Anode Resistance to Remote Earth (Dwight Equation) Calculator

Determine electrical grounding resistance (R_a) for single and multiple vertical, horizontal, and slender stand-off anodes per H.B. Dwight and Sunde.

Electrolyte & Anode Geometry

Electrolyte Resistivity ρ (Ω·cm): Seawater: ~25-30 | Soil: 1000-10000 Ω·cm

Anode Installation Geometry:

Anode Active Length L (m):

Anode Diameter or Cross-Section D (mm):

Number of Parallel Anodes (N):

Center-to-Center Spacing S (m):

Groundbed Resistance & Output

Single Anode Resistance R_a:

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Total Groundbed Resistance:

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Anode Current Output (ΔV=0.25V):

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Paralleling Efficiency:

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Groundbed Resistance Status:

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Frequently Asked Questions

What is the H.B. Dwight equation in cathodic protection?

Published in 1936, the Dwight equation R = (ρ/2πL)·[ln(4L/r) - 1] predicts the electrical grounding resistance of a cylindrical electrode discharging current into a semi-infinite uniform electrolyte.

Why does electrolyte resistivity dominate cathodic protection design?

Electrolyte resistivity spans four orders of magnitude: from 0.25 Ω·m in warm seawater to 1000+ Ω·m in dry rocky soil. Higher resistivity drastically chokes anode current output (I = ΔV/R), requiring higher impressed voltages or vastly larger anode groundbeds.

What is mutual interference between parallel groundbed anodes?

When anodes are spaced closely together, their voltage potential fields overlap, causing mutual shielding (interference) that raises groundbed resistance above the simple R/N parallel resistance ideal.