Ground Rod Depth Calculator Formula
Understand the math behind the ground rod depth calculator. Each variable explained with a worked example.
Formulas Used
Ground Resistance
resistance = rod_length_m > 0 && rod_radius_m > 0 ? soil_resistivity / (2 * pi * rod_length_m) * log(4 * rod_length_m / rod_radius_m) : 0Meets NEC 25-ohm Rule?
meets_nec = (rod_length_m > 0 && rod_radius_m > 0 ? soil_resistivity / (2 * pi * rod_length_m) * log(4 * rod_length_m / rod_radius_m) : 999) <= 25 ? 1 : 0Variables
| Variable | Description | Default |
|---|---|---|
soil_resistivity | Soil Resistivity(ohm-m) | 100 |
rod_length | Rod Length (driven depth)(ft) | 8 |
rod_diameter | Rod Diameter(in) | 0.625 |
rod_length_m | Derived value= rod_length * 0.3048 | calculated |
rod_radius_m | Derived value= rod_diameter / 2 * 0.0254 | calculated |
How It Works
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.
Worked Example
8-ft ground rod (5/8 in diameter) in 100 ohm-m soil.
soil_resistivity = 100rod_length = 8rod_diameter = 0.625
- 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.
Ready to run the numbers?
Open Ground Rod Depth Calculator