A Pt100 measures temperature through resistance, so lead wires add resistance to the measurement. The selected connection method determines how much of that lead resistance the instrument can compensate.
Use this guide to prepare a design review or RFQ. Final ratings depend on the complete product, installation, controls and operating environment.
Selection inputs that change the design
| Decision | Information to provide | Why it matters |
|---|---|---|
| 2-wire | Two conductors in series with element | Simplest, but lead resistance appears as measurement error. |
| 3-wire | Third conductor enables compensation | Common industrial choice when leads are well matched. |
| 4-wire | Separate current and voltage paths | Best lead compensation with compatible instrumentation. |
| System | Sensor, cable and input configuration | Every component must use the same scheme. |
Estimate the lead effect
Pt100 resistance changes predictably with temperature, while copper lead resistance changes with length, gauge and ambient temperature. In a 2-wire circuit the instrument sees both.
For short runs or modest accuracy needs this may be acceptable, but the error should be calculated rather than assumed negligible.
Use the connection the instrument supports
A 3-wire input estimates and cancels lead resistance when the relevant conductors are closely matched. A 4-wire measurement separates excitation and sensing paths.
Do not choose a 4-wire probe if the controller accepts only 2 or 3 wires without confirming how it will be connected and what benefit remains.
Protect signal integrity
Keep terminal connections clean and stable, and route RTD cable away from heater power and switching noise. Use cable suitable for the ambient temperature and mechanical exposure.
Verify the complete loop at commissioning using an appropriate resistance or temperature simulation procedure.
Common specification mistakes
- Calling every three-core cable a 3-wire RTD
- Ignoring unequal conductor resistance
- Connecting a 4-wire sensor incorrectly to a 3-wire input
- Routing sensor and heater cables together
RFQ checklist
- RTD curve and accuracy class
- Lead length, gauge and ambient
- 2-, 3- or 4-wire instrument input
- Cable temperature and shielding
- Commissioning test method
Review the matching Temperature Sensors product range, then send this checklist with your drawing for an engineer-reviewed quotation.
Frequently asked questions
Why is a 3-wire Pt100 common?
It provides practical lead compensation with one fewer conductor than a 4-wire system.
Can a 2-wire Pt100 be calibrated out?
A fixed correction may help at one condition, but lead resistance changes with cable temperature and connections.
Does 4-wire always improve the process?
It reduces lead-resistance error, but total accuracy still includes element, mounting, calibration and instrument error.