A single driven ground rod tests at 40 ohms resistance to earth. What does the NEC require?
Not quite. The answer is A: A second electrode must be installed and bonded to the first, since a single rod exceeding 25 ohms doesn't meet the exception that allows just one
250.53(A)(2) Exception allows a single rod, pipe, or plate electrode to stand alone only if it tests at 25 ohms or less; otherwise, a second electrode must be installed and bonded to the first, regardless of what the combined resistance measures out to afterward. At 40 ohms, this rod fails that exception and needs a supplemental electrode.
Reference: NEC 2023 250.53(A)(2) (Single Rod, Pipe, or Plate Electrode Exception)
Checked against multiple published sources
An ECM code quiz based on the 2023 NEC states the 25 ohm single-electrode exception at 250.53(A)(2), and an ExpertCE article describes the same rule at 250.53(A)(2). Neither is the code text.
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