RTD Wire Color Code and Instrument Cable Colours
Identify RTD leads by colour for 2, 3 and 4 wire Pt100 and Pt1000 sensors, and check the wiring with a simple resistance measurement.
- Measure between two leads of the same colour: only lead resistance, usually below 1 Ω
- Measure between a red and a white lead: element plus leads, about 108 to 110 Ω for a Pt100 at room temperature
RTD lead colours (IEC 60751)
| Connection | Lead colours |
|---|---|
| 2 wire | Red White |
| 3 wire | Red Red White |
| 4 wire | Red Red White White |
How RTD Leads Are Coloured
IEC 60751 marks the two ends of the sensing element with different colours: red on one end and white on the other. Extra leads for 3 and 4 wire connections repeat the colour of the end they connect to.
Some manufacturers use the opposite arrangement (one red, two white) for 3 wire sensors. A resistance check always confirms which leads belong together.
Instrument cable colours
Multi pair instrument cables usually have twisted pairs in black and white (or black and blue) with numbered cores, an individual screen per pair and an overall screen. Cables for intrinsically safe circuits are commonly light blue so they can be identified and segregated, as allowed by IEC 60079-14.
Frequently Asked Questions
What are the wire colours of a 3 wire Pt100?
Normally two red and one white (IEC 60751), with the two red leads joined to the same end of the element. Some suppliers use two white and one red.
How do I identify RTD wires with a multimeter?
Leads of the same colour show almost zero ohms between them; red to white shows the element resistance, about 100 Ω at 0 °C for a Pt100.
Why are intrinsically safe cables blue?
Light blue is the customary colour for IS circuits so they are easy to identify and keep separate from non IS wiring.
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