A 150-horsepower, three-phase squirrel-cage induction motor is connected to 440 volts. The motor nameplate lists the following information:Amps 175
SF 1.25
NEMA code BThe conductors are to be copper with type THHN insulation. The short-circuit protective device is to be an inverse-time circuit breaker. The termination temperature rating is not known.Determine the conductor size and overload size.
What will be an ideal response?
The conductor size is determined from the current listed in NEC Table 430.250 and then increased by 25 percent.180 × 1.25 = 225 amperesNEC Table 310.15B is used to determine conductor size. Type THHN insulation is located in the 90 degrees C (194 degrees F) column. Since the motor nameplate indicates a NEMA code B, the conductor size will be selected from the 75 degrees C (167 degrees F) column in accord with NEC 110.14(C). The conductor size will be 4/0 AWG.The overload size is determined from the nameplate current. Because the motor has a marked service factor of 1.25, the motor nameplate current will be increased by 25 percent.175 × 1.25 = 218.75 amperes.
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