The minimum size aluminum feeder conductors for a post office requiring a 125-amp subpanel is ___.

The minimum size aluminum feeder conductors for a post office requiring a 125-amp subpanel is ___.

• A. 1 AWG
• B. 2/0 AWG
• C. 2 AWG
• D. 1/0 AWG

Answer: (B)

Sizing a feeder means picking conductors whose ampacity meets or exceeds the panel’s feeder rating. For an aluminum feeder to a 125-amp subpanel, you must use conductors that can carry at least 125 A. With modern insulation (75°C rating), 2/0 AWG aluminum has about 135 A of ampacity, which covers the 125 A requirement. A smaller size like 1/0 AWG aluminum is typically around 120 A at 75°C, which is not enough for a 125-amp feeder, and 2 AWG or smaller would carry even less. So the minimum aluminum size that reliably meets or exceeds 125 A is 2/0 AWG. If the installation used a lower temperature rating, you’d need to size even larger, but with common 75°C-rated insulation, 2/0 is the standard minimum.

Sizing a feeder means picking conductors whose ampacity meets or exceeds the panel’s feeder rating. For an aluminum feeder to a 125-amp subpanel, you must use conductors that can carry at least 125 A. With modern insulation (75°C rating), 2/0 AWG aluminum has about 135 A of ampacity, which covers the 125 A requirement. A smaller size like 1/0 AWG aluminum is typically around 120 A at 75°C, which is not enough for a 125-amp feeder, and 2 AWG or smaller would carry even less. So the minimum aluminum size that reliably meets or exceeds 125 A is 2/0 AWG. If the installation used a lower temperature rating, you’d need to size even larger, but with common 75°C-rated insulation, 2/0 is the standard minimum.