How to Calculate Circuit-Breaker Rating || Circuit breaker amp size

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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[SaiKumar-wkmk]

Very valuable information. Nice presentation. Thank you so much. 🙏

[boshboshish]

Hi there, I'd like to know what is the difference between AT and AF breaker rating . thank you

[learnandearnbysivaji]

plz give any standard reference to show saftey factor is 1.25 which helps in our work and also for our clients.

[rcsagar]

How to find cb making current from given mva and kv

[mujeeb]

Will each pole of mcb will draw 94.5 a or its addition of all four. Will each pole of mcb with bare 94.5 amps.

[rbabu]

If you have any short cuts for getting circuit breaker without formula

[florind.]

please add specs for short circuit rating of the breaker.

[Pgan]

How to determine and calculate the Breaking Capacity of MCCB or MCB

[umangraval]

My system some times takes upto 400 amps.. motor is 10 kw 96 volt system. So can u plz help me for chosing mcb??

[EonTide]

Would the wire size also be rated for 100 amps ?

[nabeeljizany]

But i think difference in case at startin of motor when use star delta starter and case when use direct on line starter

[HariHari-jrpg]

REQUIRED SIZING OF SPARES TO COTROL PANEL LIKE CONTACT ORS, OLR, SF UNIT & MAIN & CONTROL CIRCUIT WIRES

[nathanielindonilla]

How did you get the 75.96?i cant get it

[kelvin.Epeterkio-nwnd]

Pls, how did u get 1.25 as safety factor?

[bemerchlimited]

Hi, I bought a circuit breaker with rating 12-100v, 32A. Kindly tell me at what point does it trip 12v*32A or 100v*32A.

[infiniteintelligence]

is that a train i can hear in bkground...?

[mengnggapo]

i think this is wrong for motors, we need the FLA of the motor which can be seen in NEC, dividing directly will result in low value of AMpere, which means low value of your chosen circuit breaker.

[dingdangdoogle]

Hello, please can you explain the current rating calculation???

50 x 1000/ √3 x 380 ???
how do you get 75.96 A??

√3 is 1.73???

[EonTide]

You all have to keep in mind this formula was for three phase systems not single phase

[nasirjabr]

Safety factor =125 % = 1.25 not %25 (0.25) as shown in "red strip" circuit breaker= Ix 25% safety factor