Impressed Current Anode Bed Resistance
Dwight's single-anode resistance and Sunde's multiple-anode bed, beside the naive parallel figure.
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
Checked against published worked examples from: US Department of Defense.
Details, formula, and sources
ScopeThe 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.
Formula or tableDwight 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.
Source and editionDwight's and Sunde's groundbed relations for uniform soil. A Wenner resistivity survey at the bed's own depth and the CP designer govern.
Free accessPublic relations published in the CP literature.
What governsEstimate. AHJ and licensed professional govern.
API field namessoil_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