2-Wire Current Loop vs. 4-Wire Current Source: What’s the Difference?

4–20 mA signals are widely used in industrial automation and instrumentation for transmitting analog sensor data over long distances. You can use them to measure pressure, temperature, flow rate, level, position, speed, etc.

Though they serve a similar purpose, their design and applications differ significantly. In this video we will talk through the differences in wiring each one.

2 Wire Current Loop. This is the most common and efficient method for transmitting analog signals over long distances. In this setup the transmitter and sensor share the same two wires to both power the device and carry the signal. Power typically comes from a remote power supply or PLC input and it is loop-powered, meaning the sensor draws minimal power from the current flowing through the loop. This is widely used for field transmitters like pressure, temperature, and level sensors.

Advantages:

Simpler wiring
Lower installation costs
Ideal for long cable runs and hazardous environments

4 Wire Current Source. This separates the power and signal lines. In this setup the device has two wires for power and two for the current signal output. It is self-powered (also called active or externally powered). Common in devices that require more power than a loop can supply or where signal isolation is needed.

Advantages:

More power for complex sensors or displays
Better isolation from noise
Useful in lab environments or for standalone devices

Choosing Between the Two

Use 2 wire current loops when wiring simplicity, efficiency, and low power use are key, especially in field installations.
Use 4 wire sources when devices need more power or operate independently of the loop.

2 Wire Current Loop vs 4 Wire Current Source

2-Wire Current Loop vs. 4-Wire Current Source: What’s the Difference?