Ground rod installation and the 25-ohm rule
A single ground rod must be backed up by a second electrode, such as another rod at least 6 ft away, unless a test shows 25 ohms or less to earth. Rods need at least 8 ft in contact with soil.
What the code requires
Ground rods, pipes and plates must be free of nonconductive coatings such as paint or enamel, and where practicable they go below the permanent moisture level. A rod or pipe needs at least 8 ft in contact with soil. Where rock is hit, it may be driven at an angle of no more than 45 degrees from vertical or buried in a trench at least 30 in. deep. The top must be flush with or below grade unless it is protected.
A single rod, pipe or plate must be supplemented by another electrode, such as a second rod. Multiple rods must be at least 6 ft apart, and an informational note says spacing at twice the rod length works better. The bonding jumper that is the only connection to a supplemental rod never needs to be larger than 6 AWG copper or 4 AWG aluminum or copper-clad aluminum.
Plates go at least 30 in. below the surface, and a ground ring at least 30 in. deep. A metal water pipe used as an electrode must be supplemented by another electrode. Where rod, pipe or plate electrodes are used, each electrode of one grounding system, including lightning protection, must be at least 6 ft from any electrode of another grounding system.
On the job
- A missing second rod, or two rods closer than 6 ft, is a common correction. Relying on the 25-ohm exception instead requires a resistance test.
- Rods not driven their full 8 ft, or tops left exposed above grade without protection, get flagged.
- A water pipe electrode with no supplemental electrode is another frequent fail.
Exceptions that matter
- A single rod, pipe or plate with a resistance to earth of 25 ohms or less does not need a supplemental electrode.
What changed in 2023
The 2023 edition moved the no-coating rule to the top of 250.53(A), retitled 250.53(E) for the supplemental electrode bonding jumper, and added copper-clad aluminum as an option for that jumper.
Related sections
- NEC 250.10
- NEC 250.50: Bonding all grounding electrodes together
- NEC 250.52: What counts as a grounding electrode
- NEC 250.64
- NEC 250.66: Sizing the grounding electrode conductor
- NEC 250.68: Grounding electrode connections and the 5 ft rule
- NEC 250.70
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CSLB #354550 · C-10 Electrical · Licensed since 1978 · $25,000 contractor's bond