Free Earthing Tool
Earth Resistance Calculator (Pipe / Rod Electrode)
Estimate the earth resistance of a pipe-earthing electrode from soil resistivity, electrode size and the number of pits, using the IS 3043 / Dwight formula. An estimate to plan with — always confirm on site with an earth-resistance tester.
Engineering check required: This result is indicative only. Verify it with a licensed electrical engineer or electrical consultant.
| Resistance, single electrode | — |
|---|---|
| Resistance, 1 pit | — |
The required earth resistance comes from your project specification and the statutory requirements for the installation. Confirm the achieved value on site with an earth-resistance tester.
Please read. These figures are general projections, not a design. They are estimates based on published formulae, standard weight data and assumed uniform soil conditions — real sites vary, and the numbers here may not be 100% accurate. They are offered as a working aid only. Global Electric Company accepts no liability for decisions taken on the basis of these results: always have them checked and signed off by your site engineer, electrical consultant or a qualified professional, and confirm earth resistance by measurement on site before commissioning. Where these tools and IS 3043 disagree, IS 3043 wins.
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How earth resistance is calculated
For a single vertical pipe or rod electrode, this calculator uses the Dwight / IEEE-80 rod formula, the form most widely used in Indian earthing practice:
n pits in parallel ≈ R ÷ n … F(n) × R ÷ n
where ρ = soil resistivity (Ω·m), L = electrode length (m) and d = electrode diameter (m). Pipe nominal bore is converted to outside diameter: 40 NB = 0.0483 m, 50 NB = 0.0603 m, 80 NB = 0.0889 m. Parallel pits are shown as a range, not a single figure. R ÷ n is a best case that assumes each pit sees undisturbed soil — real pits sit in one another’s resistance field, so they never quite reach it. The upper figure applies a published combining factor F(n) (roughly 1.16, 1.29 and 1.36 for 2, 3 and 4 pits at a spacing of about one electrode length). Design to the upper figure. Results assume uniform soil and must be confirmed with an earth-resistance tester on site.
Earth resistance vs soil resistivity (3 m × 40 NB pipe)
Estimated resistance of a single 3 m, 40 NB GI pipe electrode:
| Soil resistivity ρ (Ω·m) | Resistance (Ω) |
|---|---|
| 30 | 8.3 |
| 50 | 13.8 |
| 100 | 27.6 |
| 200 | 55.3 |
| 500 | 138.2 |
| 1000 | 276.3 |
Parallel pits (3 m × 40 NB, ρ = 100 Ω·m)
| Pits in parallel | Best case, wide spacing (Ω) | Realistic, spacing ≈ length (Ω) |
|---|---|---|
| 1 | 27.6 | 27.6 |
| 2 | 13.8 | 16.0 |
| 3 | 9.2 | 11.9 |
| 4 | 6.9 | 9.4 |
The left column is the textbook R ÷ n. The right column is what you should actually design to: it accounts for the mutual resistance between pits at normal spacing. Four pits do not give you a quarter of one pit.
Soil resistivity guide (indicative, Karnataka)
| Soil type | Typical ρ (Ω·m) |
|---|---|
| Marshy / treated ground | 20 |
| Moist clay / loam | 50 |
| Red soil / laterite | 100 |
| Sandy / mixed | 200 |
| Murram / weathered rock | 500 |
| Hard / dry rock | 1000 |
Indicative ranges only — actual resistivity varies widely with moisture and season. Measure on site for design.
Frequently asked questions
How do I calculate earth pit resistance?
What earth resistance is acceptable per IS 3043?
How many earth pits do I need?
Why does my measured earth resistance differ from the calculated value?
Send the Electrode Schedule for Today’s Rate
Once the pit count is settled, send the electrode type and size, the quantity and your delivery location. The installed resistance is still a site measurement — we quote the material.