It’s helpful to be reminded of the basic physics underlying the basis for the temperature rating of conductors . . . .
A conductor sitting at room temperature, with no current running through it, will be the same temperature as the room. But when the cable is loaded down with a current, that current will begin to generate heat internally within the conductor. The heat generated inside the conductor will be transferred into the surrounding insulation, jacket, and environment.
The heat-transfer relationship between those four physical elements is complex.
Consequently, the overall cable temperature will begin to rise above the ambient temperature. It’s this temperature rise above ambient that impacts a cable’s ampacity: how many degrees above ambient temperature the cable can operate at before degrading (see Mukund R. Patel, Shipboard Electrical Power Systems, p.70).
A cable rated at 90ºC is, roughly speaking, able to generate an extra 50 degrees of internal heat before damaging the insulation if the ambient temperature is 40ºC. A cable rated at 75ºC is able to generate an extra 35 degrees of internal heat if the ambient temperature is 40ºC.
To look at it another way, a cable rated for 60ºC that is installed in an environment whose ambient temperature is 60ºC has zero ampacity.
This paper prepared by the CDC presents a detailed technical analysis of cable heat transfer. It develops differential equations and compares the calculated results to experimental results.