4–20 mA Loop Wiring Types
Transmitters are classified based on how they are powered and how the signal loop is formed. Correct identification prevents wiring mistakes and equipment damage.
Two-Wire Transmitter (Loop Powered)
- Same two wires provide power and signal
- Most common in pressure, level, flow transmitters
- Minimum loop voltage must be maintained
Advantage: Simple wiring. Limitation: Voltage drop must be calculated carefully.
Three-Wire Transmitter
- Separate power supply return
- Common signal ground shared
- Used in some older or specialty transmitters
Four-Wire Transmitter (Externally Powered)
- Separate power input (often 230 VAC or 24 VDC)
- Signal output independent of power
- Common in gas analyzers and large instruments
Advantage: Strong signal drive capability. Limitation: More wiring and higher cost.
Quick Comparison
| Feature | 2-Wire | 3-Wire | 4-Wire |
|---|---|---|---|
| Power & Signal Same Pair | ✔ Yes | ✖ No | ✖ No |
| Most Common | ✔ Yes | Rare | Analyzers |
| Voltage Drop Critical | ✔ Yes | Moderate | Low Concern |
Loop Voltage Consideration
Total loop resistance must allow sufficient voltage for transmitter operation.
Available Voltage = Supply Voltage − (Loop Current × Total Resistance)
If voltage at transmitter falls below minimum operating requirement, signal instability occurs.
Field Rule
Always confirm wiring type before replacing a transmitter. A 2-wire device cannot be directly replaced with a 4-wire model without circuit modification.