Temperature de-rating is a major concern.
In Las Vegas, Nevada, USA, everything has to be up-sized -- and the authorities want every conductor above ground whenever this is at all practical.
In Kuwait it has to be a hot as Las Vegas. You have to design for the heat -- worst case.
So it's also important to use insulation that has the highest temperature rating that's financiall viable. (It being understood that MI is totally impractical.)
The last time I looked cross-linked polyethylene was the favored insulation. ( XHHW-2 for Americans )
I would expect that the local trade has already resolved the insulation type for your climate.
BTW, 125A for a 50kVA transformer doesn't add up.
I'm suspicious that your climate forces everyone to use oversized thermal circuit breakers. You may be running far fewer amps than us temperate climate electricians are used to.
Without actually knowing your loading (amps & volts) I hesitate to run the numbers.
