Chris Pearson said:

3 x 16 mm² into 25 mm conduit.

I think it's less of a function of the diameter of the conduit, more the lack of flexibility in the conductors themselves vs the bends (and also taking into account the radius of those bends, but even so, as has been said, more than a couple and it's difficult).

I think that Graham is correct: irrespective of the diameter, the cables must touch the walls on the inside of any bends.

I think that once the cables have been drawn in so that there are 3 side by side, it is just a matter of effort. Part of the problem is exerting enough force whilst up a ladder, especially when going sideways. Kinks do not help.

The difficulty with the 3rd cable is that it presumably has to slide between the other two around any bends. If the cables get a bit twisted, it gets so much more difficult.

I have found that a couple of yards at a time is the maximum if any degree of control is to be maintained; which is why it's up the ladder, lube, down the ladder, move the ladder, up the ladder, pull, etc. Patience is the key!

I think that Graham is correct: irrespective of the diameter, the cables must touch the walls on the inside of any bends.

I think that once the cables have been drawn in so that there are 3 side by side, it is just a matter of effort. Part of the problem is exerting enough force whilst up a ladder, especially when going sideways. Kinks do not help.

The difficulty with the 3rd cable is that it presumably has to slide between the other two around any bends. If the cables get a bit twisted, it gets so much more difficult.

I have found that a couple of yards at a time is the maximum if any degree of control is to be maintained; which is why it's up the ladder, lube, down the ladder, move the ladder, up the ladder, pull, etc. Patience is the key!

Chris Pearson said:

The difficulty with the 3rd cable is that it presumably has to slide between the other two around any bends. If the cables get a bit twisted, it gets so much more difficult.

From experience, it's often easier to draw in a set of conductors with a draw wire, than an individual conductor after other conductors are already present, which perhaps supports this theory. However, I think also, the difference pulling a 1.5 sq mm vs a 16 sq mm in both instances will speak for itself - even if there is loads of room for either: the 16 sq mm will struggle round the bends (to bend itself) far more than the 1.5 sq mm.

Coming back to the cables' flexibility, getting 3 x 16 mm² to go round a bend together is a big ask. I tried it and I am simply not strong enough.

Getting it out afterwards without at least partly dismantling the conduit first will be at least as difficult.

Guidance Note 1, Appendix A, Tables A3 and A4 (or OSG Appendix E, Tables E3 and E4) provides us the following for 16 sq mm insulated (not sheathed) in 25 mm conduit:

• Straight run: 10 m  run should be OK
• One bend: 10 m run should be OK
• Two bends: 10 m run should be OK.
• Three bends: 4.5 m maximum run
• Four bends: 2.5 m maximum run

I think three and four bends would not be easy full-stop, with Class 1 (Solid) or Class 2 (Stranded / rigid) 16 sq mm conductors - perhaps a classic case of where practical experience wins over theory.

Another factor is that a typical minimum bend radius of H07V-R is 4 × diameter, or 28 mm for 16 sq mm - so a tight 90 elbow really won't do the job !