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Circuit Breaker Selection

Shawn A

Hi

I have to decide on a circuit breaker for a 22 KW, 3 phase 400V Air compressor. 
Load current = P / (1.732 * V *PF) =22000 /(1.732*400*0.8)
= 39.69A

Therefore a 40A Type D CB is used.

These are my questions:
1. Do we have to include the PF if not given in the equipment datasheet for current calculation.
2. Is this the right way to find the size of MCB? i will also be including an overload relay. Is there anything else i need to lookout for?

Thank you in advance for your help.

Parents
  • +1

    For a  running current of 39.6 amps I feel that a 40 amp MCB is too small for two reasons. Firstly a margin of 1% between normal load current and MCB rating may prove unreliable. If the actual voltage at the motor falls to only 360 volts, (supply voltage 5% low and another 5% voltage drop in the wiring)

    And secondly consider starting current, this might trip a 40 amp MCB. A D type as is suggested is fairly resistant to starting currents, but still seems a bit small.

Reply
  • +1

    For a  running current of 39.6 amps I feel that a 40 amp MCB is too small for two reasons. Firstly a margin of 1% between normal load current and MCB rating may prove unreliable. If the actual voltage at the motor falls to only 360 volts, (supply voltage 5% low and another 5% voltage drop in the wiring)

    And secondly consider starting current, this might trip a 40 amp MCB. A D type as is suggested is fairly resistant to starting currents, but still seems a bit small.

Children
  • 0 in reply to broadgage

    Hi thank you for replying. I understand what you explained. you mentioned about the voltage drop and how it wont be reliable to size a CB based on load current. Can you please suggest how else to calculate or guide me where i can find the material for the same.

    Thank you again.

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