Anode Output Formula for Cathodic Protection

Anode output calculations estimate how much current an anode may deliver under assumed field conditions. In practice, output depends on anode material, electrolyte resistivity, installation geometry, backfill, circuit resistance, and driving voltage.

The simplified study approach commonly uses Ohm's law to relate driving voltage, resistance, and current output.

Simplified Formula

I = V / R

Symbol Meaning Common Unit
I Anode current output amps
V Driving voltage volts
R Circuit resistance ohms

What the Calculation Represents

The simplified calculation estimates current output from an assumed driving voltage and resistance. It is useful for study and rough checking, but actual anode output should be verified in the field when possible.

Galvanic anode systems can change output as polarization develops, soil moisture changes, connections deteriorate, or anodes deplete.

Worked Example 1: Single Anode Output

A galvanic anode has an assumed driving voltage of 0.5 volts and a circuit resistance of 2 ohms. Estimate the current output.

I = V / R

I = 0.5 / 2

I = 0.25 amps

I = 250 mA

The estimated output is 0.25 amps, or 250 mA.

Worked Example 2: Higher Resistance

A similar anode has the same 0.5-volt driving voltage, but circuit resistance is 5 ohms. Estimate the output.

I = V / R

I = 0.5 / 5

I = 0.1 amps

I = 100 mA

The higher resistance reduces estimated output to 100 mA. This illustrates why soil resistivity, anode installation, backfill, and connection quality matter.

Worked Example 3: Multiple Similar Anodes

Four similar anodes are each estimated to output 150 mA. Estimate the total output if they are operating similarly.

Total Output = Number of Anodes x Output per Anode

Total Output = 4 x 150

Total Output = 600 mA

Total Output = 0.6 amps

The estimated total output is 0.6 amps. In the field, individual anode outputs may not be equal, so branch current measurements are useful when available.

CP Interpretation Notes

  • Anode output calculations are estimates, not proof of CP adequacy.
  • Higher resistance generally reduces current output for the same driving voltage.
  • Actual output may change over time as polarization, moisture, and anode condition change.
  • Uneven anode output can affect current distribution.
  • Structure potentials and applicable criteria are still required to evaluate protection.

Common Mistakes

Assuming Equal Output

Multiple anodes may not share current equally because resistance and installation conditions can vary.

Ignoring Resistance Changes

Dry soil, failed splices, depleted anodes, and poor backfill contact can increase resistance and reduce output.

Treating Output as Criteria

Anode output supports system evaluation, but CP criteria depend on structure response and valid field measurements.

Practice Problems

  1. An anode has 0.6 volts driving voltage and 3 ohms resistance. What is the estimated output?
  2. An anode outputs 0.2 amps. What is that value in milliamps?
  3. Six similar anodes each output 75 mA. What is the estimated total output in amps?

Related Pages

Field Use Reminder

Anode output estimates are useful for planning and troubleshooting, but actual output should be confirmed with field measurements when possible because resistance, moisture, backfill, and anode condition can change over time.