When selecting: insulation, moisture, abrasion, flexing, chemical attack, temperature extremes, and other adverse environments some considerations must be evaluated. Insulations are rated for a maximum continuous use temperature. This maximum use temperature is for the insulation material, not the temperature at which the thermocouple is operating. That temperature limit is based on the thermocouple design and calibration.
Thermocouple extension grade wire has approximately the same thermoelectric properties as thermocouple wire but is only guaranteed accurate within a limited temperature range. Typically, under 400 degrees Fahrenheit. All thermocouple wire is supplied to your specified length (plus or minus 10 percent).