How we can size circuit breaker for three phase motor power circuit?

Asper nec code art430 First go to nec430.250 for3 phases motor at voltage and horse power you can select flc or starting current Second step to calculat cable size: flc *1.25 you will get the minimum ampere for cable Third step go to cable catalogue as saudi cables and choose proper cable size If you want to select breaker flc*2.5 and you will get maximum over current protection take the next breaker standard

Motors Power (kW) is mentioned on Motor Tag Plate from which we can get the Full Load Current i.e the Current that will be drawn by the Motor at startup. Motors draw maximum current at startup therefore we shall consider Full Load Current in our Breaker Sizing Calculation. If FLA is not mentioned on Motor, we can calculate it from following formulae:

 

Power = (root3) x (Voltage) x (Current)

 

Add20% into the Current obtained and this shall be the Breaker Size.

 

There are certain limits on which type of Breakers can be used for which type of Rating therefore this shall also be taken into consideration.