Introduction to 4-20mA Current Loop Wiring
The 4-20mA current loop is the workhorse of industrial instrumentation. Pressure transmitters, temperature sensors, level sensors, and flow meters all commonly use this standard. However, one source of confusion is that sensors and transmitters come in 2-wire, 3-wire, and 4-wire configurations — and wiring them incorrectly can damage equipment or produce erroneous readings.
This guide explains each configuration clearly so you can wire any 4-20mA device with confidence.
2-Wire (Loop-Powered) Sensors
A 2-wire transmitter is also called a loop-powered device. It draws its operating power directly from the 4-20mA signal loop — no separate power supply connection is needed at the sensor.
How It Works
The power supply (typically 24VDC) is wired in series with the loop. The transmitter modulates the current flowing through the loop between 4mA (zero measurement) and 20mA (full-scale measurement). The receiving device (PLC input card or indicator) reads this current, often by measuring the voltage drop across a precision resistor (usually 250Ω).
2-Wire Wiring Steps
- Connect the positive terminal of the 24VDC supply to the transmitter's + terminal.
- Connect the transmitter's − terminal to the AI+ (analog input positive) terminal of the PLC.
- Connect the PLC's AI− (analog input common) terminal back to the negative of the 24VDC supply.
Note: The loop must be a single unbroken series circuit. Any break will cause the signal to drop to 0mA — a detectable fault condition.
3-Wire Sensors
A 3-wire transmitter has a separate power supply connection and a dedicated signal wire. This is typical of sensors that require more than a few milliamps to operate — often sensors with displays, processors, or active conditioning circuits.
3-Wire Wiring Steps
- Connect the supply positive (PWR+) wire (usually red) to the +24VDC supply.
- Connect the supply negative / common (PWR−) wire (usually black or white) to the 0V / supply negative.
- Connect the signal output wire (usually green or blue) to the AI+ terminal of the PLC.
- Connect the PLC's AI− (common) to the same 0V supply negative — completing the signal return path.
4-Wire Sensors
A 4-wire transmitter is essentially the same as a 3-wire device but with both supply and signal fully isolated from each other. This is the configuration used when the transmitter electronics are fully isolated, providing maximum noise immunity and flexibility. It is common in high-precision industrial instruments.
4-Wire Wiring Steps
- Connect PWR+ and PWR− to the 24VDC supply (wires 1 and 2).
- Connect Signal OUT+ to AI+ on the PLC (wire 3).
- Connect Signal OUT− to AI− on the PLC (wire 4).
- The signal ground and supply ground may or may not be tied together — check the datasheet.
Quick Comparison Table
| Configuration | Wires | Power Source | Best For |
|---|---|---|---|
| 2-Wire | 2 | Loop-powered | Simple sensors, long runs |
| 3-Wire | 3 | External supply | Sensors needing more power |
| 4-Wire | 4 | External, isolated | High-precision, isolated instruments |
Common Wiring Mistakes to Avoid
- Reversing polarity on a 2-wire loop will cause 0mA output and may damage the transmitter.
- Grounding the signal wire at both ends creates a ground loop that causes noise and offset errors.
- Exceeding loop compliance voltage — ensure your supply voltage is sufficient to drive the loop resistance (sensor + cable + receiver resistor combined).
- Mixing up a 3-wire NPN/PNP switching sensor with a 4-20mA transmitter — they use similar connectors but are electrically incompatible.
Summary
Choosing the right wiring configuration depends on the sensor type and your system requirements. When in doubt, always consult the sensor's datasheet and identify whether it is loop-powered or requires a separate supply. Correct wiring ensures accurate, reliable measurements and protects both the sensor and the PLC input cards.