The following are based on the OMEGA technical reference page. Other vendors may have different specifications for maximum lengths.
What is Thermocouple Wire?
Wire that is used in a thermocouple from the point of sensing (probe) to the point of cold junction compensation (cjc end) where the signal is measured (RI hardware). Different thermocouple types (e.g. J, K, T, E, etc) use different mixtures of metals in the wire.
What is the difference between Thermocouple grade and Extension grade wire?
Thermocouple grade wire is wire that is used to make the sensing point (or probe part) of the thermocouple. Extension grade wire is only used to extend a thermocouple signal from a probe back to the instrument reading the signal.
What is the maximum length of thermocouple wire?
There are many factors that can impact the usable length of a thermocouple. As a guideline, under 100 feet (~ 30 meter) with 20 AWG (American Wire Gauge) or thicker wire in an area free of electromagnetic interference usually is fine.
Check out the AWG to metric conversion chart.