How to Calculate Grounding Resistance? (Rod Electrode)

Grounding resistance of the vertical rod electrode R = ρ/(2πL)·(ln(8L/d)−1) It is found using: soil resistivity (ρ), rod length (L) and diameter (d). For example, for ρ=100 Ω·m, 3 m length, 16 mm diameter, R ≈ 33.5 Ω. Soil resistivity is determined by measurement (Wenner).

R = ρ/(2πL)·(ln(8L/d) − 1) [Ω]
Example calculation: ρ=100 Ω·m, 3 m long, 16 mm diameter rod → R = 100/(2π×3)·(ln(8×3/0.016)−1) ≈ 33.5 Ω. Longer/more rods are needed for lower resistance.

Grounding Resistance (Rod Electrode)

Calculate the approximate grounding resistance of a single-rod electrode from the soil resistivity and rod dimensions.

Grounding resistance
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The ρ used
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Notes: R = ρ/(2πL)·(ln(8L/d)−1) (BS 7430 / IEEE 142 — vertical bar). Soil resistivity ρ is determined by measurement (Wenner 4-prong method). And it varies depending on the season/humidity; typical values are only indicative. The result is for a single bar. The exact value will be verified by field measurement.
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How is it calculated?

  1. Enter the soil resistivity (Ω·m) (from the measurement).
  2. Enter the rod length (m) and diameter (mm).
  3. Press Calculate; the approximate grounding resistance will be displayed.

Frequently Asked Questions

How can grounding resistance be reduced?
Longer rods, multiple parallel rods (dropping to less than 1/n due to interaction), deep well electrodes, or applications that increase conductivity.
How is soil resistivity measured?
In the field, measurements are taken using the Wenner 4-probe method; the value varies depending on humidity and season.
What should the grounding resistance be in ohms?
The target varies depending on the application; generally, ≤ 2–5 Ω is targeted for many facilities, but the binding value is determined according to the relevant regulation and system type.

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