In a conduit with 9 conductors, the wire to Load #3 is 6 AWG, the wire to Load #2 is 1 AWG, and the wire to Load #1 is 2/0 AWG. Each of the loads is 3Ø, 3-wire. In the high-temperature area where the temperature is 133°F, how much current can the wire to Load #3 carry?

• A. 32
• B. 26
• C. 43
• D. 37

Answer: (D)

The lesson here is how to derive ampacity when multiple current‑carrying conductors share a raceway in a high ambient temperature. You start with the conductor’s base ampacity from the insulation’s temperature rating, then apply two deratings: one for the number of current‑carrying conductors in the conduit, and one for the ambient temperature.

For a 6 AWG copper conductor with common high‑temperature insulation (like THHN), the base ampacity in the 90°C column is about 75 A. In this conduit there are nine current‑carrying conductors (three 3‑wire three‑phase loads), so the derating factor for number of conductors is about 0.70. The ambient temperature is 133°F (about 56°C), which requires another derating factor related to temperature, which is also around 0.70 for this situation.

Multiply the factors: 75 A × 0.70 × 0.70 ≈ 37 A. Therefore, the wire to Load #3 can carry about 37 A in that high‑temperature, fully loaded conduit.