What is the minimum size of the ungrounded conductors in each conduit feeding Panel A?

What is the minimum size of the ungrounded conductors in each conduit feeding Panel A?

• A. 3/0
• B. 2/0
• C. 4
• D. 4/0

Answer: (A)

The main idea is sizing feeder conductors to match the panel’s rating using the conductor ampacity in the 75°C column for copper in conduit. If Panel A is fed by a 200-amp feeder, the ungrounded conductors in each conduit must have an ampacity of at least 200 A. Looking at common copper sizes with THHN insulation in the 75°C column, 2/0 AWG is about 115 A, 4 AWG is about 85 A, 4/0 AWG is about 230 A, and 3/0 AWG is around 200 A. So 3/0 copper is the smallest size that meets the 200 A requirement. That’s why 3/0 copper is the correct minimum. Larger sizes like 4/0 are allowed but not the minimum, and smaller sizes would not suffice for a 200 A feeder.

The main idea is sizing feeder conductors to match the panel’s rating using the conductor ampacity in the 75°C column for copper in conduit. If Panel A is fed by a 200-amp feeder, the ungrounded conductors in each conduit must have an ampacity of at least 200 A. Looking at common copper sizes with THHN insulation in the 75°C column, 2/0 AWG is about 115 A, 4 AWG is about 85 A, 4/0 AWG is about 230 A, and 3/0 AWG is around 200 A. So 3/0 copper is the smallest size that meets the 200 A requirement. That’s why 3/0 copper is the correct minimum. Larger sizes like 4/0 are allowed but not the minimum, and smaller sizes would not suffice for a 200 A feeder.