There are no NEC rules for this.
If it was my job I would figure the voltage drop based on no more than the calculated load. I would not base it on the service size. Just my opinion.
The calculated load is always higher than the 'real' load.
But 600' feet is a long way to stay within the recommended 3% for a feeder.
Using 150 amps I come up with 350 Kcmil CU or one 600 Kcmil AL (or two 300 Kcmil AL) to maintain 3% or less drop.
If you run the same load but allow 5% drop it gets better.
One 4/0 CU or one 350 AL
By the way just for fun, 200 amps at 600 feet 240 volts with only 3% drop requires these conductors.
One 500 Kcmil CU or one 750 Kcmil AL
Bob
