Until a couple of years ago the IET published a 'commentry' that described some tests of cable ageing with over temperature and has formuae for estimated the life shortening effect of overloading.
The URL used to be
https://www.theiet.org/Publish/WireRegs/Commentary-UpdateApr04.pdf
Sadly the webpage has moved as part of one of the tidy-ups, and for whatever reason the Wayback machine never archived it. Before I pester the webmasters on Monday if they have it on back-up tapes I'd like to ask on the off chance if anyone happens to have kept a copy they could upload ?