Shunt Current Formula for Cathodic Protection

Shunts are used throughout cathodic protection work to measure current in rectifiers, bonds, galvanic anode leads, and individual impressed-current anode circuits. The calculation is simple, but the interpretation depends on the shunt value and polarity.

Formula

I = V / R

I = mV drop × shunt factor

Symbol	Meaning	Common Unit
I	Current through the shunt	amps
V	Voltage drop across the shunt	volts or millivolts
R	Shunt resistance	ohms
Factor	Current per millivolt	A/mV

Worked Example 1: Using Shunt Resistance

A shunt has a resistance of 0.01 ohm. A meter reads 25 mV across the shunt. What current is flowing?

25 mV = 0.025 V

I = V / R

I = 0.025 / 0.01

I = 2.5 A

The current through the shunt is 2.5 amps.

Worked Example 2: Using A/mV Factor

A shunt has a factor of 0.1 A/mV. The measured drop is 38 mV. What current is flowing?

I = mV × factor

I = 38 × 0.1

I = 3.8 A

The current through the shunt is 3.8 amps.

Direction of Current Flow

Current magnitude is only part of the answer. The polarity of the millivolt reading helps determine direction. This matters when evaluating bonds, diode/current-reversing switch behavior, interference bonds, and galvanic anode output.

Always record lead placement, meter polarity, magnitude, and units so the reading can be interpreted later.

Interpretation Notes

Rectifier shunts are commonly used to verify or replace panel ammeter readings.
Bond shunts help determine whether current is draining, returning, or flowing in an unintended direction.
Individual anode shunts can reveal uneven current distribution or inactive anodes.
Very small millivolt readings require a stable meter connection and careful polarity control.

Common Mistakes

Using mV as V

When using I = V/R, convert millivolts to volts first.

Wrong Shunt Factor

Different shunts use different resistance and A/mV values. Do not assume all shunts are the same.

Ignoring Polarity

A magnitude-only reading may miss the most important part of the test: direction.