Grounding calculator: earth electrode resistance (PUE)

A grounding system's resistance must not exceed the limit (4 Ω for most electrical installations). The calculator estimates the resistance of one vertical electrode and of a group, and how many electrodes you need to drive to meet the limit. Soil resistivity is taken with a seasonal (climate) coefficient.

Solid rods are driven electrodes; dashed ones show how many more are needed to meet the limit. Depth and length are shown to scale.

Input data

Soil resistivity, ρSoil resistivity — how well the soil conducts current. Depends on type and moisture: sand conducts poorly (high ρ), clay conducts well (low ρ). or pick a typical soil Ом·м Climate zone (coefficient ψ)Seasonal (climate) coefficient — a margin for soil that conducts worse once frozen in winter. The harsher the climate zone, the higher it is. Electrode length, L м Electrode diameter, d мм Burial depth of the top, h м Number of electrodes, n шт Utilization factor, ηUtilization factor — nearby electrodes interfere with each other, so a group performs worse than the sum of its parts. Depends on their number and spacing. 1 for a single electrode; ~0.6–0.85 for a group Required resistance Ом

Result

Resistance of the group — Ом

Fill in the fields — the verdict will appear here

Resistance of one electrode — Ом

Electrodes needed to meet the limit — шт

An estimate for vertical electrodes; a horizontal connecting strip is not accounted for — it slightly lowers the total. The utilization factor η depends on the number of electrodes and spacing between them (taken from PUE/reference tables).

How it's calculated

We take soil resistivity as the design value: the table value multiplied by the climate coefficient, because frozen soil conducts worse.
We calculate the resistance of one vertical electrode — it depends on its length, diameter, and burial depth.
A group of electrodes isn't just divided by their count: neighboring electrodes interfere with each other, so the utilization factor goes into the denominator.

Example: Loam at 100 Ω·m in climate zone III, a 3 m electrode with a 16 mm diameter: one electrode gives 49.99 Ω, four with a factor of 0.7 give 17.85 Ω. To meet the 4 Ω limit, you'd need 18 of them.

R₁ = ρ·ψ/(2π·L)·[ln(2L/d) + 0.5·ln((4t+L)/(4t−L))], t = h + L/2; Rgroup = R₁/(n·η). Limits — PUE Ch. 1.7.

This is an estimate

Verify final values against current standards, the project design, and equipment nameplates.

Explained in the article Сопротивление заземляющего устройства: нормы и методы измерения

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