The resistance of a grounding electrode to earth controls how well fault current and lightning are dissipated. This calculator estimates that resistance using the Dwight formula for a driven rod and a parallel-efficiency model for multiple rods, then checks the NEC single-rod 25-ohm rule.
How ground-rod resistance is estimated
For one vertical rod, the Dwight formula is:
R = (rho / (2 pi L)) * ( ln(4L / d) - 1 )where:
rhois soil resistivity in ohm-metres,Lis the buried rod length in metres,dis the rod diameter in metres.
Resistance falls as the rod gets longer (more contact area with deeper, often moister soil) and as resistivity drops, but it changes only slightly with rod diameter — doubling the diameter barely moves the result, which is why driving a longer rod beats buying a fatter one.
For multiple rods in parallel, the combined resistance is higher than a
simple R / N because each rod’s resistance area overlaps its neighbours. The
tool applies an efficiency factor (for example about 0.86 for two rods spaced one
rod-length apart):
R_combined = R_single / (N * efficiency)Worked example
An 8 ft × 5/8 in rod in moist loam (rho = 100 ohm-m):
L = 8 ft = 2.44 m,d = 0.625 in = 0.0159 m.ln(4 × 2.44 / 0.0159) = ln(614) ≈ 6.42.R = (100 / (2π × 2.44)) × (6.42 − 1) ≈ 6.52 × 5.42 ≈ 35 Ω.
That single rod exceeds 25 Ω, so NEC 250.53(A)(2) calls for a supplemental
electrode. Add a second rod at least 6 ft away and the combined resistance drops
to roughly 35 / (2 × 0.86) ≈ 20 Ω, which passes.
How soil and length move the number
Resistance tracks soil resistivity almost linearly and rod length inversely, so the same 5/8-inch rod behaves completely differently across sites. Approximate single-rod resistance from the Dwight formula:
| Soil (resistivity) | 8 ft rod | 10 ft rod |
|---|---|---|
| Wet organic clay (10 Ω·m) | ~3.5 Ω | ~2.9 Ω |
| Moist loam (100 Ω·m) | ~35 Ω | ~29 Ω |
| Sandy/gravel (300 Ω·m) | ~105 Ω | ~87 Ω |
| Dry sand or rock (1000 Ω·m) | ~350 Ω | ~290 Ω |
Two lessons fall straight out of the table. First, soil dominates — moving from loam to dry rock multiplies resistance by 10 while a longer rod only shaves 15–20%. Second, this is why the 25-ohm rule is easy in wet clay and nearly impossible in rocky or desert soil, where installers turn to chemical (ground-enhancement) electrodes, deeper driven rods, or larger radial/ground-ring systems instead of chasing it with more 8-ft rods.
Field realities the idealized model omits
- Real soil is layered. The Dwight formula assumes uniform resistivity, but most sites have a moist conductive layer over dry or rocky substrate (or vice versa). A rod that penetrates into a wetter deep layer can read far lower than the surface soil predicts — the reason a 10-ft rod sometimes beats the table by a wide margin.
- The measured value drifts seasonally. Frozen ground in winter and drought in summer both spike resistivity. Codes and standards expect the electrode to work in the worst season, so a September test in dry soil is more meaningful than one after spring rain.
- The 25-ohm rule is a floor, not a goal. NEC 250.53(A)(2) only requires a second electrode if a single rod exceeds 25 Ω; it never requires proving the pair is below 25 Ω. Lightning protection (NFPA 780) and sensitive electronics routinely target ≤5 Ω, which needs engineered grounding, not the code minimum.
- Two rods too close barely help. Spacing below one rod length makes the resistance areas overlap so heavily that a second rod adds little — the efficiency factor collapses. Space supplemental rods at least a full rod length (ideally two) apart.
Notes and tips
- Space supplemental rods at least one rod-length apart — closer spacing wastes copper because the resistance areas overlap heavily.
- Soil moisture and temperature swing the number seasonally; design for the dry, worst-case season.
- The 25-ohm rule is a code minimum, not a performance target. Lightning protection and sensitive electronics often demand 5 ohms or less.
Sources and references
- NFPA 70 — National Electrical Code, Article 250 (grounding and bonding) — the 250.53(A)(2) supplemental-electrode / 25-ohm rule
- IEEE Std 142 (Green Book) — Grounding of Industrial and Commercial Power Systems — the Dwight formula and multi-rod efficiency
- IEEE Std 81 — Measuring earth resistance (fall-of-potential method) — the field test this estimate does not replace
Maintained by the Gera Tools editorial team. The Dwight formula and parallel-efficiency factor model idealized, uniform soil; real soil is layered and moisture varies seasonally, so always confirm with a fall-of-potential or clamp-on test and follow the current NEC. Not a substitute for a licensed electrician. Last reviewed 2026-07-02.
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