Subpanel and grounding
I have a question about grounding a subpanel, and despite searching through old posts, I don't see that my specific question has been addressed. Here's the scenario:
I am building a shop in my basement which will be served by a 60amp subpanel. My main panel is almost full, so I am installing a small second subpanel next to it to feed the shop sub, and to be available for future expansion if needed. I am running 6-3 to the small sub and then another run of 60 feet or so of 6-3 to the shop sub.
In both subs, I have isolated the neutral and ground bars. The ground from the 6-3 is attached to the ground bar in both subs and then to the ground in the main panel. My friend, who is a journeyman electrician, has suggested that we run a second, independent ground (6AWG THHN) from the sub ground bar to the water service entrance (where the main panel ground is connected.)
Is this second ground ok? Any advantages or disadvantages to doing this?