Ib =30/1.73x415x0.9=46 A
In = 50 or 60 A
用50/(FgxFixFa)=50/0.9×0.9×0.9= 69 Amps
Assume the length is short and comply with the 4% voltage drop, the cable size is about 10 mm square or better to use a16 mm sq if the the length is suspiciously too long.
Then if this cable is very critical we need to check the voltage drop, the Ib<In<Is too.
This is only outlying the procedure that we are providing the answers .
There may be other things that we need to check on tripping time, the earth loop impedance, the adiabatic equation for short circuiting of the cable is so critical to be reviewed.