4–20 mA Loop Voltage Calculator

Check a two-wire current loop’s supply, transmitter requirement, receiver burden and cable resistance at the chosen current.

Enter your values

Supply voltage (V)Transmitter minimum (V)Design current (mA)Total receiver burden (Ω)Complete cable loop (Ω)

Try an example

Your result

Voltage headroom6 V

Total resistive drop (V): 6
Voltage at transmitter (V): 18
Total resistance budget (Ω): 600

Calculated

Positive headroom in this resistive model. Check worst-case device specifications.

The supply voltage must cover every series burden

How this answer is calculated

These steps use your current values. Change an input to follow a different calculation.

1
Add the series resistance
250 + 50 = 300 Ω
2
Find the voltage lost in the loop
20 ÷ 1000 × 300 = 6 V
3
Check what remains for the transmitter
24 − 6 − 12 = 6 V

How to use this on a job

The transmitter can regulate current only while it has enough voltage left to operate. Every series resistance spends part of the supply. A loop that behaves at 4 mA can run out of voltage near the top of the range.

01
Use the lowest supply voltage expected at the loop and the transmitter’s minimum operating voltage from its data sheet.
02
Enter the design current, total receiver resistance and complete outgoing-plus-return cable resistance. If a device specifies a fixed voltage drop rather than a resistor, this simple resistive model is not sufficient for that device.
03
Check the remaining transmitter voltage and headroom. A negative margin cannot support the chosen operating point in this model. Test at the maximum current the transmitter may produce, including a configured alarm current when applicable.

Work through one example

margin = Vₛ − I × (Rload + Rcable) − Vminimum

At 20 mA, a 250 Ω receiver and 50 Ω complete cable loop drop 6 V. A 24 V supply leaves 18 V at the transmitter. If the transmitter needs 12 V, the headroom is 6 V. Reduce the supply to 15 V and the headroom becomes −3 V; the loop cannot sustain that current under these assumptions.

Before you trust the answer

Enter both cable conductors, not just the one-way length. Temperature, supply tolerance, barriers and isolators can change the real budget. This is a DC resistive calculation; it does not establish intrinsic safety, isolation suitability or HART impedance compliance.

Voltage across a 250 Ω input resistor

Current	Voltage drop	Calculation
4 mA	1 V	0.004 × 250
12 mA	3 V	0.012 × 250
20 mA	5 V	0.020 × 250

Questions that come up on the job

Why does the measurement stop rising near the top?

A two-wire transmitter needs a minimum voltage to operate. As current rises, more voltage drops across the receiver and cable. If too little remains, the transmitter may be unable to drive the requested current. Check the budget at the highest required current, not only at 4 mA.

Which cable resistance should I enter?

Use the complete current path, including the outbound and return conductors. If a cable table gives resistance for one conductor per metre, account for both lengths. Add other resistive series burdens to the load. Barriers and isolators may need a more detailed manufacturer-specific model.

TRYPLC FIELD REFERENCE / 01