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.

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.

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.

Thank you for clearing it for me. Just a doubt. The figure 110%, (5%+5%) you got in accordance with BS 7671 right?

I am not aware of any SPECIFIC regulatory requirement to allow a 10% margin but would presume that this comes under accepted good practice/good design.

It depends a bit on the nature of the load and reasonably expected use. For a load expected to be used continually throughout the working day I would allow a 10%margin as described.  For a load used only briefly I might not bother.