Among the options for the indoor installation, which is the conductor size specified as the transformer primary?

• A. 3/0 kcmil
• B. 2/0 kcmil
• C. 300 kcmil
• D. 250 kcmil

Answer: (D)

Sizing the transformer primary conductor is all about carrying the primary current without overheating. The primary current is found from the transformer’s rated VA divided by the primary voltage. For a typical indoor installation, you use ampacity values for conductors at a 75°C insulation rating (the common inside-building standard) to determine the needed size.

If the transformer is around 125 kVA at 480 V, the primary current is about 260 A. A 250 kcmil copper conductor has an ampacity near 255 A at 75°C, which just meets that current without overheating. Smaller sizes would be undersized for this current, risking excess temperature rise, while a larger size (like 300 kcmil) would work but isn’t necessary and adds cost and room requirements.

So 250 kcmil is chosen because it provides enough ampacity to safely carry the primary current in an indoor setting, using the typical 75°C ampacity standards.