ECN Forum Forums Code Related Discussion NEC & other Code issues Ground wire Size to water Line.

According to the table, anything larger that 1,100 kcmil requires 3/0 Copper (or 250 kcmil Aluminum).
Since you're running parallel sets, you need to run that size ground wire with each set.

Calculations are:
(6) 600 MCM = 3,600 MCM
3,600 MCM > 1,100 MCM