Thermocouple Wire: JX and KX Thermocouple Grade Cable

This article is meant to explain the various types of thermocouple wire as well as explain the difference between thermocouple grade and thermocouple extension wire.

First, it is good to understand what a thermocouple sensor does. A thermocouple sensor is a device that is used to measure temperatures. Many types of industries require the monitoring of temperatures in certain areas. Some examples would be iron and steel industries where a specific temperature needs to be maintained in the production of the metals. Many times thermocouple sensors are used for safety reasons. If a temperature reading is outside of what is deemed safe for the room it is measuring, it can alert the computer to shut down operations to prevent a fire or explosion.

In terms of wire, there are two general types of available thermocouple wire. One is called thermocouple grade and the other is thermocouple extension.

Thermocouple grade wire is used in the probe part of the thermocouple sensoring device. Thermocouple extension wire is simply used to relay the signal from the probe (sensor) back to the actual instrument used in reading the temperature. Due to the fact that the extension wire is not exposed to the heat or elements as much as the thermocouple grade style wire, it is less expensive and more typically a stocking item for most specialty wire and cable distributors. You can’t use thermocouple extension wire in a thermocouple grade application. However, you can use thermocouple grade in an application that requires thermocouple extension wire.

Most thermocouple wire uses solid conductors with one red conductor representing the negative lead. The most prevalent sizes are 16 and 20 AWG but you will find some 24 AWG as well. A good way to identify the type of thermocouple wire is by asking the jacket and conductor colors. There are many styles of thermocouple wire and here are a few of the most popular.

Type J – Type J, or JX extension grade, is mainly used on older equipment that can’t accept a modern thermocouple. The positive conductor is iron while the negative is constantan.

Type E – Type E, or EX extension grade, is used mostly found in cryogenics, or cold temperature measuring environments. The positive conductor is chromel and the negative is constantan.

Type K – Type K, or KX extension grade, is the most popular general use thermocouple wire. It is affordable and used in various types of sensors. The positive conductor is chromel and the negative is alumel.

Type T – Type T, or TX extension grade, is somewhat limited in temperature range but is a good option to use for things like electrical generators. The positive conductor is copper and the negative is constantan.

These are only four types of thermocouple wire but are typically the most popular. There are other types such as B, R, S, N, C and M. Some of these are used in very high temperature environments which can exceed 1700 degrees Celsius depending on their jacket and insulation types. These higher temperature products can be expensive and hard to locate for shorter lengths.

It is also worth noting thermocouple extension wires are manufactured in multiple pairs under one jacket. This allows an individual to purchase one cable to handle multiple thermocouple needs with one simple pull. The pair counts can go up as high 36 pairs for stocking items.

The three key questions to ask when someone is looking for thermocouple wire are:

1. What style/size of thermocouple wire do you need? If they do not have the style, try to find out the jacket and conductor colors to be able to identify it.

2. What are the temperature requirements for this application?

3. Is the wire going to be used on the actual thermocouple device or simply as an extension to the probe?

View detailed specs for various thermocouple cable by visiting the product page at