Formula and Inputs
Loop margin = supply voltage - transmitter minimum voltage - cable resistance drop - barrier and shunt voltage drop.
VoltCheck24 Engineering Calculator
4-20mA Loop Voltage Margin Calculator
Check transmitter voltage margin, loop resistance, shunt resistor loading, and cable losses for industrial 4-20mA current loops.
How to use this calculator
- Collect the values below from the equipment nameplate, data sheet, or field measurement.
- Enter values using one consistent unit system and use worst-case operating conditions.
- Review the result, margin, and engineering note before selecting hardware.
- Cross-check the result with the related calculator and save the review for your project record.
Input checklist
- Supply voltage and worst-case voltage
- Transmitter minimum operating voltage
- Cable length and conductor resistance
- Barrier and shunt resistance
Industrial Use Cases
Use this tool for pressure, flow, temperature, and level transmitters connected to PLC or DCS analog input cards.
Engineering Note
Verify the transmitter minimum operating voltage, barrier entity parameters, shunt resistor tolerance, and worst-case supply voltage from manufacturer data.
FAQ
Why is 250 ohms common in a 4-20mA loop?
A 250 ohm shunt converts 4-20mA into 1-5V using Ohm’s law, which matches many analog input modules.
What causes a 4-20mA loop to fail at 20mA?
The loop may run out of voltage compliance because cable, barrier, shunt, and device losses exceed the available supply margin.
Preliminary engineering reference only. Confirm final selections against current standards, project specifications, and manufacturer data.