Estimates the resistance-to-earth of a single vertical driven ground rod from soil resistivity and rod geometry, and screens the result against the common practical 25Ω threshold used for single-electrode NEC 250.53(A)(2) Exception decisions. For a full multi-rod/mesh grid design with touch/step voltage checks, use the Earthing Grid Design calculator.
Standards followed
BS 7430:2011 §11.2.2 — single-rod electrode resistance formulaNEC 250.53(A)(2) Exception — common 25Ω practical threshold (not a universal code limit for every electrode type)
What it can do
- Single-rod resistance-to-earth from soil resistivity, rod length and diameter.
- Grading band (LOW/NORMAL/HIGH/VERY HIGH) and pass/fail against a user-settable target resistance.
- Uses the identical formula already verified in the Earthing Grid Design calculator's BS 7430 rod-sizing path.
Example problem
ρ=100 Ω·m, 3m rod, 16mm diameter (8mm radius).
- R = (ρ/2πL) × (ln(4L/a) − 1)
- R = (100/(2π×3)) × (ln(4×3/0.008) − 1) = 5.305 × (ln(1500) − 1) = 5.305 × 6.313 ≈ 33.5 Ω
≈33.5 Ω — above the 25Ω rule-of-thumb, so a supplemental electrode would typically be needed.
Single-rod resistance is very sensitive to soil resistivity, which varies with moisture and season — a measured fall-of-potential test result should always take precedence over a calculated estimate for final acceptance.