Adding ground rods to 100A sub-panel

The 100 amp sub-panel in my workshop doesn't have separate ground rods; it's feed with 4-conductor wire from the main panel in the house about 75 ft away, buried in 2" conduit (don't remember the exact size off hand but it's aluminum and I think it's 2-2-2-4, seems like it was labeled as "mobile home feeder" at the Borg when I bought it). The neutral bus is isolated from ground in the sub-panel. So hopefully that's all correct...

From reading the other "100 amp subpanel" thread, it seems evident that I should probably have ground rods for my sub-panel. So I'm planning to add a pair of 8 ft rods, looks like the rules say they need to be at least 6 ft apart. Do they need to be any certain distance away from the building?

How should I approach getting the ground wire to the outside of the workshop? My shop is a pole barn with a (thick) concrete floor and finished / insulated walls; the feeder for the panel comes up through the floor via 2" conduit. So I guess I'll need to drill a hole down through the floor and excavate under the floor from outside to get the ground wire out.

Does the wire need to be in conduit through the concrete floor? All the way to the first ground rod, or only far enough to get it outside of the building? How far below the surface do the ground rods need to be and what type of wire should I use to bond the rods together and for the main ground conductor back to the panel? I thought I saw a reference to using #6 THHN stranded wire from the panel to the ground rod and #6 solid bare copper between the rods. Is that correct?

I seen to recall that solid wire is used for grounding. You should run it long enough to reach the second ground wire all in one piece. The ground rods are meant to be driven all the way in, but if electricians did that the inspector could not see if they are there or not! The further in the better. Can you just drill a small hole large enough for the wire?

I don't believe ground wire needs to be in conduit at all.

My two ground rods connects with solid copper wire through the rim joist. In my area, the inspector said nothing about the need for conduit. He did require 2 of them spaced apart. I think the spacing is about 8 feet. A tip is to drive the rods using a large hammer drill on as ladder. I had very rocky soil and it worked well for me.

Bob

I drove my two rods into the earth with a fence post driver until it got too close to the ground then used a small sledge to pound it the rest of the way. My ground line is continuous bare copper 4CU but I ran it through the same 2.5" conduit through my concrete.

ground rods in that situation are mostly for lightning protection. I'd use them. wire doesn't need to be in conduit. I'd find easiest route that doesn't leave so much exposed wire on the side of the building to get hit by something. I'd use solid copper for all of it. You can buy the rods, clamps, and wire from a big box store. Good luck with getting them in.