Conductor Size

This has been discussed dozens if not hundreds of times on CR4; you can do a "Search all of CR4." There is more to it than some formulas, and there is no "the formula."

First you should know how much current will flow

its a 3 Phase 750 KVA Generator

In that case, knowing the voltage might be first....

voltage will be 415, frequency 50 Hz, PF 0.8

Assuming that it is a 415V generator, the following are the recommended earthing conductor sizes:

Neutral Earthing:

i) 1 1/2" x 1/4" (40mm x 6mm) Electrolytic Bare Copper Conductor or Strip

OR

ii) 2 x 225 sq.mm or 1 x 500 sq.mm PVC Insulated Single Core Stranded Aluminium Conductor

Body Earthing:

i) 1" x 1/8" (25mm x 3mm) Bare Copper

OR

ii) 185 sq.mm PVC Insulated Stranded Aluminium

OR

iii) 2" x 1/4" (50mm x 6mm) Galvanised Iron

how we can calculate this

If you want to actually do the sizing very precisely, then you have to calculate the fault current at the motor terminal for a single-line-to-ground fault, duly taking into account the fault contributors and all the network impedances in line upto the point of fault. Then you calculate the conductor sizing by following the maximum permissible current density for the material chosen.

But, as there would be number of motors in an industry, it is a laborious process to undertake such an excercise. So, engineers' associations have given a ready-reckoner, assuming the nomimnal cable sizes used for the given HP/kVA. The above recommendation is from one such ready-reckoner.

In many jurisdictions, the local codes will prescribe the minimum conductor size to use. These sizes are based on standard motors and/or other loads and will suffice for virtually all applications..

British Standard 7671.

Canadian Electrical Code, table 16.