RTD Wiring Diagram: 2-Wire, 3-Wire, and 4-Wire Pt100 Lead Resistance Compensation
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An RTD wiring diagram shows how 2-wire, 3-wire, and 4-wire Pt100 connections handle lead resistance to deliver accurate temperature measurement across industrial distances.
A resistance temperature detector (RTD) measures temperature by exploiting the predictable increase in electrical resistance of a pure metal — most commonly platinum — with rising temperature. The Pt100 RTD is the international standard: at 0 °C its resistance is exactly 100.00 Ω, and it rises approximately 0.385 Ω per °C (defined by the Callendar–Van Dusen equation and standardised in IEC 60751). A Pt1000 RTD has ten times the base resistance (1000.00 Ω at 0 °C) with proportionally higher sensitivity.
The fundamental measurement challenge is that the cable connecting the RTD element to the measuring instrument adds its own resistance — and this lead resistance adds directly to the measured value, introducing a systematic temperature error. Copper cable at 20 °C has a resistance of approximately 0.017 Ω per metre per millimetre-squared of cross section; a 10-metre run of 0.5 mm² cable adds approximately 0.68 Ω each way, representing a temperature error of about 1.8 °C for a Pt100 — an unacceptable error in process measurement.
A 2-wire RTD connection makes no attempt to compensate for lead resistance. The measuring instrument sees RTD resistance plus both lead resistances in series. This is acceptable only for very short cable runs (under 2 metres) or where measurement accuracy of ±2 °C or better is not required.
A 3-wire RTD connection uses three conductors: two to carry the excitation current (one wire each direction) and a third sense wire running alongside one of the current-carrying conductors. The instrument reads the voltage across the RTD plus one lead, then subtracts the resistance of the third (sense) wire — which equals the lead resistance of that one conductor — thereby compensating for one lead. This assumes the three leads are identical in resistance, which is true for matched cables at the same temperature. 3-wire is the most common industrial standard and provides accuracy of ±0.1 °C to ±0.5 °C over typical cable lengths.
A 4-wire RTD connection is the most accurate method. Two wires carry excitation current, and two separate wires measure the voltage directly across the RTD element only. Because the voltage-sensing circuit draws negligible current (high-impedance input), no current flows through the sense leads and therefore lead resistance has zero influence on the measurement. 4-wire Kelvin connection achieves accuracy of ±0.01 °C or better and is used in laboratory standards, pharmaceutical validation, and precision process control.
How to wire rtd wiring diagram
- Identify the RTD element type and wiring configuration Check the sensor body label or datasheet for element type (Pt100 or Pt1000), number of leads (2, 3, or 4), and accuracy class (Class A: ±(0.15 + 0.002|T|) °C; Class B: ±(0.30 + 0.005|T|) °C per IEC 60751). Confirm the transmitter or instrument input matches the RTD type and wiring configuration.
- Select cable specifications for the cable run Use screened twisted-pair cable with a conductor cross-section of at least 0.5 mm² for runs up to 20 metres, 1.0 mm² for 20 to 100 metres. The cable screen (shield) should be grounded at one end only (typically at the instrument end) to prevent ground loop currents from affecting the measurement. Use the same cable cross-section for all leads — mismatched cross-sections in a 3-wire installation create unequal lead resistances and introduce error.
- For 2-wire connection Connect one RTD lead to terminal F+ (force/current supply) and the other to F− (return) on the transmitter. Label the connection clearly as 2-wire and note the cable length and resistance if recording measurement uncertainty. Add the lead resistance value to the transmitter offset correction if the instrument supports it.
- For 3-wire connection Connect the current-supply lead (red, or marked +) to terminal I+ on the transmitter. Connect the return lead (white, or marked −) to terminal I−. Connect the sense wire (the third conductor, running alongside either current lead) to terminal S. Ensure the sense wire is the same material and cross-section as the current-carry leads. The transmitter internally subtracts the sense lead resistance from the total reading.
