#130 - Sizing Motor Protection Circuit Breakers and Motor Contactors

Example of Motor branch circuit with Motor Protection Circuit Breaker, Motor Contactor and wiring sizing selection :

Motor: 380V AC, 3Ph, 1.5kW (2hp), FLA 4A, SF 1.15

Functions:

> Disconnection > Branch circuit short-circuit protection > Motor control > Motor overload protection

a) Get Full-Load Current (FLC) from UL508A, table 50.1 based on Motor plate horsepower and operating voltage: FLC = 4.3A

b) Select the appropriate motor contactor based on FLC manufacturer's catalog

c) Calculate the motor over-load protection that trips at 1.25xFLA AND FLA is in range of over-load operating range under normal conditions:

FLA x 1.25 = 5A

Note: If SF >= 1.15 THEN OL = FLA x 1.25 else OL = FLA x 1.15

d) Calculate wiring size according to UL508A, table 28.1 in which the ampacity is 1.25xFLC

1.25xFLC = 5.375A

Therefore, the AWG 14 can be selected, as it has an ampacity of 15A (Copper at 60degC)

e) Calculate short-circuit protection based on UL508A, table 31.1, with 2.5xFLC OR following manufacturer's recommendation for a product such as a Motor Circuit Breaker. For branch circuit protection, see circuit breakers that are according to UL489.

2.5xFLC = 10.75A

Note: Wire specification can limit this to a lower value.

References:

> WEG Motor Protection Circuit Breaker > WEG Motor Contactor > Siemens SIRIUS Motor Circuit Breaker