I don't usually comment on technical things but this one - I gotta say this:
Please don't use a separate grounding connection to the planet(earth) to the sub-panel.
We've run into a couple of cases where this happened and it leads to all kinds of problems. Corrosion of ground connections is one of them, depending on the soil conditions that you might run into. It's a problem that happens almost right away - within a few days sometimes, the connectors tying into the earth (water pipe, ground rod/plate - whatever)grow a lovely white insulating substance on them and the lights and all the household electronics start to go all weirdo on everybody as the EGC's go a bit resistive and start to get significant voltage differences between the EGC and the neutrals in the separate panels.
This is not to mention the possible damage to the metal water pipes that might be tied into the vicinity, which could be a whole other set of problems down the road. (Not for you, mind you, just the poor unlucky turd herder that can't figure out how come the pipes are rotting away so quickly).
We now know it's best to just run good old #6 or #4 copper, in your case, if it's a longer run, right back to the main panelboard and tie it all together neatly in one spot to one connection to good old planet earth.
