Size of PVC Electrical Conduit

[nivra_kudos]

I would like to know the reason why the maximum size permitted for PVC conduit is 6" diameter only as per NEC Article 352.20(B). Can you give me the reason or disadvantages if I use 8" diameter of PVC conduit for ductbank? Thanks.

 

[iwire]

The first question that comes to my mind is what would you plan on putting in that 8" raceway?

The moment you go beyond 3 current carrying conductors in a raceway you suffer derating issues.