- For 4-wire connection Connect one current-carry lead to I+ and one to I−. Connect one voltage sense lead to V+ (or S+) and one to V− (or S−). The V+ sense lead connects to the same RTD terminal as the I+ current lead, and V− connects to the same terminal as I−. Do not interchange sense and current leads — this would inject excitation current through the sense circuit and defeat the Kelvin measurement principle.
- Connect and ground the cable screen Terminate the cable screen (drain wire or foil shield) at the transmitter or instrument earth terminal. Leave the other end of the screen unterminated (floating) and insulated. This prevents circulating ground loop currents while still providing capacitive shielding against electromagnetic interference.
- Verify measurement accuracy Place the RTD in an ice-water bath (0 °C reference) and verify the transmitter or display reads within the expected accuracy class tolerance. For a Pt100 Class B sensor in 3-wire connection, expect a reading within ±0.30 °C of 0 °C. If the error is larger, check for lead resistance mismatch (3-wire) or a cold junction compensation error.
Specifications
| Pt100 resistance at 0 °C | 100.00 Ω (IEC 60751) |
|---|---|
| Pt100 temperature coefficient (α) | 0.003851 Ω/Ω/°C (mean value 0–100 °C) |
| Pt100 sensitivity | ≈ 0.385 Ω/°C |
| IEC 60751 Class A accuracy | ±(0.15 + 0.002 × |T|) °C |
| IEC 60751 Class B accuracy | ±(0.30 + 0.005 × |T|) °C |
| Typical measurement temperature range (Pt100) | −200 °C to +600 °C (element dependent) |
| Typical excitation current | 0.5 mA – 2 mA (balance between signal level and self-heating) |
| 4-wire Kelvin connection achievable accuracy | ±0.01 °C (with Class AA element and precision transmitter) |
Safety warnings
- RTD circuits operate at very low signal voltages (millivolts) and low current (1 mA). While not inherently dangerous to personnel, RTDs are frequently installed in high-temperature processes, pressurised vessels, and hazardous areas. Always follow the site's process safety management procedures and obtain a permit to work before installing or removing sensors in live processes.
- In ATEX or IECEx classified hazardous areas, use only RTD sensors and transmitters certified for the zone and gas group of the installation. Connect zener barriers or galvanic isolators as required by the hazardous area installation certificate. Do not substitute non-certified components.
- Ensure the process is at safe pressure and temperature before removing or installing an RTD thermowell. High-pressure processes can eject a thermowell assembly with lethal force. Follow the plant's pressure isolation and lock-out-tag-out (LOTO) procedure.
- Ground loops created by connecting the cable screen at both ends can inject common-mode noise into the measurement circuit, causing erratic readings. Ground the screen at one end only (instrument/transmitter end) to maintain measurement integrity.
Tools needed
- Digital multimeter with 4-wire (Kelvin) resistance measurement capability or a precision resistance bridge
- Calibrated temperature reference (e.g. dry-block calibrator or reference thermometer traceable to a national standard)
- Cable stripper and wire ferrule crimping tool
- Insulated screwdrivers (flathead) for terminal block connections
- Multimeter or milliammeter (for verifying 4–20 mA loop output)
- Insulation resistance tester (for verifying cable insulation before connection)
- Cable labels and thermal-transfer printer (for permanent identification)
Common mistakes
- Using unmatched cable cross-sections in a 3-wire installation: if the sense wire and current-carry wires are different cross-sections, they have different resistances per unit length. The 3-wire compensation assumes all three leads are equal — unequal leads introduce a systematic error that cannot be zeroed out.
- Confusing the current-carry and voltage-sense leads in a 4-wire connection: if excitation current is injected through the sense leads, the measurement loses its Kelvin accuracy and reverts to a 2-wire-equivalent result with full lead resistance error.
- Grounding the cable screen at both ends: this creates a loop antenna that picks up magnetically induced noise (particularly from adjacent power cables and motors) and injects it into the measurement, causing reading instability and drift.
- Selecting the wrong RTD element type in the transmitter configuration: configuring a transmitter for Pt1000 when a Pt100 is connected results in a severely incorrect temperature reading. Always verify the transmitter input type setting matches the sensor.
