Earth Rod Resistance Calculator
Estimate the earth resistance of a single vertical rod electrode from the soil resistivity and rod dimensions, using the Dwight formula.
- R = ρ ÷ (2πL) × (ln(8L ÷ d) − 1) = 100 ÷ (2π × 3) × (ln(1,500) − 1) = 33.49 Ω
Effect of rod length
| Rod length | Resistance |
|---|---|
| 1.5 m | 59.63 Ω |
| 2.4 m | 40.39 Ω |
| 3 m | 33.49 Ω |
| 4.5 m | 23.76 Ω |
| 6 m | 18.58 Ω |
| 9 m | 13.11 Ω |
| 12 m | 10.21 Ω |
Dwight Formula for a Single Rod
ρ is soil resistivity (Ω·m), L the driven length and d the rod diameter (m). Typical soils range from 10 Ω·m (wet clay) to over 1,000 Ω·m (rock or dry sand).
Worked example
A 3 m, 16 mm rod in 100 Ω·m soil: R = 100 ÷ (2π × 3) × (ln(1500) − 1) = 33.5 Ω.
Frequently Asked Questions
What earth resistance is required?
It depends on the system and local rules; values of 1 Ω for substations, 5 Ω for instrument earths and 10 Ω for general installations are commonly specified.
How do I reduce earth rod resistance?
Drive longer rods, add more rods spaced apart, use a ground mesh, or treat the soil with low resistivity backfill such as bentonite.
How is soil resistivity measured?
With the Wenner four pin method using an earth tester; the reading varies with moisture and season.
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