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Six 15-HP continuous-duty motors are connected to a common 480-volt feeder. The nameplate of each motor indicates: Full load amps 18.6, NEMA Code B, and SF 1.10. The conductors supplying each motor are to be copper with THHN insulation, and the feeder conductors supplying all motors are to be copper with THHN insulation. What size conductor should be used to supply each motor and what size conductor should be used for the feeder conductor

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The size conductor that should be used to supply each motor is 12 AWG and the size conductor  that should be used for the feeder conductor is 1/0AWG.

Size conductor

a. Using NEC table 430.250

Running connect for 150HP  connected to 460V = 21 A

Conductor size=25% current to 125% current (Increase)

Increase current=21×125/100

Increase current=26.25 A

Nearest standard conductor size using NEC table 310.15(B)(16)=25 A

Since the motor contain NEMA design code the conductor will be choosing from 75° column which means that the size conductor that should be used to supply each motor is 12AWG copper conductors.

b. Feeder conductor

Using NEC section 430.24  plus ampere rating of other motors

Running current

Using NEC Table 430.250

Running current=26.25+21+21+21+21+21

Running current=131.25 A

The value that is greater than 131.25 A  under  75° column of NEC Table 310 (B)(16) is 150 A which means that the size conductor that should be used for the feeder conductor is 1/0 AWG conductors.

Therefore the size conductor that should be used to supply each motor is 12 AWG and the size conductor  that should be used for the feeder conductor is 1/0AWG.

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