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Question:

What is the minimum size THWN branch circuit conductor required for a 20 HP, 480-volt, 3 Phase Induction type squirrel cage motor with nameplate FLI of 26.5 amps?

A 10 AWG
Explaination

Refer: NEC Table 430.250
FLI of 20 HP 3 phase motor at 460 volts = 27 amperes
Refer: NEC 210.20(A)
Conductor ampacity = 27 x 1.25 = 33.75
Refer: NEC Table 310.15(B)(16)
Minimum conductor size =10 AWG THWN with 35 amperes capacity is suitable conductor