Earth Electrode & EPR Estimator
Soil resistivity and electrode geometry to an earth electrode resistance — grid, driven rod, rods in parallel or buried tape — with the BS 7671 Reg 411.5.3 TT condition (Ra × I∆n ≤ 50 V) and an earth potential rise estimate. A design-stage magnitude in uniform soil, not an earthing study.
BS 7671 gives no method for calculating an electrode resistance — it requires the resistance to be measured (Reg 643.7.2). Everything here is a design-stage estimate that the measurement supersedes.
Uniform soil is assumed throughout. Real ground is layered, seasonal and anisotropic, and resolving that needs a survey sweeping several probe spacings and a computed model. This page does not assess touch or step voltage — those limits live in BS EN 50522 and ENA TS 41-24, which are named here and not reproduced. Rods in parallel use an empirical combining efficiency that is flagged unverified in the VERIFY register below.
What the Resistance Actually Depends On
Almost all of an electrode's resistance sits in the first few metres of soil around it — the shell-resistance integral converges — which is why local ground matters far more than ground a hundred metres away, and why area beats copper. Driving a rod deeper helps and making it fatter barely does: in Re = ρ/(2πL)·[ln(8L/d) − 1], the length sits outside the logarithm and the diameter inside it. For a grid, the leading term ρ/(4r) is the exact resistance of a disc of the same area, so adding more mesh inside a fixed footprint has sharply diminishing returns.
The resistivity you feed it is the weakest link. Start from a real survey — the soil resistivity survey calculator reduces Wenner and Schlumberger field readings — and read what earth potential rise is and how a resistivity survey works before relying on a single figure.