Reason for conduit fill rules?

[templdl]

The fill rules have to do with preventing physical damage of the conductors.

The derating rules deal with the heat issue.

Has there been any consideration given regarding the insulation used today? I know that the older TW insulation as compared to the current THHN and THWN is thinner making the wire smaller in diameter and the outer surface seems to be as lot tougher and smoother which should make it an easier pull.
Do you have any thoughts about that?

 

[MD88]

A lot of good points, it's appreciated. What we actually decided to do is do a test pull with old cable that's about the same size. We'll see how smoothly it goes and base the decision on that. Will update with the results.