Q. What is the maximum ground resistance required by the NEC for the grounding electrode system?
A. There is no maximum ground resistance specified for the grounding electrode system. However, if a single ground rod is used as the required grounding electrode system, and it’s resistance exceeds 25 ohms, then it must be augmented by one additional electrode located no less than 6 ft from the original ground rod (250.56).
Example: If the first ground rod has a ground resistance of 100 ohms, you only need to add one additional ground rod, regardless of the resistance of the two ground rods.