need to know what size groung conductor to use for the main ground to the water pipe for a 1600 amp service with 4 sets of 600 mcm cables to the utility transformer.

With the information given, table 250.122 shows 4/0 if the egc is copper, or 350 if it's al/cu. 250.122 (F) talks about conductors in parallel also. This should get you started.

You use 250.66, not 250.122
I read it as the max required will be 3/0 copper or 250 MCM al.

Thanks, and apologies for the mix-up, 250.122 is for grounding raceway's and equipment. Seen that first part, where it says minimum size egc, and the rest is history.

If under the Chicago code its 18-27-250.66 page 93
copper (300) in a 1 1/4 inch conduit or tubing.