Current Requirement for Cathodic Protection

Quick Definition

Required current is calculated as:

In symbols:

Where:

• I is required current.
• A is the area used in the estimate.
• J is the assumed current density.

The current-density units must match the area units.

When Current Requirement Is Used in CP Work

Current requirement may be used when estimating:

• current for an exposed or bare area;
• current for a selected tank bottom area;
• current for a selected pipeline area;
• current for comparison between different areas;
• current requirement before selecting or reviewing CP equipment.

This page does not prescribe what current density to use.

This page does not determine whether a structure is protected.

This page does not replace structure-to-electrolyte potential measurements, current distribution review, or design judgment.

Formula Reference

Plain-language form:

Variable and Unit Table

Use matching units:

• ft2 × A/ft2 = A
• ft2 × mA/ft2 = mA
• m2 × A/m2 = A
• m2 × mA/m2 = mA

Common conversions:

• 1 A = 1000 mA
• 1 mA = 0.001 A

How to Use the Formula

First, identify the area used in the estimate.

The area may be:

• total surface area;
• exposed area;
• bare area;
• tank bottom area;
• pipeline outside surface area.

Next, identify the assumed current density.

The current density must be supported by the project, design basis, test method, engineering review, or controlling source. Do not treat an example value as a universal requirement.

Finally, multiply area by current density.

If the current density is in mA/ft2, the answer will be in mA.

If the current density is in A/ft2, the answer will be in A.

Worked Example 1 — Required Current in Milliamps

An estimated exposed area is 50 ft². A basic arithmetic exercise uses an assumed current density of 10 mA/ft². What current does that represent?

Known values:

• A = 50 ft2
• J = 10 mA/ft2
• I = ?

Use:

Substitute:

Calculate:

Convert if needed:

Answer: The estimated current is 500 mA, or 0.500 A.

Field meaning: This shows the arithmetic relationship between area, current density, and current. It does not mean that 10 mA/ft² is a required design value.

Worked Example 2 — Required Current in Amps

An estimated area is 200 ft². A current density of 0.02 A/ft² is used for a basic calculation. What required current does that represent?

Known values:

• A = 200 ft2
• J = 0.02 A/ft2
• I = ?

Use:

Substitute:

Calculate:

Answer: The estimated current is 4 A.

Field meaning: The result depends directly on both the area and the assumed current density.

Worked Example 3 — Estimate Exposed Area First

A pipeline has a total outside surface area of 2,000 ft². An estimate assumes 3% exposed area. A basic arithmetic exercise uses 15 mA/ft² for the exposed area. What current does that represent?

Known values:

• total area = 2,000 ft2
• exposed percentage = 3%
• current density = 15 mA/ft2

Convert percentage to decimal:

Calculate exposed area:

Use:

Substitute:

Calculate:

Convert if needed:

Answer: The estimated current is 900 mA, or 0.900 A.

Field meaning: Both the exposed-area percentage and current-density value are assumptions. Changing either one changes the result.

Worked Example 4 — Solve for Current Density

A CP review estimate uses 3 A of current over 150 ft² of selected area. What current density does that represent?

Known values:

• I = 3 A
• A = 150 ft2
• J = ?

Use:

Substitute:

Calculate:

Answer: The current density is 0.02 A/ft².

Field meaning: This describes current per square foot for the selected area. It does not prove protection.

Common Mistakes

1. Using an unsupported current-density value. Example values are for arithmetic practice unless a project, design, or source says otherwise.
2. Confusing total area with exposed area. The selected area strongly affects the current estimate.
3. Mixing amps and milliamps. A/ft2 and mA/ft2 produce different current units.
4. Treating an estimate as a field result. A current requirement estimate is not the same as measured CP performance.
5. Assuming current requirement proves protection. Protection must be evaluated with proper field measurements and criteria.
6. Forgetting percent-to-decimal conversion. 3% is 0.03, not 3.
7. Using area units that do not match current-density units. Do not multiply square feet by a current density stated per square meter.
8. Using too many decimals. Current requirement estimates often depend on assumptions.

Practice Problems

1. An estimated exposed area is 100 ft². A basic arithmetic exercise uses 5 mA/ft². What current does that represent?
2. An area is 250 ft². A current density of 0.01 A/ft² is used. What current does that represent?
3. A total area is 1,500 ft². If 2% is assumed exposed, what exposed area is used?
4. Using the exposed area from Problem 3, and an assumed current density of 20 mA/ft², what current does that represent?
5. Convert 750 mA to amps.
6. A current of 2 A is used over 100 ft². What current density does that represent?
7. If the assumed current density doubles and the area stays the same, what happens to the estimated current?
8. Does a current requirement calculation by itself prove that a structure satisfies CP protection criteria?

Practice Problem Answer Key

Related Learning

Use these pages to continue studying related concepts:

• Pipeline Surface Area
• Tank Bottom Surface Area
• Current Density
• Coating Breakdown Factor
• Soil Resistivity
• Ohm’s Law
• Current Requirement Testing

Related formulas:

• Current Density
• Pipeline Surface Area
• Tank Bottom Surface Area
• Coating Breakdown Factor
• Ohm’s Law