A single 3 m rod misses the common 25 Ω target; doubling its length gets there, while a fatter rod barely helps.
| Soil resistivity | 100 Ω·m (uniform) |
| Rod | 16 mm diameter, driven vertically |
| Target | 25 Ω (NEC 250.53(A)(2) single-electrode exception) |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| 3 m × 16 mm rod | ≤ 25 Ω | 33.5 Ω | ✗ FAIL |
| 6 m × 16 mm rod | ≤ 25 Ω | 18.6 Ω | ✓ PASS |
Key insight: Resistance falls almost in proportion to rod length but only weakly with diameter, because diameter appears inside a logarithm. Deeper rods also tend to reach moister, lower-resistivity soil, so in practice the gain is often larger than this uniform-soil estimate.
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Open Ground Resistance Calculator calculator →Only if they are spaced at least about one rod length apart. Closer rods share the same soil and their combined resistance is noticeably higher than half of one rod.