Sizing Motor Feeders

The feeder cable shall have the ampacity of 1.25 x the FLA of the motor. The breaker may be sized at 250% of the FLA. This is not required. It could be set at a lower value. The reason is so the breaker will not trip under starting conditions. This is for 1 motor. How many motors do you have?

Short-Circuit and Ground -Fault Protection 150% to 300% NEC table 430.52

times FLC NEC tables 430.247, 248, 250 (round up next size)

Single Motor Conductors 430-22A 125% NEC tables 430.247, 248, 250

Overloads 430.32A 115%- 125% Heater (ok round up) max 140% with marked tempertuure rise of 40 degree C or less (fuse round down) type times FLA (Name

plate)

Temp rise and service factor changes can change things.

Its a very good answer.

where can we get the copy of NEC? Kindly throw some light on probable wesites.

Regards,

PG

thanks for mentioning the code tables no. and for your comment

In sizing up feeder wire and circuit breaker in more than one motor you have to consider as per NEC Standard :

a) The full load Current according to the type of lock rotor involve

1) Squirel Cage type of rotor - 250% - 300% of the Full load current as per NEC Standard

2) Wound Type rotor - 125% -250% of Full load current as per NEC Standard

b) The size of Main Breaker (CB) - The Highest Full load ampere x the type of lock rotor of the motor plus the sum of the remaining FLA only of the

motors.

c) The size of Main Feeder - The Highest Full load ampere with lock rotor x 125% plus the sum FLA only of the remaining motors

d) For individual sizing of the motor:

1) Size of Wire - FLA x 125% round up to nearest higher size available in the table

2) Size of Circuit Breaker (CB) - FLA x the Type of lock rotor and round up to the nearest higher size available in the Table

thanks for your answer for both protection and feeder sizes as per NEC, for diffrent type of motors.