3-Wire RTD Temperature Sensor Wiring Made Clear

A resistance temperature detector (RTD) measures temperature by tracking the predictable change in resistance of a sensing element, commonly platinum. A Pt100 has 100 ohms at 0°C, while a Pt1000 has 1,000 ohms at the same reference point. The instrument converts that resistance into a temperature reading.

Three-wire RTD temperature sensor wiring is popular because it reduces the error caused by lead resistance without needing the extra conductor used by a four-wire circuit. It is widely found in process control, refrigeration, HVAC equipment, industrial ovens, water systems and engine monitoring.

For Australian installers, the practical details matter. Long cable runs, hot plant rooms, outdoor switchboards, salty coastal air and the 12 V or 24 V systems used in utes and boats can all affect reliability. Correct terminal identification and a sound connection are more important than simply matching wire colours.

How a three-wire RTD works

A three-wire sensor uses two conductors connected to one side of the RTD element and a third conductor connected to the other side. The measuring transmitter or PLC assumes that the two same-side wires have approximately equal resistance. It then subtracts the lead resistance from the measurement.

This arrangement is less accurate than a four-wire RTD, but it is considerably better than a two-wire sensor for ordinary industrial measurements. A typical Pt100 with copper extension cable can produce a noticeable error over a long distance, especially when the cable passes through warm areas or runs beside high-current equipment.

The compensation method depends on the input device. A compatible temperature transmitter, data logger or PLC analogue input should be configured specifically for a three-wire Pt100 or Pt1000. A standard resistance input may display an incorrect value if it does not support lead compensation.

Identifying the sensor leads

Wire colours are not universal, so do not rely on colour alone. Many IEC-style RTDs use two matching-colour wires for the duplicated side of the element and one different-colour wire for the other side. Some manufacturers use red and white combinations, while others use black, red or custom colour codes.

The most reliable check is a resistance test with the sensor disconnected from the control circuit. Measure between each pair of wires. The two wires belonging to the same side will usually show a very low resistance between them, while measurements involving the opposite side will show the RTD’s nominal resistance plus the cable resistance.

At room temperature, a Pt100 will often measure around 107–110 ohms, depending on the actual temperature. A Pt1000 will commonly read around 1,070–1,100 ohms. Record the measurements before landing the wires, and label the conductors at both ends.

Basic wiring arrangement

At the sensor end, connect both matching-side conductors to the two terminals associated with one end of the RTD element. Connect the remaining conductor to the other RTD terminal. At the instrument end, place the matching pair into the terminals marked for the compensated or duplicated side, often labelled A and A’, with the single conductor connected to B.

Terminal markings vary between manufacturers. Some transmitters use 1, 2 and 3; others use A, B and B’, or use symbols in the wiring diagram. Always follow the device manual rather than copying a terminal arrangement from another brand.

Use a twisted, screened instrumentation cable where the run is exposed to variable-frequency drives, contactors, ignition systems or radio transmitters. Keep the RTD cable separate from mains wiring. In Australian workshops and sheds, routing a low-level sensor cable alongside a compressor feed or welding circuit can create unstable readings.

Cable length and connection quality

Three-wire compensation assumes the two compensated leads are similar in length, gauge, material and temperature. Avoid extending only one conductor. If a junction is necessary, use the same type of cable and make all three connections in the same enclosure.

Copper conductors are common, but their resistance changes with temperature. A loose terminal, corroded crimp or damp junction box can add resistance and cause a false high temperature. Outdoor installations in coastal Queensland, Western Australia or around Sydney’s harbour need sealed cable glands and terminals suitable for moisture and salt exposure.

Use ferrules or approved terminals where appropriate, and avoid twisting bare wires under a screw terminal. In a marine installation, tinned copper cable and adhesive-lined heat-shrink can help protect connections; related navigation circuits can be reviewed in this marine wiring guide.

Checking the reading before commissioning

With the sensor disconnected, compare the measured resistance with the expected RTD curve. A Pt100 should be close to 100 ohms at 0°C and approximately 138.5 ohms at 100°C. A Pt1000 follows the same temperature relationship multiplied by ten. Exact values should come from the sensor’s IEC 60751 data or manufacturer’s calibration sheet.

After connection, compare the displayed temperature with a trusted reference, such as a calibrated probe placed beside the RTD. Allow both sensors to reach thermal equilibrium. Do not test a surface-mounted probe against air temperature, because the two sensors may be measuring different conditions.

A reading near zero ohms usually indicates a short circuit. An open-circuit or over-range display points to a broken conductor, loose terminal or failed element. A stable but incorrect value often indicates wrong terminal assignment, an incorrect Pt100/Pt1000 setting or a two-wire configuration selected by mistake.

Industrial, automotive and marine applications

RTDs suit applications where stable and accurate temperature measurement matters, including hot-water storage, refrigeration, pump skids, switchboards, hydraulic systems and food-processing equipment. In Australian commercial premises, the sensor may be connected to a building management system, a DIN-rail transmitter or a programmable controller.

Automotive and marine systems need extra care because vibration and electrical noise are common. A 24 V truck, earthmoving machine or touring setup may use a transmitter that sends a 4–20 mA signal rather than exposing the RTD directly to a dashboard gauge. Confirm whether the sensor input is isolated and whether the cable shield should be grounded at one end only.

For boats operating around Fremantle, Cairns or the Whitsunday coast, protect the probe cable from bilge water, galvanic corrosion and engine heat. A stainless-steel probe may be appropriate, but the sheath material, response time and chemical compatibility still need to match the installation.

Safety and documentation

Turn off and isolate the equipment before opening a panel. An RTD circuit is low voltage, but the terminals may be inside a cabinet containing hazardous mains or control voltages. Follow the applicable Australian requirements, site procedures and manufacturer instructions; fixed electrical work should be handled by a licensed electrician.

Keep a record of the sensor type, element value, cable route, terminal numbers and calibration result. A small wiring sketch inside the enclosure can save considerable time when a technician services the system months later. Mark whether the installation uses Pt100 or Pt1000, since connecting the wrong type can produce a believable but inaccurate reading.

Use a clear diagram, verify each conductor with a meter and configure the input before applying power. Wiring Cloud provides practical pinouts, schematics and reference diagrams to help you document the circuit and troubleshoot it confidently in the workshop, plant room, home project or out on the water.