Referencing a motor feeder from a 120/240-volt, 3Ø, 4-wire panelboard, what is the smallest conductor size allowed for that feeder?

• A. 4/0 AWG
• B. 250 kcmil
• C. 3/0 AWG
• D. 300 kcmil

Answer: (A)

Sizing a motor feeder is about ensuring the conductors can carry the motor’s load with a margin for starting, so you size the feeder ampacity at 125% of the motor’s full-load current. Once you know that required ampacity, you pick the smallest conductor size that has an ampacity equal to or greater than that value, using NEC tables (310.16, with 75°C insulation as a common reference).

From there, for a 120/240-volt, three-phase, four-wire panelboard, the copper conductor that meets typical motor-starting and load conditions with the next practical size above the minimum is 4/0 AWG. It provides enough ampacity while remaining the smallest option among the standard large-conductor sizes listed. The other options listed are larger conductors and would not be the smallest size that satisfies the ampacity requirement.