How to Calculate Circuit-Breaker Rating:

Example:

Motor – 40 kw
Voltage – 380 V
Power Factoe-0.8
Circuit Breaker-?

Answer:
First we Convert KW to KVA
KVA = KW/ P.F.
KVA = 40/0.8
KVA = 50

Now we calculate current rating
Current (I) = Conneted Load/√3 x Voltage
Current (I) = 50 x 1000/√3 x 380
Current (I) = 75.96 A

Circuit Breaker = Current x 25 % Safety Factor
Circuit Breaker = I x 1.25
= 75.96 x 1.25
= 94.5 A

Cicuit Breaker will be100 A

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