I have seen a range of 'solutions' to this over the years mostly retrofitted and most would be classed as bodges.  These include: forced air cooling of the ducts (diificult to arrange and requires substantial space and power); filling the ducts with water and arranging a flow to waste (once set up effective but cost of water cannot be neglected) and there is the risk of flooding the switch room if there is a leak.  Cables need to be suitable for long term immersion too.

The best answer is to redesign the last few metres to avoid the need for ducts or failing that transition to bigger cables for the ducted portion.  Easy to say but you would need a substantial marshalling / joint chamber fitted with busbars and appropriate separators/insulation. 

If this is all indoors and dry could you use busbar chamber in a pupose made cable trench/ duct.  These tend to have higher ratings as the bars are normally bare though you would have to take account of the restricted ventillation..  

I have seen a range of 'solutions' to this over the years mostly retrofitted and most would be classed as bodges.  These include: forced air cooling of the ducts (diificult to arrange and requires substantial space and power); filling the ducts with water and arranging a flow to waste (once set up effective but cost of water cannot be neglected) and there is the risk of flooding the switch room if there is a leak.  Cables need to be suitable for long term immersion too.

The best answer is to redesign the last few metres to avoid the need for ducts or failing that transition to bigger cables for the ducted portion.  Easy to say but you would need a substantial marshalling / joint chamber fitted with busbars and appropriate separators/insulation. 

If this is all indoors and dry could you use busbar chamber in a pupose made cable trench/ duct.  These tend to have higher ratings as the bars are normally bare though you would have to take account of the restricted ventillation..