I'm not current on the NEC but is used to be that if the installer put in
ONE grounding rod and the resistance to ground was greater than 25 ohms, he
was required to put in a second ground rod.
If a second rod is in place, there was no "resistance" requirement.
"Smart" electricians quickly figured out that it was easier/cheaper/etc. to
just plan on installing two rods in the first place and just not care about
the resistance value.
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