What 4–20 mA means and why it does not start at zero

4–20 mA is the most widely used signal in industrial instrumentation. A pressure, level or temperature transmitter turns what it measures into a current: 4 mA is 0 % of its range and 20 mA is 100 %. The PLC measures that current and converts it back into bar, metres or degrees.

Why current and not voltage

In a loop, the same current flows through every element in series. That is why it is not affected by the length of the cable or the resistance of the terminals, which would make a voltage signal drop. A current signal also resists the electrical noise from nearby motors and drives better.

Why it starts at 4 mA and not 0

To detect faults. If the scale started at 0 mA, a broken wire would look like a reading of zero. With the "live zero" at 4 mA, 0 mA is a fault, not a measurement.
To power the transmitter. In two-wire transmitters the signal itself powers the electronics: with less than about 4 mA they could not work.

The NAMUR NE 43 recommendation sets the limits many devices use: between 3.8 and 20.5 mA is a valid reading (even if slightly out of range), and below 3.6 mA or above 21 mA indicates a failure.

Try it

Enter your transmitter’s range and the current you measure, or the process value to find the current it should give:

Range start (value at 4 mA) Range end (value at 20 mA) Unit Measured current (mA) Or: process value

value = start + (I − 4) / 16 × (end − start)

Two-wire and four-wire

Two-wire (loop-powered): the transmitter has no supply of its own. The 24 V supply, the transmitter and the PLC input are in series, in a single loop. This is the most common type.
Four-wire: the transmitter has a separate supply (for example 230 V or 24 V) and drives the current on two other wires. Used for devices that need more power, such as some flowmeters or analysers.

A two-wire loop has a resistance limit: the supply must provide enough voltage for the transmitter plus the drop across the input and the cable. For example, with 24 V and a transmitter that needs 12 V, the maximum load is (24 − 12) / 0.02 = 600 Ω.

How to measure it

By opening the loop and putting the multimeter in series, on the mA range.
With a milliamp clamp meter, without opening anything.
By measuring the voltage across a known resistor: with 250 Ω, 4–20 mA becomes 1–5 V.

If the PLC is a Siemens S7-1200 or S7-1500, the analog card turns 4–20 mA into a number from 0 to 27648: the Siemens analog scaling calculator explains how to convert it into real units.

Typical mistakeThinking 0 mA means "value zero". In a 4–20 mA loop, 0 mA means no current is flowing: a broken wire, a loose terminal or a loop with no power.

Other guides

Overload relay or motor protection circuit breaker: which one? · PNP or NPN sensor: the difference and how to tell which you have · What is a contactor and what is it used for?

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