Ground Rod Resistance Calculator
Enter soil resistivity and rod size to get single and multi-rod resistance and the rods needed for your target.
Step-by-step
- Enter valid values to begin.
Dwight formula for one vertical rod in uniform soil. Groups use RN = (R + (N−1)Rm)/N with Rm ≈ ρ/(2πs), an approximation valid for s ≥ L. Soil is assumed uniform; real soil is layered and varies with moisture and temperature. The soil-type list gives typical values only and the resistivity field is what is used. Measure the installed electrode (fall-of-potential) before relying on the estimate.
Ground Rod Resistance: Why the Soil Does the Work
Core Engineering Principles
Techs assume a thicker rod must be better. It isn’t. Current fans out into the earth, and nearly all the resistance sits in the first few metres of soil, where current density is highest. Copper-clad steel conducts far better than dirt, so the metal contributes almost nothing. The answer is soil resistivity times a geometry factor, which is why one rod reads 5 Ω in wet clay and 200 Ω in dry gravel.
Geometry still matters, and the lesson is length over thickness. Resistance falls roughly as 1/L but only with the logarithm of diameter, so doubling the diameter buys a few percent while doubling the length nearly halves the value and reaches moister, more stable soil. Frozen or dried-out topsoil can double a reading, so we design for the worst month. Added rods sit inside each other’s field, so spacing closer than the rod length gives sharply diminishing returns.
RN = (R + (N − 1) × Rm) / N • Vrise = Ig × RN
Group formula is approximate and intended for spacing s ≥ L.
NEC & Standard References
NEC 250.52 lists the permitted grounding electrodes, including driven rods, concrete-encased electrodes, ground rings and plate electrodes. NEC 250.53(A)(2) requires a single rod, pipe or plate electrode to be supplemented by an additional electrode unless it measures 25 Ω or less. NEC 250.56 addresses the resistance of rod, pipe and plate electrodes, and 250.53 covers installation and bonding. IEEE Std 80 and IEEE Std 142 discuss substation and industrial grounding design, and IEEE Std 81 describes the fall-of-potential method for measuring earth resistance. Confirm the edition your jurisdiction has adopted.1. Single rod: 100 / (2π × 3) = 5.305, and ln(8 × 3 / 0.016) − 1 = ln(1500) − 1 = 6.313, so R = 33.5 Ω.
2. That is above 25 Ω, so a second electrode is needed.
3. Rm = 100 / (2π × 3) = 5.31 Ω.
4. Two rods: R2 = (33.5 + 5.31) / 2 = 19.4 Ω, a 42% reduction.
The two-rod group meets 25 Ω. A third rod gives 14.7 Ω, a smaller gain.
- Spacing under one rod length. Fields overlap and the formula turns optimistic, so space rods at least one length apart.
- Frozen or dry soil. Seasonal swings can double resistance, so design for the driest, coldest case.
- Bond the systems together. Separate rods for power, telecom and lightning create potential differences in a fault, so bond them with building steel and metal water pipe.
- Low resistance does not clear faults. A 25 Ω rod at 120 V passes only a few amps, which will never trip a breaker. Fault clearing depends on a low-impedance equipment grounding conductor back to the source, not on the earth.
- Check with a meter. Real soil is layered, so verify with a fall-of-potential test.