4–20 mA Loop Wiring: Two, Three and Four Wires
Identify two-, three-, and four-wire transmitter arrangements, distinguish loop power from signal measurement, and record the exact input and output requirements.
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Compare transmitter arrangements
A wire count describes an arrangement, not a terminal assignment. Start by finding which device supplies loop energy and which regulates or measures current. The functional paths below omit shields and protective conductors; they are reading aids, not installation diagrams.
| Arrangement | Power and signal relationship | Confirm in both manuals |
|---|---|---|
| Two-wire loop-powered | Operating power and signal share the series loop. | Transmitter minimum voltage, receiver burden, loop supply and polarity. |
| Three-wire separately powered | Supply positive, common and signal are commonly separate connections. | Whether the output sources or sinks current, and how signal common relates to supply common. |
| Four-wire separately powered | A separate power pair operates the device; a signal pair carries the output. | Whether the signal is active or requires external loop power; isolation is not implied by wire count. |
Match the transmitter to the input
Read the input module channel diagram before connecting a transmitter. A passive current input measures current but does not necessarily power the loop. A powered input may supply the transmitter. Manufacturers use active/passive terminology differently: write down “supplies loop voltage” or “measures current only” beside the actual terminals.
- Identify the complete transmitter and input module models and channel mode.
- Trace one continuous signal-current path, including its return.
- Locate every power source and confirm the documented combination.
- Check input burden, transmitter voltage requirement, isolation and common-mode limits.
Do not place two supplies in a signal path merely because both drawings show a positive terminal. A shared common can also defeat an assumed isolation boundary.
Keep a loop record
Record the loop ID, transmitter range and units, channel address, source of loop power, terminal references and manual revisions. Attach the actual wiring drawing and record any isolator or barrier in the path. Shield treatment belongs to the product and site design, not to a rule based on the number of signal wires.
Use test records for measured values and the cable schedule for endpoints. Signal scaling and available loop load remain separate calculation tasks.
For the separate numerical task, use signal scaling, loop load resistance. Use the equipment data identified above as inputs within each tool’s stated scope.