5. Cathodic Protection

5.10 Anode-to-Earth Resistance

Anode Earth Resistance of a Single Vertical Anode

The anode earth resistance of a single vertical anode is calculated using Dwight equation

where Rv = resistance of one vertical anode to earth
P = resistivity of backfill
L = length of the anode (or backfill) in ft
d = diameter of anode in ft

The resistance of two or more vertical anodes in parallel can be calculated by Sunde equation:

where S = spacing between the anodes in ft
N = number of anodes


From the data provided below, calculate the resistance of one vertical anode to earth.

Soil resistivity 100 ohm-cm
Backfill resistivity 5 ohm-cm
Backfill dimensions 8.5" x 28"    (0.7083' x 2.333')
Anode dimensions 5" x 5" x 20.5"   (0.470' x 1.7081')
First determine the resistance from the anode to the outer edge of backfill columns Rv(i)
Determining the resistivity of the vertical anode to backfill:
(use the dimensions of the backfill and the resistivity of the backfill as shown above)
Rv,1 = Rvi - Rv,backfill

       = 0.3614 - 0.2538

       = 0.1076 ohm

Next, calculate the resistance of the anode to soil (use backfill dimensions and resistivity of the soil)
To get the total resistance of one anode to earth

Rv,total = Rv,1 + Rv,2

            = 0.1076 + 5.074

            = 5.181 ohm