....The Code requires you to space rods
at least 6 feet apart [250.53(B)]. However,
this spacing is a minimum?and far from
ideal. When using the typical 8-foot or 10-
foot ground rod, you get the best results by
spacing the rods at least 16 or 20 feet apart,
respectively. This is much greater than the
Code minimum 6-foot spacing.
Ground rods spaced less than two
rod-lengths apart will interfere with each
other because their effective resistance areas
will overlap (Figure 2A). For reference, see
IEEE-142 and Soares Book on Grounding.
The overlap increases the net resistance of
each rod, making the grounding electrode
system less effective than if the rods were
spaced farther apart (Figure 2b)......