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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallFor an RTD, the number of wires determines how much the measurement is affected by resistance in the cable. A two-wire connection is simplest but includes both lead resistances in the reading; three-wire wiring can compensate for much of that error when the leads and measuring circuit meet the required assumptions; and four-wire Kelvin sensing largely separates lead resistance from the measurement. The right choice depends on the allowable error, cable conditions, sensor, and whether the readout supports that wiring.
How the three RTD wiring configurations differ
An RTD measures temperature through a change in electrical resistance. The measurement instrument therefore needs to determine the sensing element’s resistance without mistaking resistance in the connecting wires for resistance in the sensor.
| Configuration | How it connects and measures | Main advantage | Key limitation and suitable use |
|---|---|---|---|
| 2-wire | One lead connects to each end of the sensing element. The measured resistance includes both leads. | Simple, with fewer conductors and typically lower cost. | Lead resistance and its changes with temperature appear as measurement error. More suitable for short runs or applications where that error is acceptable; a Pt1000’s higher resistance can make a given lead resistance proportionally smaller. |
| 3-wire | One lead connects to one side of the element and two leads to the other. A compatible circuit uses the additional connection to estimate or compensate for lead effects. | A common practical balance between connection count and accuracy. | Compensation depends on the relevant lead resistances matching and on the circuit’s implementation. Mismatch, damaged wiring, or circuit limitations leave residual error. |
| 4-wire | Two leads connect to each end. One pair carries excitation current; the other senses voltage across the element. | Kelvin sensing minimizes the influence of lead resistance on the calculated resistance. | Requires four connections and a readout that performs a true four-wire measurement. |
Texas Instruments summarizes the trade-off in A Basic Guide to RTD Measurements: “Four-wire RTDs yield the most accurate measurements, but are the most expensive RTD configuration.” Here, “most accurate” refers to rejecting lead-resistance effects among these wiring options; other sources of measurement error remain.
Why lead resistance affects a 2-wire reading
In a two-wire circuit, the instrument sees the sensing element and both lead resistances in series. It cannot distinguish those cable resistances from the RTD itself. Added resistance normally makes a Pt100 appear warmer because its resistance rises as temperature rises.
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The size of the resulting temperature error depends on total lead resistance and the sensor’s resistance-temperature characteristic. Cable length alone is not enough to predict it: conductor gauge and material, routing, wire temperature, and sensor type also matter.
Analog Devices gives an illustrative example in its May 21, 2021 RTD selection and design article: a 25-foot run of 24 AWG copper in the described two-wire Pt100 setup has about 1.3 Ω of lead resistance and an estimated temperature error of about 3.38°C, using approximately 0.385 Ω/°C for that example. This is an example, not a universal result for all Pt100 installations.
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A one-time offset adjustment may correct a stable lead contribution, but it cannot reliably account for a contribution that changes as the cable temperature changes. Analog Devices explains this limitation in its CN0383 circuit note.
How 3-wire compensation works—and where it can fall short
Three-wire measurement uses an extra lead and a compatible input circuit to compensate for lead resistance. The circuit may compare or balance measurement paths; the exact method varies by instrument. Analog Devices’ CN0383 design uses matched excitation currents. Its note says, “For the 3-wire measurement, the second excitation current actively compensates for the lead resistance”—a description of that circuit, not a guarantee about every three-wire instrument.
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Compensation works to the extent that the relevant lead resistances match. Differences in conductor resistance, contact resistance, or wiring condition can leave an error. Beckhoff’s RTD measurement documentation likewise describes line compensation in the context of identical supply lines and notes the relevance of lead and contact resistance. Follow the requirements for the specific input rather than assuming that three wires automatically eliminate cable error.
What four-wire Kelvin sensing changes
In a four-wire connection, one pair supplies current through the RTD, while a separate pair measures voltage directly across the sensing element. With a suitable high-impedance voltage input, little current flows through the sense leads, so their voltage drop has little effect on the measured element voltage. The instrument can therefore calculate resistance with much less influence from the excitation leads.
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This is the strongest of the three configurations for rejecting lead-resistance error, but it does not remove RTD tolerance, self-heating, instrument error, calibration error, electrical noise, or installation-related error. The readout must also support a true four-wire measurement; having four wires at the sensor does not by itself make a four-wire measurement possible.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose wiring for the whole measurement system
Start with the allowable error and the conditions of the complete installation, rather than choosing by wire count alone.
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- Consider 2-wire for a short connection or modest accuracy requirement when the expected lead contribution is acceptable. A Pt1000 can make the same lead resistance a smaller fraction of the sensor resistance, but it is not a universal replacement for four-wire wiring.
- Consider 3-wire when you want a common industrial compromise and can use a compatible input with appropriately matched leads.
- Consider 4-wire when minimizing lead-resistance influence matters and the measurement device explicitly supports true four-wire sensing.
Compare the required accuracy, run length and temperature variation, element type (such as Pt100 or Pt1000), conductor matching, instrument capability, terminal count, installation complexity, and cost. A four-wire arrangement may reduce one important error source, but it cannot compensate for an unsupported input or other measurement errors.
Check the sensor and readout before wiring
- Identify the sensor. Confirm the nominal resistance type (for example, Pt100 or Pt1000), element tolerance or class, temperature range, probe construction, and the number and identification of its leads.
- Verify the input topology. Check the controller, meter, or input-module manual to confirm whether it supports 2-, 3-, or true 4-wire RTD measurement. Many industrial controllers and measurement devices cannot perform a true four-wire measurement, as TE Connectivity’s RTD overview notes.
- Follow the terminal diagram. Map the sensor leads to the documented input terminals. Do not infer terminal assignments from wire color alone: conventions vary by manufacturer and product.
- Inspect connections and cable conditions. Check for damaged conductors and sound contacts, especially with two- or three-wire measurements where lead or contact resistance can influence the result.
- Plan the transition to field wiring. A connection head and terminal block commonly provide the transition between sensor leads and field wiring, according to TE Connectivity.
If you are buying a probe, specify the needed sensor type and lead configuration, then verify that the intended input accepts it. A four-wire Pt100 probe is appropriate only when paired with compatible four-wire measurement hardware.
Interpreting published accuracy and resolution figures
Accuracy and resolution are different specifications, and figures from a reference circuit should not be treated as general guarantees for an RTD installation. Analog Devices’ CN0383 reports better than ±1°C overall system accuracy after two-point calibration for its stated two-wire AD7124-4/AD7124-8 implementation over −50°C to +165°C, and for its stated three-wire implementation over −50°C to +200°C. Those results apply to the circuit note’s Class B Pt100 context, calibration, linearization, and measurement setup.
The same note reports typical peak-to-peak resolution of 0.0043°C (17.9 bits) in its full-power, sinc4, 50 SPS configuration, and 0.0092°C (16.8 bits) in its low-power, post-filter, 25 SPS configuration. These design-specific resolution results do not mean the system is accurate to those fractions of a degree.
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