Loop power budget for 4-20 mA loops

Check the voltage a 4-20 mA loop leaves the transmitter.

AS/NZS 3008.1.2 AS/NZS 3008.1.1

Inputs

Worked example with the default inputs. The live calculator replaces it when the page loads.

Loop supply voltage: 24 V
Transmitter minimum voltage: 12 V
Loop current: 20 mA
Input resistance: 250 Ω
HART resistor: 0 Ω
Other series resistance: 0 Ω
Cable length, one way: 500 m
Conductor resistance: From the conductor size
Conductor size: 1 mm²
Standard: NZ · AS/NZS 3008.1.2

Voltage margin

6.48V

Voltage at the transmitter: 18.48V
Loop resistance: 275.8Ω
Cable resistance: 25.8Ω
Maximum loop resistance: 600Ω
Maximum cable length: 6,782m

This result is a design aid. Verify it against the current standard.

Calculation steps

Conductor resistance25.8Ω/kmAS/NZS 3008.1.2:2017 Tables 34 to 35
Cable resistance, both conductors25.8Ω
Series resistance250Ω
Loop resistance275.8Ω
Loop current20mA
Voltage drop in the loop5.52V
Voltage at the transmitter18.48V
Voltage margin6.48V
Maximum loop resistance600Ω
Maximum cable length6,782m

How the tool checks the loop

A 4-20 mA loop is one series circuit. The loop supply drives the current through the transmitter, the cable and the receiver input. The transmitter needs a minimum voltage at its terminals to work.

Conductor resistance. The tool reads the resistance of a copper conductor for the size you pick. It can also take the value from the cable data.
Cable resistance. The current goes out on one conductor and comes back on the other. So the tool doubles the one-way length.
Loop resistance. It adds the cable resistance to the input resistance, the HART resistor and any other series parts.
Voltage at the transmitter. It multiplies the loop current by the loop resistance and subtracts that drop from the supply voltage.
Voltage margin. It subtracts the transmitter minimum voltage. A margin of 0 V or more passes.
Maximum length. It gives the longest one-way run that keeps the margin at 0 V or more.

Worked loops

Each line opens the tool with its values. Every line uses a 24 V supply, a 12 V transmitter minimum voltage and a 250 Ω receiver input.

Pressure transmitter, 500 m of 1 mm² at 20 mA. The loop passes with a margin of 6.48 V.
HART loop, 400 m of 1 mm² at 22 mA. A 250 Ω HART resistor sits in the loop. The loop passes with a margin of 0.55 V.
HART loop, 500 m of a 36 Ω/km pair at 22 mA. The resistance comes from the cable data. The loop passes with a margin of 0.21 V.
The same HART loop at 700 m. The loop fails with a margin of -0.11 V. Shorten the run, use a larger conductor or raise the supply.

New Zealand and Australia

The Standard select picks the part of AS/NZS 3008.1 the tool reads for the conductor resistance. Part 1.2 is the New Zealand part, 2017 edition. Part 1.1 is the Australian part. For Australia, the Edition select picks 2025 or the superseded 2017 edition. All three print the same resistance values. The impedance table entry says how AmpSize checked them.

Related checks

The comms cable selector checks the cable construction and the run length for the interface.
The DC cable size calculator sizes the 24 V d.c. supply cable to the transmitter panel.
The segregation checker checks the spacing between the loop cable and power cables.

Questions

Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.

What sets how far a 4-20 mA loop can run?

The supply voltage, less the transmitter minimum voltage, is the budget. The loop current through every series resistance spends it. The cable adds the resistance of both conductors, so a longer run or a thinner conductor spends more.

Why check at more than 20 mA?

A transmitter can drive more than 20 mA, for example an upscale alarm near 22 mA. The drop in the loop rises with the current. Check at the highest current the transmitter drives.

Where do I find the transmitter's minimum voltage?

The transmitter data sheet gives it, often as the lift-off voltage or the minimum terminal voltage. Some data sheets give a load line instead. Read the voltage at the highest loop current.

Clauses and tables this tool reads

Every clause and table, with how each was verified

Tables last checked on 27 Sept 2026.