What section of the NEC can answer the following question? A 2nd ground rod shall be installed if the first ground rod has a resistance greater than how many ohms?
Part III Grounding Electrode System and Grounding Electrode Conductors - 250.53(2) 25 ohms Exception: If a single rod, pipe, or plate grounding electrode has a resistance to earth of 25 ohms or less, the supplemental electrode shall not be required.