Basic NEC rule says that the current carrying conductor supplying power to motor loads used for continuous duty must have a current carrying capacity of not less than 125% of the rated motor full load current.
Conductors supplying two or more motor loads must have a current rating of not less than 125% of the full load current rating of the largest motor supplied plus the sum of full load current ratings of the all the other motors in the circuit.
The details related to the full load current ratings of the motor against power rating (hp power) of the motor and voltage supplied to the motor are given in NEC 2011 book under Table 430.247 to Table 430.250
Table 430.247 - Full load current, Direct Current motor
Table 430.248 -Full load current, Single phase alternating current motor
Table 430.249 - Full load current, Two phase alternating current motor (4 wire)
Table 430.250 - Full load current, Three phase alternating current motors
After determining the ampacity of the conductor from above tables, we can select the conductor size from the Table 310.15(B) for a given type of insulation required
Example: What is the size of conductor needed for 5hp 200 volts single phase motor?
Step1: From the table 430.248 full load current is 32.2 amps
Step 2: Ampacity is 125% of full load current = 1.25 X 32.2 = 40.25 amps
Step 3: From Table 310.15(B) 14 AWG THHN, TW, THW, RHW, RHH,or XHHV can be sufficient
Example : What is the size of branch feeder circuit conductor supplying power to 3 phase 3hp, 2hp and 1.5hp motors supplying voltage at 460V?
Step 1: From the table 430.250 full load currents on the 1.5hp, 2hp and 3hp motors are 3.0 amps, 3.4amps and 4.8 respectively
Step 2: From NEC section 430.24 the branch circuit conductors are sized accordingly
4.8 X 1.25 + 3 + 3.4 = 12.4 amps
Step 3:The conductors must have ampacity at least 12.4 amps and 14 AWG THHN, TW, THW, RHW, RHH,or XHHV can be sufficient
