CSLB C-10 Electrical ContractorWiring Methods and MaterialsMedium
A technician drives a single 8-foot ground rod at a residence and measures 32 ohms of resistance to earth using a ground resistance tester. Per NEC 250.53(A)(2), what must the technician do?
- AIncrease the size of the grounding electrode conductor to compensate
- BInstall a supplemental grounding electrode bonded to the first rod
- CReplace the rod with one at least 12 feet long
- DNothing further, as 32 ohms is within the acceptable range
Show answer & explanationAnswer & explanation
Correct answer: B. Install a supplemental grounding electrode bonded to the first rod
NEC 250.53(A)(2) requires that a single rod, pipe, or plate electrode with a resistance to earth exceeding 25 ohms be supplemented by an additional electrode. Since 32 ohms exceeds this threshold, a second electrode must be installed and bonded to the first.
Why the other options are wrong
- A. GEC sizing is unrelated to correcting excessive ground resistance readings.
- C. Rod length alone does not satisfy the code requirement; a second electrode is mandated instead.
- D. 32 ohms exceeds the 25-ohm threshold, so additional action is required.
Single Ground Rod 25-Ohm Rule
NEC 250.53(A)(2) requires a single rod, pipe, or plate electrode exceeding 25 ohms resistance to be supplemented with an additional electrode.
- 25 ohms is the threshold for single-rod electrodes.
- If exceeded, one supplemental electrode is required (no further testing needed once added).
- Supplemental electrode must be bonded to the first.
Memory trick: 'Twenty-five or add a friend' — over 25 ohms needs a second rod.