On a single-phase 75 kVA transformer, what is the minimum secondary conductor size?

On a single-phase 75 kVA transformer, what is the minimum secondary conductor size?

• A. 2/0
• B. 1/0
• C. 250 kcmil
• D. 4/0

Answer: (D)

The key idea is to size the secondary conductor by ampacity to handle the transformer’s secondary current, with a practical safety margin. For a 75 kVA single‑phase transformer, the current on the secondary is found by dividing the kVA by the secondary voltage. With a typical 480 V secondary, this works out to about 156 A. The conductor chosen must have an ampacity at or above that value (considering the insulation type and temperature rating, and allowing for continuous operation and some margin). Smaller conductors in the set generally have ampacities below 156 A, so they would overheat under full load. The larger option is sized to comfortably carry the 156 A (plus margin) and thus meets the requirement as the minimum that ensures safe operation. In practice you’d check the exact ampacity table for the insulation and rating used, but the principle is to pick the smallest conductor that can safely carry the transformer’s secondary current.

The key idea is to size the secondary conductor by ampacity to handle the transformer’s secondary current, with a practical safety margin. For a 75 kVA single‑phase transformer, the current on the secondary is found by dividing the kVA by the secondary voltage. With a typical 480 V secondary, this works out to about 156 A. The conductor chosen must have an ampacity at or above that value (considering the insulation type and temperature rating, and allowing for continuous operation and some margin).

Smaller conductors in the set generally have ampacities below 156 A, so they would overheat under full load. The larger option is sized to comfortably carry the 156 A (plus margin) and thus meets the requirement as the minimum that ensures safe operation. In practice you’d check the exact ampacity table for the insulation and rating used, but the principle is to pick the smallest conductor that can safely carry the transformer’s secondary current.