The 25-ohm rule has been relocated and greatly clarified. Editorial changes to this section have also been made, and the Informational Note has been revised.

250.53 Grounding Electrode Installation Requirements.

(A) Rod, Pipe, or Plate Electrodes.

(1) Below Permanent Moisture Level. If practicable, rod, pipe, and plate electrodes must be embedded below the permanent moisture level and be free from nonconductive coatings such as paint or enamel.

(2) Supplemental Electrode. A single rod, pipe, or plate electrode must be supplemented by an additional electrode that’s bonded to one of the following:

(1) The single rod, pipe, or plate electrode

(2) The grounding electrode conductor of the single electrode

(3) The neutral service-entrance conductor

(4) The nonflexible grounded service raceway

(5) The service enclosure

Ex.: If a single rod, pipe, or plate grounding electrode has an earth contact resistance of 25 ohms or less, the supplemental electrode isn’t required.

(3) Spacing. The supplemental electrode for a single rod, pipe, or plate electrode must be installed not less than 6 ft from the single electrode. (click here to see Fig. 8)

Note: The efficiency of paralleling electrodes is improved by spacing them at least twice the length of the longest rod.

Analysis: The long-standing rule that a ground rod as well as a pipe or plate electrode must have a resistance to earth of

25 ohms or less or be supplemented by an additional electrode was well understood until recent revisions to the NEC created confusion. These revisions left the Code user trying to figure out if a concrete-encased electrode required a supplement and when a ground rod is actually required. Revisions to this section now match the standard industry practice of (when required) driving two ground rods instead of testing the resistance of a single driven rod. The 25-ohm language is now written as an exception, recognizing this practice. Code users will notice that 250.56 has been deleted as a result of this change, but the technical provisions contained therein haven’t disappeared.

The Informational Note explaining the logic of spacing ground rods more than 6 ft apart, while technically true, didn’t provide any guidance as to what the spacing should be. This change clarifies that the driven rods should be at least twice the length of the longer of the two rods. For example, two rods 8 ft in length should be driven at least 16 ft apart. It’s worth remembering that this is an Informational Note, not a Code requirement [90.5(C)].