The 4-20 mA current loop is the most widely used analog signaling standard in industrial instrumentation. A transmitter varies the current in a two-wire loop in proportion to what it measures, and that current carries the value reliably over long distances. This guide explains how the loop works, why it uses 4 mA as its zero, and where it fits in SCADA.
4-20 mA Loop in one line: A 4-20 mA current loop is an analog signaling standard in which a field transmitter represents a measured value as a proportional electrical current between 4 milliamps (0 percent) and 20 milliamps (100 percent), sent over a two-wire loop to a controller.
In a 4-20 mA loop, the transmitter acts as a current regulator: it adjusts how much current flows in the loop to represent its measurement. A pressure transmitter ranged 0-1000 psi outputs 4 mA at 0 psi, 12 mA at 500 psi, and 20 mA at 1000 psi, scaling linearly in between. The receiving device, typically a PLC or RTU analog input card, passes the loop current through a precision resistor and measures the voltage across it to recover the value. Because the same current flows everywhere in a series loop, the signal is unaffected by wire length or connection resistance, which is why the standard has endured for decades.
Current signaling is chosen over voltage for two big reasons. First, current is immune to the voltage drop that would corrupt a voltage signal over the long cable runs common in the field, so a reading stays accurate hundreds of meters from the transmitter. Second, it is inherently noise-resistant in the electrically noisy environment around motors and drives. The result is a rugged, simple, self-powered loop that many two-wire transmitters can run entirely on the loop current itself.
A defining feature is the live zero: the range starts at 4 mA, not 0 mA, so a genuine zero measurement still draws current. This is deliberate. If a wire breaks or a transmitter loses power, the loop current falls to 0 mA, which is below the valid range and immediately flags a fault rather than being mistaken for a real reading of zero. That built-in diagnostic distinction between a true low value and a dead loop is a major safety advantage.
Many 4-20 mA transmitters also carry a HART signal, a small digital tone superimposed on the analog current that lets tools read diagnostics, configuration, and additional variables without disturbing the primary measurement. Once the loop current reaches the PLC or RTU, it is scaled into engineering units and stored as a tag. A SCADA platform such as Merobix then reads that tag from the controller over Modbus, OPC UA, or another protocol, so a wellhead pressure on a Merobix dashboard often traces directly back to a 4-20 mA loop in the field.
The 4 mA live zero means a true zero measurement still draws current. If the loop instead started at 0 mA, a broken wire or dead transmitter would read the same as a real zero. With a 4 mA floor, a 0 mA reading unambiguously signals a wiring or power fault, giving the loop built-in fault detection.
Current in a series loop is the same at every point, so it is immune to voltage drops across long cables and connection resistance, keeping readings accurate over hundreds of meters. Current signaling is also more resistant to the electrical noise around motors and drives, which would corrupt a low-level voltage signal in the field.
HART is a digital communication signal modulated on top of the analog 4-20 mA current. It lets configuration tools and asset-management systems read transmitter diagnostics, ranges, and secondary variables without disturbing the primary 4-20 mA measurement, so a single loop carries both the classic analog value and digital information.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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