4–20 mA Wiring – 2-Wire vs 3-Wire vs 4-Wire
Understanding transmitter wiring types is critical for troubleshooting loop failures, grounding problems, and unexpected signal behavior. Many field mistakes happen because technicians assume all transmitters are loop-powered.
1️⃣ Two-Wire (Loop-Powered) Transmitter
This is the most common industrial configuration. The same two wires provide both power and signal.
- Powered from loop (typically 24V DC)
- No separate power terminals
- Simpler installation
- Most pressure & flow transmitters use this
Typical Wiring
If the loop opens anywhere, current drops to 0 mA.
2️⃣ Three-Wire Transmitter
Three-wire transmitters have a separate power return and a dedicated signal wire.
- Separate power supply required
- Common in temperature transmitters
- Better power stability
- More wiring complexity
Typical Wiring
Loss of signal does not always mean power loss.
3️⃣ Four-Wire Transmitter
Four-wire transmitters have a dedicated power supply and an isolated 4–20 mA output.
- Externally powered (often 230V AC or 24V DC)
- Output loop is independent
- Common in analyzers and gas detectors
- More robust for high-power devices
Even if transmitter is powered, signal loop can fail independently.
Quick Comparison
- 2-Wire: Most common, loop-powered, simplest
- 3-Wire: Separate return, moderate complexity
- 4-Wire: Fully powered, independent output
Field Troubleshooting Logic
- No signal on 2-wire → Check loop continuity first
- No signal on 3-wire → Verify supply voltage
- Signal fixed at 4 mA → Check underrange or sensor fault
- Signal fixed at 20 mA → Possible saturation or wiring short
- Unstable reading → Inspect grounding and shielding