There is another point worth considering, that is if there are any bends you will need a much bigger duct than expected or the pull force will be immense or impossible. The telephone system is interesting to copy, it uses 100mm ducts for large cables but the runs are always straight with manholes at any change in direction. Ducts used to only have one multipair cable each of perhaps 40mm diameter, but they were lead sheathed types. Modern practice is to use thinner cables and therefore some constraints have gone. There is little point in using ducts less than 100mm, the cost is in digging, not the duct itself. The big question is do you need a duct? Direct burial is often perfectly satisfactory and reduces the chance of damage as the ducts usually become full of water, and sheath defects are not unknown. Correct backfill will mean that immersion changes to damp conditions, which is probably more satisfactory.
There is another point worth considering, that is if there are any bends you will need a much bigger duct than expected or the pull force will be immense or impossible. The telephone system is interesting to copy, it uses 100mm ducts for large cables but the runs are always straight with manholes at any change in direction. Ducts used to only have one multipair cable each of perhaps 40mm diameter, but they were lead sheathed types. Modern practice is to use thinner cables and therefore some constraints have gone. There is little point in using ducts less than 100mm, the cost is in digging, not the duct itself. The big question is do you need a duct? Direct burial is often perfectly satisfactory and reduces the chance of damage as the ducts usually become full of water, and sheath defects are not unknown. Correct backfill will mean that immersion changes to damp conditions, which is probably more satisfactory.
