They call it "the well of influence" and it is a circle the size of the rod depth as a radius. If you have 2 sharing the same well they are only getting the benefit of one rod. Someone decided that a 2 foot overlap on an 8' radius was the point where you were losing enough grounding ability that you should stop the bleeding and they made that the minimum.
The reason we get the pitch in seminars is for those times when we are not trying to do the "minimum" and we are driving deeper rods. At that point you also want to spread out accordingly. Using the same logic the minimum distance should be 75% of rod length, up to 200% when the wells don't touch.
When you are really serious about grounding you will be running several of these rods in a circle with a ground ring between them and long "radials" back to the center of your grounding system, usually a radio tower with a Ufer ground in the radio shack. The radials will also extend out from the ring quite a ways to make your ground plane bigger.
The only other thing I have worked with that gets grounded like that is a toll plaza.
[This message has been edited by gfretwell (edited 09-16-2004).]
