Load Rating and Cable Sizing

dear,

you should consider the sizing of load based on 630 Amp. for 3 phases. Plus 1.73 times * 630 Amp = 1089 Amp for maximum prospective un balanced current in the Neutral wire. for each feeder.

Quote "I have an Main Power Distribution Board(MPDB) which supplies power for 7 Numbers of Main Lighting distribution Board(MLDB) in which MPDB incomer is 2500A from Transformer and it supplies power to seven numbers of MLDB whose incomer is 630A"

The conductor should be size on the breaker feeding the MLDB. If the breaker is 630 amps, then the conductor should have an ampacity of 630 amps.

Do you have any 630A breaker in each of the outgoing feeder in MPDB?

If you don't have it then,

the cables between MPDB and MLDB are not protected properly against over current if it is designed with 630A.

Good point,

If there is no 630 A CB in the MPDB, the cables between panels are not protected.

Cable size determine always depending on the load value, but CB should be at the beginning of the line not at the end.

What do your local electrical wiring standards and regulations say (the ones you are designing to, as required by local law). Please consult them to get an answer which will comply with your local electrical regulations.

British Standard 7671.