But if its an MCB then it will almost certainly only have terminals rated for 70degree operation, so the cable will be designed based on its operation at 70.
Correct, but if we end up using the additional capacity, then i apply the fudge factor.
Although I suspect there are many out there that just use the rating from the table as-is without either considering those ratings apply only if you can use the higher temperature rise, nor the higher resistance when operating.
The additional resistance thing is just another factor that is easily forgotten, it doesn't replace normal design practice, just part of it.
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Touche! Do they make 90*C terminal breakers in the UK?
Yes, but usually mccb, fuse switches, or just connectors.
If you are just feeding it off an mcb in a standard board, then not as far as I know.
The other thing though is the temperature rise may not be for the full length.
If the cable runs through hot ambient areas, or through good thermal insulation then the whole length has to be downrated, - if this was 50% then yes a 100A cable may be running at 90 degC in the key area, but the actual terminations wouldn't be anything like that as it's only 50A through a 100A cable so you can use the full temperature capacity of xlpe, even with normal temperature range terminations.
It just takes a bit more thinking about, and calculation