Impressed Current Anode Bed Resistance

Dwight's single-anode resistance and Sunde's multiple-anode bed, beside the naive parallel figure.

Run the calculator

Example

You enter

Soil resistivity (ohm-cm) 5000
Anode or backfill column length (ft) 10
Column diameter (in) 8
Number of anodes 10
Centre-to-centre spacing (ft) 15
Alternative spacing (ft) 30
Alternative column length (ft) 20
Alternative column diameter (in) 6

You get

One anode to earth 9.87 ohms
The bed 1.640 ohms for 10 anodes at 15 ft
Naive parallel figure 0.986642
Interference penalty (%) 66.2176
At the alternative spacing 1.313 ohms at 30 ft (-20%)
One anode, longer column 5.84 ohms at 20 ft (-41%)
One anode, other diameter 10.6158

Details, formula, and sources

The worked bed of ten is 66% worse than parallel resistors, all of it interference, and doubling the column length cuts a single anode 41% where widening the hole barely moves it. Drill deeper, do not auger wider.

Dwight single anode R = 0.00521 rho / L x [ln(8L/d) - 1]; Sunde bed R_N = 0.00521 rho / (N L) x [ln(8L/d) - 1 + (2L/S) ln(0.656 N)], rho in ohm-cm and lengths in feet.

Dwight's and Sunde's groundbed relations for uniform soil. A Wenner resistivity survey at the bed's own depth and the CP designer govern.

Public relations published in the CP literature.

Estimate. AHJ and licensed professional govern.

Field names used by the API: soil_resistivity_ohm_cm, column_length_ft, column_diameter_in, anode_count, spacing_ft, alternative_spacing_ft, alternative_length_ft, alternative_diameter_in, single_anode_resistance_ohm, bed_resistance_ohm, parallel_resistance_ohm, interference_penalty_pct, alternative_spacing_bed_ohm, alternative_length_single_ohm, alternative_diameter_single_ohm

Soil uniform resistivity assumed; a layered profile changes the answerWenner survey
Mutual interference the Sunde term, which the naive parallel figure omitsspec-v1763 method