Cathodic Protection Groundbed

Current requirement, anode count and life, groundbed resistance (Dwight, Sunde) and rectifier sizing for impressed-current systems.

1. What this check does

Finds the protective current from surface area, current density and coating breakdown (or a measured value), the number of anodes needed to carry that current and to last the design life, the resistance of a vertical, horizontal or deep-well groundbed, and the rectifier voltage and current with a margin.

2. Inputs used

Surface area, design current density and coating breakdown factor — or a known current requirement.
Soil resistivity at the groundbed; anode or backfill column length and diameter; spacing; burial depth for horizontal beds.
Anode mass, consumption rate, utilization factor and maximum current per anode; design life.
Cable and structure-to-earth resistance, back EMF (default 2 V) and rectifier margin (default 1.25).

3. Formulas and assumptions

Current I = A · i · f A m², i mA/m² (÷1000), f coating breakdown Anodes N = max(⌈I / I_max⌉, ⌈I · C · Y / (m · U)⌉) Life Y = N · m · U / (C · I) Vertical, one R = ρ / (2πL) · (ln(8L/d) − 1) Dwight Vertical, N R = ρ / (2πNL) · (ln(8L/d) − 1 + (2L/S) · ln(0.656N)) Sunde Horizontal R = ρ / (2πL) · [ln((4L² + 4L√(s² + L²)) / (d·s)) + s/L − √(s² + L²)/L − 1] L = (N − 1)·S + anode length, s = 2 × depth Dwight Deep well active column as a single vertical electrode (Dwight) Rectifier V = I · (R_bed + R_cable + R_structure) + back EMF rating = V · margin, I · margin

Uniform soil resistivity at the groundbed; use the value measured over the anode depth.
Backfill is treated as part of the electrode (its diameter and length are used); backfill resistance itself is ignored.
Default back EMF 2 V and rectifier margin 1.25 are RokBench assumptions.
Current density, coating breakdown, consumption rate and utilization are your design inputs; UFC 3-570-01 and anode manufacturers publish typical values.

4. What is NOT checked

Protection criteria such as −850 mV CSE (AMPP/NACE SP0169) and polarization.
Current attenuation along pipelines and the need for multiple groundbeds.
Interference with foreign structures and stray current.
Galvanic (sacrificial) anode systems.

5. Reference standards

Pointers only; consult the edition adopted by your authority having jurisdiction.

UFC 3-570-01 Cathodic Protection — U.S. DoD, approved for public release
AMPP (NACE) SP0169 — control of external corrosion on buried piping
H. B. Dwight, Calculation of Resistances to Ground (1936)
E. D. Sunde, Earth Conduction Effects in Transmission Systems (1949)

6. Validation cases

Hand-calculated cases run on every change. See Validation.

7. Known issues and changes

See the Changelog. Try the tool: Cathodic Protection Groundbed.