Thermocouple Color Codes — ANSI, IEC & BS
Thermocouple and extension wire are color-coded by type and by standard. The code differs between the US (ANSI/ASTM), international (IEC), and legacy British (BS) systems — a frequent source of wiring errors. This reference gives the conductor and jacket colors for every standardized type.
The One Rule to Remember
In the ANSI/ASTM system the negative lead is always red. In the IEC 60584-3 system the negative lead is always white, and the jacket color identifies the type. Mixing the two systems is the most common cause of reversed-polarity wiring errors.
ANSI (US) Color Codes
| Type | Positive (+) | Negative (−) | Overall Jacket |
|---|---|---|---|
| J | White | Red | Black |
| K | Yellow | Red | Yellow |
| T | Blue | Red | Blue |
| E | Purple | Red | Purple |
| N | Orange | Red | Orange |
| R | Black | Red | Green |
| S | Black | Red | Green |
| B | Gray | Red | Gray |
| C | White | Red | White |
IEC (International) Color Codes
| Type | Positive (+) | Negative (−) | Overall Jacket |
|---|---|---|---|
| J | Black | White | Black |
| K | Green | White | Green |
| T | Brown | White | Brown |
| E | Violet | White | Violet |
| N | Pink | White | Pink |
| R | Orange | White | Orange |
| S | Orange | White | Orange |
| B | Gray | White | Gray |
| C | Not defined by IEC (Type C is covered only by ASTM E696) | ||
The older British standard BS 1843 used a different scheme again — for example, Type K was brown (+) / blue (−) with a red jacket. BS 1843 has been largely superseded by IEC 60584-3 (BS EN 60584-3). If you encounter equipment wired to BS 1843, verify polarity against a known reference before connecting, because its colors conflict with both the ANSI and current IEC systems.
How to Identify Polarity in the Field
- ANSI: find the red lead — it is always negative. The other colored lead is positive.
- IEC: find the white lead — it is always negative. The jacket color tells you the type.
- If a reading moves in the wrong direction (reads low when heated), the leads are reversed — a classic polarity error.
- Extension and compensating wire follow the same type color code; see extension & compensating wire.