Presumably you have looked at the actual soil conditions, rather than the conservative defaults in Appx 4. By my reckoning, assuming typical values for the UK, if you've gone up a size for volt drop etc., you're getting closer. Conversely, with a cable run that long followed by a need for burial I assume this is outside... is the cable ladder arrangement protected from solar gain / do your calculations allow for it?

You could also consider direct burial, encasing the duct in concrete (reducing the STR, given that it appears to be under the foundation anyway) or casting cable pits in and outside the substation so you've only got the cable entry to worry about. Or indeed keep the cable entry above ground and go through the wall.

If you're sizing for 70°C at the terminals, might be worth allowing the conductors to run up to 90°C in buried sections as long as they're far enough away from the terminations.

Presumably you have looked at the actual soil conditions, rather than the conservative defaults in Appx 4. By my reckoning, assuming typical values for the UK, if you've gone up a size for volt drop etc., you're getting closer. Conversely, with a cable run that long followed by a need for burial I assume this is outside... is the cable ladder arrangement protected from solar gain / do your calculations allow for it?

You could also consider direct burial, encasing the duct in concrete (reducing the STR, given that it appears to be under the foundation anyway) or casting cable pits in and outside the substation so you've only got the cable entry to worry about. Or indeed keep the cable entry above ground and go through the wall.

If you're sizing for 70°C at the terminals, might be worth allowing the conductors to run up to 90°C in buried sections as long as they're far enough away from the terminations.