Most studies we perform are aimed a finding the max current that could be carried without exceeding the cable temperature (usually 90 or 105°C).
We use a software that can give us the max current for any temperature and cable type and configuration. The lower the temp, the lower the current naturally.
Clients usually try to get the smallest cable that can supply the load while staying below the max temp allowable by the manufacturer.
For any cable the ampacity it is not a rating but the maximum conductor permissible temperature is in order to maintain the integrity of insulation in contact with this conductor.
The ampacity is the maximum current intensity which, in actual installation conditions, shall not determine the conductor temperature to be over the permissible. It is changed from configuration to configuration for the same cable.
The heat is produced by copper losses and dielectric losses and it is evacuated through insulation and other outer material as jacket, serving, surrounding air or earth or other. The vicinity of other source of heat as an other phase core in the same cable or another cable in the same cable tray or trench could increase the conductor temperature also.
Derating factor is stated for a standard configuration as cable in air or buried in earth or in exposed or concealed conduit and for a certain environmental temperature for air- for instance 40 degrees Celsius and 25 for earth- for another environmental temperature , another depth or for another number of same cables in the same routing. But always is related to the maximum permissible conductor temperature.
One could calculate the maximum current for another temperature less than maximum permissible, of course, but it is not a usual job as economical it is not interesting. An appreciation of increased reliability as the conductor temperature is lower could be done, nevertheless, but I am not sure you could prove how many years the cable would last more.
