This is a continuation of a recent thread I started but I decided to post this question separately because I felt like it stood on its own

I am either missing something obvious or misinterrupting the Code in this regard. NEC Figure 310.60(C)(3) for detail 2 (3 electrical ducts) and detail 3 (6 electrical ducts) shows a centerline-to-centerline spacing of 7.5" between ducts. My experience from long days past suggests this is the minimum spacing between 4" PVC ducts. In the case of larger ducts such as 5", I recall seeing the spacing increase 0.5"-1". In the project I'm currently involved with, the underground ducts for the duct-bank arrangement in question are 6". This leads me to believe the engineered duct-bank must have an even larger spacing then what NEC shows. I understand this spacing and duct-bank arrangement/layout is subject to what the EoR dictates in their design but where does the 7.5" in the Code come from? Does the Code say anywhere what the assumption is for size of embedded conduit?