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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.