#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 (A) > Motor branch circuit short-circuit (over-current) protection (B) > Motor control (C) > Motor overload protection (D)

Notes:

> (A) and (B) are typically the same part, e.g., Miniature Circuit Breaker, 3 Poles, C Characteristic, 13A Rated Operational Current (Ie), 10000 (Rated Short-Circuit Capacity), ~230/400V (Rated Voltage (Ur). > (C) starts/stops the motor. > Power to the control circuit is generally taken between (B) and (C). > (A) and (C) must be horsepower rated. > (D) the heater element senses the overload current. A solder-pot relay is a melting alloy type of thermal overload. A bimetallic relay uses a strip of dissimilar metals.

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

Notes: - Wire specification can limit this to a lower value. Typical sizes are: 3A - 0.5mm2 6A - 1mm2 10A - 1.5mm2 13A - 1.5mm2 16A - 2.5mm2 20A - 4mm2 25A - 6mm2 32A - 6mm2 40A - 10mm2 63A - 16mm2 80A - 25mm2 100A - 35mm2 125A - 50mm2 160A - 70mm2 200A - 95mm2 240A - 120mm2 300A - 150mm2 315A - 185mm2 400A - 240mm2 - Thermal overheating is proportional to the current squared multiplied by the time.

References:

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