Fieldeng
Calculate shunt current from a millivolt reading.
Divide the measured voltage across a DC shunt by its known resistance: I = V / R. If the shunt is labeled with a millivolt and amp rating, use I = measured mV × rated A / rated mV.
In cathodic protection work, a shunt lets you calculate current from a voltage reading. Keep the shunt rating and measured drop with the result so the next person can understand how you obtained it.
Worked example: a 50 mV / 10 A shunt
These are illustrative values. Suppose the shunt is rated 50 mV at 10 A, and the measured drop is 12.5 mV. The rating describes two quantities together; 50 mV alone does not tell you the current.
- Find the scale factor. 10 A ÷ 50 mV = 0.2 A/mV.
- Apply the reading. 12.5 mV × 0.2 A/mV = 2.5 A.
- Check the proportion. 12.5 mV is one quarter of the rated voltage drop, so the current is one quarter of 10 A.
The same example using resistance
Convert the rated voltage to volts before dividing by amps:
R = 0.050 V ÷ 10 A = 0.005 Ω
Now convert the measured voltage and apply Ohm’s law:
I = 0.0125 V ÷ 0.005 Ω = 2.5 A
Both methods give the same result. Dividing 12.5 directly by 0.005 would give a result 1,000 times too large because it treats millivolts as volts.
Enter it in Fieldeng
Open Shunt Current in the app. Enter 0.005 Ω for shunt resistance and 12.5 mV for shunt voltage. The result is 2.5 A. The Shunt Current reference lists the formula, inputs, output, and assumptions.
If using camera capture, confirm the sign, decimal point, and units before using the estimate. Save the result with an optional location and asset. Include the shunt rating and any relevant measurement notes in the record.
Checks that keep the result useful
- Use the actual shunt rating. A different amp rating changes the scale factor even when the rated millivolt output is the same. Use calibration data when supplied.
- Preserve polarity. A negative reading produces a negative current with the same resistance. Interpret direction using the lead orientation and measurement setup.
- Keep the assumptions visible. This example assumes DC, a known positive resistance, and a linear shunt operated within its specified conditions. Tolerance and temperature affect the real measurement.
- Separate calculation from assessment. A calculated current does not establish whether a CP system is adequate or compliant.
This is a calculation example, not a live-circuit measurement or installation procedure. Follow the equipment instructions and your organization’s safe-work procedures.
Sources and related tools
Weschler Instruments: Using DC Current Shunts explains rated millivolt outputs and the resistance relationship. Bourns: Ammeter Shunts FAQ describes measuring current through the voltage drop across a precision resistor. The values above are an original worked example.
For a known current and resistance, see Shunt Voltage. For the rest of the field workflow, see Fieldeng’s cathodic protection tools.