1. NACE
Companion to the Peabody Book
October 26, 2000
Revision 1.1
Dwight's Equation for Single Vertical Anode Resistance to Earth - inches
Soil resistivity in ohm-cm 10,000 ohm-cm
Rod length in inches 84.0 inches
Rod diameter in inches 8.00 inches
Resistance of vertical rod in ohms 25.5 ohms
Dwight's Equation for Single Vertical Anode Resistance to Earth - feet
Resistivity of backfill material (or earth) in ohm-cm 10,000 ohm-cm
Length of anode in feet 7.0 feet
Diameter of anode in feet 0.667 feet
Resistance of one vertical anode to earth in ohms 25.5 ohms
Dwight's Equation for Multiple Vertical Anodes in Parallel - feet
Soil resistivity in ohm-cm 10,000 ohm-cm
Number of anodes in parallel 10 each
Length of anode in feet 7.0 feet
Diameter of anode in feet 0.667 feet
Anode spacing in feet 10.0 feet
Resistance of vertical anodes in parallel to earth in ohms 4.51 ohms
NOTICE
For these equations to be valid the soil must be homogeneous and L >> d. With low resistance backfill, as
coke, d is taken as the diameter of the coke and L as the length of the backfill provided it does not extend
too much greater than the length of the anode (about 1.5 times the coke diameter).

2. NACE
Companion to the Peabody Book
October 26, 2000
Revision 1.1
Modified Dwight's Equation for Multiple Anodes Installed Horizontally
Resistance, in ohms, of horizontal anode to earth
Resistivity, in ohm-cm, of backfill material (or earth)
Length of anode in feet
Twice depth of anode in feet
Diameter of anode in feet
10,000 ohm-cm
7.0 feet
12.0 feet
0.667 feet
22.49 ohms
NOTICE
For these equations to be valid the soil must be homogeneous and L >> d. With low resistance backfill, as
coke, d is taken as the diameter of the coke and L as the length of the backfill provided it does not extend
too much greater than the length of the anode (about 1.5 times the coke diameter).

3. NACE
Companion to the Peabody Book
October 26, 2000
Revision 1.1
Galvanic Anode Life
Mag Anode Zinc Anode
Weight = 240 lbs Weight = 240 lbs
Efficiency = 0.50 Efficiency = 0.90
Utilization Factor = 0.85 Utilization Factor = 0.85
Current = 2.0 A Current = 2.0 A
Mg Life = 5.9 yrs Zn Life = 3.9 yrs
Consumption Solution
Output² Rate² Potential³
Anode¹ (amp-hr/lb) (kg/amp-hr) Efficiency² (Cu-CuSO4)
Zinc 370 23.7 90% -1.1 V
Std. Mg 500 17.4 50% -1.4 to -1.6 V
Hi-Pot Mg 500 17.4 50% -1.7 to -1.8 V
1. Anodes installed in suitable chemical backfill.
2. Current efficiency with current density. The shown efficiency, and the resulting consumption rate, are
at approximately 30 milliamps/ft² of anode surface. Efficiencies are higher at higher current densities and
lower at lower current densities.
3. The potentials are solution potentials. When calculating driving potentials, the difference between the
protected structure and the anode, allow for anode polarization. Anode polarization is also influenced by
current density at the anode surface. For magnesium polarization allow for 0.1 V anodic polarization.
Zinc in a proper backfill is not usually subject to significant anodic polarization and the solution potential
may be used.

4. NACE
Companion to the Peabody Book
October 26, 2000
Revision 1.1
Rectifier Total Circuit Resistance
Ground bed resistance (ohms) 3.08 ohms
Cable resistance (ohms) 0.097 ohms
Pipeline/structure to earth resistance (ohms) 7.53 ohms
Total circuit resistance (ohms) 10.71 ohms
Deep Anode Ground Bed Resistance
Effective soil resistivity (ohm-cm) 10,000 ohm-cm
Anode length (feet) 40 feet
Anode diameter (feet) 0.667 feet
Resistance to earth of a vertical single anode (ohms) 6.73 ohms
Rectifier Efficiency
K = Meter constant DC Amps = 1.00 A
N = # of revolutions of disk DC Volts = 2.00 V
T = Time in seconds K= 0.01
N= 4
T= 10 sec
Efficiency = 27.78%

5. NACE
Companion to the Peabody Book
October 26, 2000
Revision 1.1
Impressed Current - # of Anodes Required
Wt = Weight per anode (pounds) Wt = 60 lbs
CR = Consumption rate (lbs./amp-year) CR = 0.75 lbs/A-yr
DL = Desired life (years) DL = 20 yrs
Current = Current required (amps) Current = 15.00 A
UF = Utilization factor UF = 0.60
# anodes = 7 each
# of Anodes Required Based on Current Discharge
* from manufacturer data
MD = Maximum discharge per anode (amps) MD = 2.50 A
Current = Current required (amps) Current = 15.00 A
# anodes = 6 each
Cable Resistance
Resistance per 1000 feet (Mft) 0.254 ohms/Mft
Length in feet (sum of positive and negative cables) 384 feet
Cable resistance 0.097 ohms