- Placing the RTD cable in the same conduit as power wiring: power cable electromagnetic fields induce voltages into the low-level RTD circuit. Use dedicated instrument cable conduit or tray, maintain at least 300 mm separation from power cables, and use metal-screened cable.
Troubleshooting
- RTD reading shows a fixed offset (too high or too low) at all temperatures
- Cause: Lead resistance error (2-wire connection), mismatched lead resistances (3-wire), or incorrect transmitter offset calibration Fix: For 2-wire: measure the total round-trip lead resistance with an ohmmeter and enter it as an offset correction if the transmitter supports it. For 3-wire: measure resistance of each individual lead with the RTD disconnected; they must be equal to within 0.1 Ω. Re-run mismatched cable if necessary. For transmitter offset: zero the transmitter against a known reference temperature.
- RTD reading is unstable and noisy
- Cause: Cable screen grounded at both ends, RTD cable routed near power cables, or high contact resistance at terminal connections Fix: Verify the cable screen is grounded at one end only. Re-route cable away from power sources if possible, or install in screened metal conduit. Clean and retighten all terminal block connections — oxidised or loosely clamped terminals add variable contact resistance. Check for moisture ingress at cable glands.
- Transmitter shows open-circuit or over-range fault
- Cause: RTD element burned out (open circuit), broken cable conductor, or loose terminal connection Fix: Measure resistance between RTD leads at the sensor head with the circuit isolated. A Pt100 at ambient temperature should read approximately 109 Ω at 25 °C. An open-circuit reading (infinite resistance) indicates a failed element or broken wire. Check each cable conductor for continuity. Replace the sensor if the element is confirmed open-circuit.
Frequently asked questions
What is the temperature error caused by lead resistance in a 2-wire RTD?
For a Pt100, every 0.385 Ω of additional resistance appears as 1 °C of measurement error. A 10-metre run of 0.5 mm² copper cable adds approximately 1.36 Ω total (0.68 Ω each way), resulting in a positive offset error of approximately 3.5 °C. This systematic error is fixed for a given cable length and temperature but cannot be corrected without knowing the exact lead resistance.
Can I connect a 4-wire RTD to a 3-wire input on my transmitter?
Yes. Leave one of the two sense wires unconnected at the transmitter, or link it to the corresponding current-carry terminal. The transmitter will then operate in 3-wire mode with the remaining three conductors. You lose the accuracy benefit of 4-wire connection but the RTD element itself is not damaged. Check the transmitter manual for the correct terminal assignment.
What is the difference between a Pt100 and a Pt1000 RTD?
Both are platinum RTDs following the same resistance-temperature characteristic defined by IEC 60751. The Pt100 has 100 Ω at 0 °C; the Pt1000 has 1000 Ω. The Pt1000 is ten times more sensitive (3.85 Ω/°C vs 0.385 Ω/°C), making it less susceptible to lead resistance errors in 2-wire installations. Pt1000 sensors are common in HVAC and building automation; Pt100 dominates industrial process instrumentation.
How do I identify which wires to use on a 3-wire RTD?
RTD leads are colour-coded per international standards, though these vary by region. IEC 60751 specifies: one red wire (force, or excitation current supply), one white wire (return/sense), and one third wire that may be red or white. The manufacturer's datasheet or sensor label identifies which terminals are the current leads and which is the sense lead. Measure resistance between each pair of wires with the sensor at a known temperature to identify the RTD element.
What is the effect of self-heating on RTD accuracy?
The excitation current passed through an RTD element generates heat (P = I²R), raising the element temperature above the medium being measured — this is called self-heating error. For a Pt100 with 1 mA excitation current: power = (0.001 A)² × 100 Ω = 0.1 mW. Self-heating error depends on how effectively the sensor is thermally coupled to the medium. In poorly conducting media (air, gas), self-heating of 0.5 °C or more is possible with high excitation currents. Use the minimum excitation current that provides an adequate signal-to-noise ratio.