A 300-amp breaker feeds a 225 kVA transformer whose secondary leads go to a panelboard with a 200-amp main; the secondary run is 13 feet. The conductors must be at least which size?

A 300-amp breaker feeds a 225 kVA transformer whose secondary leads go to a panelboard with a 200-amp main; the secondary run is 13 feet. The conductors must be at least which size?

• A. 2/0 AWG
• B. 350 kcmil
• C. 4/0 AWG
• D. 250 kcmil

Answer: (D)

The main idea is to size feeder conductors for the current they must carry, including a margin for continuous loading. The panelboard has a 200 A main, so the feeder from the transformer to the panel can be expected to carry up to 200 A under typical operation. When a circuit can carry continuous loads, NEC requires the conductors to be sized at 125% of the protection rating to prevent overheating. That means you need conductors capable of carrying 1.25 × 200 A = 250 A. Now look at the options and their typical ampacities. The smallest conductor that can reliably carry at least 250 A is the 250 kcmil copper size, which has an ampacity around 255 A (at 75°C in common tables). The other options are smaller and do not reach 250 A, so they wouldn’t meet the required capacity. The larger 350 kcmil would also work but isn’t the minimum needed. The short 13-foot run doesn’t change the sizing for ampacity; it might affect voltage drop but not the minimum size needed to handle the current safely.

The main idea is to size feeder conductors for the current they must carry, including a margin for continuous loading. The panelboard has a 200 A main, so the feeder from the transformer to the panel can be expected to carry up to 200 A under typical operation. When a circuit can carry continuous loads, NEC requires the conductors to be sized at 125% of the protection rating to prevent overheating. That means you need conductors capable of carrying 1.25 × 200 A = 250 A.

Now look at the options and their typical ampacities. The smallest conductor that can reliably carry at least 250 A is the 250 kcmil copper size, which has an ampacity around 255 A (at 75°C in common tables). The other options are smaller and do not reach 250 A, so they wouldn’t meet the required capacity. The larger 350 kcmil would also work but isn’t the minimum needed.

The short 13-foot run doesn’t change the sizing for ampacity; it might affect voltage drop but not the minimum size needed to handle the current safely.