I have a new 24'-24' wood structure detached garage on a concrete slab. I installed a 30 amp sub panel with 4 - #10 gauge THHN wire through conduit. Two hots, a neutral and ground going back to the Main panel. The neutral and ground are separated at the sub panel, do I need a grounding rod even though I am grounding back at the Main panel? If so am I supposed to attach the ground copper wire to the very small ground bus bar inside that sub-panel, and what size wire?
...And you didn't put a concrete-encased conductor (UFER ground) in the slab? Sigh. That's the easy way out and the most effective ground you can make in most places.
Yes, you need a grounding electrode (ground rod) local to the detached building. Connecting to reinforcing steel in the slab would have provided a very good grounding electrode, but if it's already poured, that ship has sailed.
6 AWG solid is the default minimum size for a connector to a grounding electrode. 8 AWG might be acceptable given that the feed size is 10 AWG, but I'm having trouble tracking down the exact language where that is mentioned. It's one of the messier (or more confusing) sections of the code (IMHO.)