Free Ground Rod Depth Calculator
Calculate the grounding resistance of a single rod driven into soil, based on rod dimensions and soil resistivity.
ohm-m
ft
in
Ground Resistance
0.0 ohms
Meets NEC 25-ohm Rule?0
Ground Resistance vs Soil Resistivity
Ground Rod Resistance
R = (rho / (2 x pi x L)) x ln(4L / a)
Where rho is soil resistivity in ohm-meters, L is the rod driven depth in meters, and a is the rod radius in meters. The NEC requires a single ground rod to achieve 25 ohms or less; otherwise a second rod is required.
Example Calculation
8-ft ground rod (5/8 in diameter) in 100 ohm-m soil.
- 01L = 8 x 0.3048 = 2.438 m
- 02a = 0.3125 x 0.0254 = 0.00794 m
- 03R = 100 / (2 x pi x 2.438) x ln(4 x 2.438 / 0.00794)
- 04R = 6.527 x ln(1228) = 6.527 x 7.11 = 46.4 ohms
- 05Exceeds 25 ohms; install a second rod at least 6 ft away.
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