I'd very much like to install a ground rod for my garage's electrical system (since the previous owner didn't have it done) but I have a concern. I've been told that the ground rod should be driven into the ground to a depth of about 8 feet. Is this basically the minimum depth?

My concern is that the entire area I live in is sitting on fairly shallow limestone, and I very well may hit limestone before getting anywhere near 8 feet. When the PO had a septic tank installed they had to put a small mound of earth over it because they couldn't get it any deeper into the ground.

If I strike rock at, say, 4 or 5 feet will be that be deep enough to be effective, or am I just wasting my time? Is there a standard procedure for a situation of this kind?

Any input appreciated!!