What does mean "Thermal capacity of conductor"?

Answer 1

Thermal capacity of a conductor (a.k.a. "ampacity") is the maximum rating of the conductor without damaging the conductor or its insulation. This rating is generally calculated based on maximum allowable temperature.

The current flowing through the conductor, combined with the resistance of the conductor generates heat. The ambient temperature and surroundings also affect the ultimate ampacity.

If a cable is rated at 90 °C, the ampacity of the cable is based on the flow of current that will generate a temperature of 90 °C. These ratings can change depending on whether the conductor is in conduit, direct buried in the ground or suspended outdoors in the air.

For overhead conductors, the two biggest concerns from running the conductor too hot are excess sag and annealing. Since the conductors expand as they heat up, the conductors may sag too low and cause clearance (safety) problems. Annealing is changes to the micro-structural of the metal conductor that reduces its physical strength -- making it very brittle.

For insulated cables, the biggest concern is reducing the insulation's life by running it too hot. It's the heat that kills electrical insulation.

Answer 2

i guess it would mean the heat it can withstand before failing.