Dear all,

i want to size breaker/contactor for a ac induction motor say drawing 20amps at normal operation/load but my confusion is that when ac motor start it draws about 5 times its running current momentary so is that OK to size the breaker with the motor running current will it not trip at start up of the motor.

second there are confusing current label on contactor/breaker like I(th) and others so which current specification to pick

 

Contactors are usually rated in HP as well as current rating.
The O/L/ breaker is usually set for the motor plate rating for FLA and the time delay feature is built in, fuses are sized for at least 1.5x the FLA.
Max.

 

so it means the breaker disconnect the motor from the circuit when the motor took full load current

 

No, it will only trip if the time delay is exceeded when the FLA current draw or inrush time is exceeded.
It was once common for the O/L's to have heaters which had to be selected based on the size of the motor, now they are generally electronic and are set by dial setting for suitable rating.
Max.

 

I understand now thank you MaxHeadRoom