Circuit Breaker Selection

Question

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.

broadgage · Accepted Answer

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.

broadgage · Answer

For a potentially long hour load, I would consider it good practice to size a fuse or an MCB at at least 110% of the calculated or stated running current, rounded up to the next standard size. This allows for supply voltage variations and manufacturing tolerances. So for a 39.6 amp load use a 45 amp fuse or MCB as a minimum. 
 Additional upsizing may be required for motors due to the starting current. This depends on the starting method used, and guidance may be obtained from the supplier of the equipment.

Circuit Breaker Selection

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