What is the Difference Between RTD PT100 2-Wire, 3-Wire, and 4-Wire? A Complete Guide
Resistance Temperature Detectors (RTDs) are widely used in industrial applications for their accuracy, repeatability, and stability in temperature measurement. Among RTDs, the PT100 is the most common type. The term “PT100” indicates a platinum resistance element with 100 ohms resistance at 0°C. But beyond the sensing element itself, how the RTD is wired — as a 2-wire, 3-wire, or 4-wire configuration — has a significant impact on measurement accuracy.
This guide breaks down the key differences between these wiring types and helps you understand which configuration is best for your specific application.
Understanding the Basics of PT100 RTD Sensors
An RTD functions by correlating the resistance of the sensing element with temperature. As temperature increases, the resistance of the platinum wire increases in a known, repeatable way. The PT100 sensor’s resistance typically increases about 0.385 ohms/°C (for standard RTDs with a European alpha coefficient).
However, the accuracy of the reading depends not just on the sensor but also on how the signal travels from the sensor to the transmitter or measuring instrument. This is where wire configuration comes into play.
PT100 Wiring Configurations
1. 2-Wire RTD
How It Works:
The simplest configuration uses two wires: one connected to each end of the sensing element. The current passes through both the lead wires and the element.
Pros:
- Lowest cost
- Easy installation
Cons:
- Lead wire resistance adds to sensor resistance
- Measurement error increases with longer cables
- Not recommended for precision applications
Typical Use:
- HVAC systems, short-distance measurements, and non-critical temperature monitoring
2-Wire RTD Wiring Diagram
2. 3-Wire RTD
How It Works:
A third wire is added, creating a reference point to compensate for lead wire resistance. The system assumes the resistance in both lead wires is equal and subtracts it from the measurement.
Pros:
- Good accuracy with moderate cost
- Compensates for lead wire resistance
Cons:
- Assumes equal resistance in all wires (may still introduce small error)
- Slightly more complex installation
Typical Use:
- Industrial process control, moderate-precision applications
3-Wire RTD Wiring Diagram
3. 4-Wire RTD
How It Works:
Two wires carry the current through the sensor, and two separate wires measure the voltage drop across the sensor. This completely eliminates the effect of lead resistance on the voltage measurement.
Pros:
- Highest accuracy
- Fully compensates for wire resistance, regardless of length
Cons:
- More expensive
- More complex wiring and termination
Typical Use:
- Laboratory measurements, calibration systems, high-accuracy industrial processes
4-Wire RTD Wiring Diagram
Comparison Table: 2-Wire vs 3-Wire vs 4-Wire RTD
| Feature | 2-Wire RTD | 3-Wire RTD | 4-Wire RTD |
|---|---|---|---|
| Accuracy | Low | Moderate | High |
| Lead Resistance | Not Compensated | Partially Compensated | Fully Compensated |
| Cost | Low | Moderate | High |
| Installation | Simple | Moderate | Complex |
| Typical Use | Basic Monitoring | Industrial Processes | Labs, Precision Control |
Selection Tips
- Choose 2-wire for simple, low-budget applications where accuracy isn’t critical and cable lengths are short.
- Use 3-wire for most industrial applications — it offers a good balance of cost and accuracy.
- Go with 4-wire when maximum precision is required, such as in calibration equipment or laboratory setups.
Conclusion
While all PT100 RTDs use the same sensing element, the wiring configuration dramatically affects performance. Understanding the differences between 2-wire, 3-wire, and 4-wire setups allows you to make informed decisions based on application needs, budget, and accuracy requirements.
In high-stakes environments where accuracy and consistency are non-negotiable, the extra cost and complexity of a 4-wire system are easily justified. Meanwhile, 2-wire and 3-wire configurations still have important roles in less demanding applications.
The key is to match the wiring configuration to the process’s sensitivity to temperature measurement error.
Investing in the right RTD wiring setup ensures long-term measurement stability, accurate control, and optimal process performance.