# J — Thermocouple type letter to alloy pair, ANSI/IEC color code and tolerance class

For J, positive-leg alloy is Iron; negative-leg alloy is Constantan; ansi/astm positive-leg color is White; ansi/astm negative-leg color is Red; ansi extension-grade jacket color is Black, verified against its source on 2026-08-19.

- **Type letter:** J _(verified: appears in the quote below)_
- **Positive-leg alloy:** Iron _(verified: appears in the quote below)_
- **Negative-leg alloy:** Constantan _(verified: appears in the quote below)_
- **ANSI/ASTM positive-leg color:** White _(verified: appears in the quote below)_
- **ANSI/ASTM negative-leg color:** Red _(verified: appears in the quote below)_
- **ANSI extension-grade jacket color:** Black _(verified: appears in the quote below)_
- **Source document:** ANSI/ASTM Thermocouple Color Codes _(verified: appears in the quote below)_
- **Usable temperature range:** –210 to 1200°C _(per assets.omega.com, not stated by the source above)_
- **IEC 60584 Class 1 tolerance:** –40°C to +375°C ±1.5°C 375°C to 750°C ±0.004⋅ |t| _(per tc-inc.com, not stated by the source above)_
- **IEC 60584 Class 2 tolerance:** –40°C to +333°C ±2.5°C 333°C to 750°C ±0.0075 ⋅ |t| _(per tc-inc.com, not stated by the source above)_
- **IEC 60584-3 positive-leg color:** Black _(per tc.co.uk, not stated by the source above)_
- **IEC 60584-3 negative-leg color:** White _(per tc.co.uk, not stated by the source above)_
- **IEC 60584-3 outer sheath colour:** Black _(per tc.co.uk, not stated by the source above)_

## What the source says

> Type | Positive conductor | Negative conductor | Positive color | Negative color | Extension-grade jacket | Thermocouple-grade jacket shown in legacy assets | T | Copper | Constantan | Blue | Red | Blue | Brown | J | Iron | Constantan | White | Red | Black | Brown |

## Sources disagree

More than one authority states this, and they do not state the same
thing. Both are reproduced with the source each came from.

### Source document

thermometricscorp.com says source document is **ANSI/ASTM Thermocouple Color Codes**, as of 2026-08-19.

> Type | Positive conductor | Negative conductor | Positive color | Negative color | Extension-grade jacket | Thermocouple-grade jacket shown in legacy assets | T | Copper | Constantan | Blue | Red | Blue | Brown | J | Iron | Constantan | White | Red | Black | Brown |

Source: https://thermometricscorp.com/thermocouple-color-codes.html

assets.omega.com says source document is **ANSI and IEC Color Codes**, as of 2026-08-19.

> J J IRON CONSTANTAN – Fe COPPER- in Oxidizing at High – – Temperatures. –210 to 1200°C –8.095 to – (magnetic) NICKEL Not Recommended for –346 to 2193°F 69.553 Cu-Ni Low Temperatures.

Source: https://assets.omega.com/pdf/test-and-measurement-equipment/temperature/sensors/thermocouple-probes/tc_colorcodes.pdf

tc-inc.com says source document is **Thermocouple Tolerances**, as of 2026-08-19.

> Thermocouple Output Tolerances to IEC 60584-1 | Type | + Leg | - Leg | 100ºC | 500ºC | 1000ºC | Continuous | Short Term | Type | Class 1 | Class 2 | K | NICKEL - CHROMIUM Also known as: Chromel™, Thermokanthal KP*, NiCr, T1*, Tophel™ | NICKEL - Aluminum (magnetic) Also known as: Ni-Al, Alumel™, Thermokanthal KN™, T2™, NiAl™ | 42 | 43 | 39 | 0 to +1100ºC | -180 to +1350ºC | Temperature Range Tolerance Value Temperature Range Tolerance Value | –40°C to +375°C ±1.5°C 375°C to 1000°C ±0.004 ⋅ |t| | –40°C to +333°C ±2.5°C 333°C to 1200°C ±0.0075 ⋅ |t| | T | COPPER | COPPER - NICKEL Also known as: Constantan, Advance™, Cupron™ | 46 | – | – | –185 to +300°C | –250 to +400°C | Temperature Range Tolerance Value Temperature Range Tolerance Value | –40°C to +125°C ±0.5°C 125°C to 350°C ±0.004⋅ |t| | –40°C to +133°C ±1.0°C 133°C to 350°C ±0.0075 ⋅ |t| | J | IRON (magnetic) Also known as: Fe | COPPER - NICKEL Also known as: Nickel-Copper, Constantan, Advance™, Cupron™ | 54 | 56 | 59 | +20 to +700°C | –180 to +750°C | Temperature Range Tolerance Value Temperature Range Tolerance Value | –40°C to +375°C ±1.5°C 375°C to 750°C ±0.004⋅ |t| | –40°C to +333°C ±2.5°C 333°C to 750°C ±0.0075 ⋅ |t|

Source: https://www.tc-inc.com/thermocouple/thermocouple-tolerances.html

tc.co.uk says source document is **Thermocouple Colour Code Standards**, as of 2026-08-19.

> What is the Colour Code for a Type J Thermocouple? In the UK and Europe, the most widely used colour code for Type J thermocouple cable is to the international IEC 60584-3 standard which is as follows: Positive Leg = Black, Negative Leg = White, Outer Sheath (Jacket) = Black, Thermocouple Connector Colour = Black

Source: https://www.tc.co.uk/thermocouple-information/thermocouple-colour-code.html


## Source

- https://thermometricscorp.com/thermocouple-color-codes.html
- https://assets.omega.com/pdf/test-and-measurement-equipment/temperature/sensors/thermocouple-probes/tc_colorcodes.pdf
- https://www.tc-inc.com/thermocouple/thermocouple-tolerances.html
- https://www.tc.co.uk/thermocouple-information/thermocouple-colour-code.html

Last verified: 2026-08-19. Review by: 2028-08-18.
Part of [Thermocouple type letter to alloy pair, ANSI/IEC color code and tolerance class](https://referencesource.org/thermocouple-type-color-code-crosswalk/).