Though the melting point of copper theoretically imposes a limit on wire ampacity, the materials commonly employed for insulating conductors melt at temperatures far below the melting point of copper, and so practical ampacity ratings are based on the thermal limits of the insulation. (allaboutcircuits.com)

Determination of the actual thermal resistivity of the soil is very important since the thermal characteristics of the surrounding soil and the laying depth of the cable circuit can alter the circuit ampacity by more than 50%. (Kema)

In general most terminals are rated for 75 degrees C. Conductors with 90 degree C insulations can be used on these terminals provided they are not used at an ampacity higher than the 75 degree C. ampacity. However, the 90 degree insulation ampacity can be used for derating purposes. (electriciancalculators.com)