperature correction; the end result picks a protection size that just fits the adjusted ampacity.