Yes, I understand that, strictly speaking, it isn't always necessary to have any neutral, let alone a full-size one, serving a motor control center. I also understand that it is not the purpose of the code to plan for future uses. But....
I would strongly encourage the installation of a full-size neutral anyway- even if all loads are three-phase motors. Or, perhaps I should say, especially if all loads are three-phase motors.
Why? Because we're not in the '60's anymore. Today's production equipment more and more uses variable speed controls and electronic components. The two don't mix well- that's why there is such a concern these days over "power quality" and "harmonics." In some cases, it is even necessary to double the size of the neutral.
Sure, the place may have only simple, old-fashioned three-phase motors today....but what about tomorrow? Or, will the PoCo hit him with 'power factor' penalties, and he'll want to add a bank of capacitors? Then he'll be glad you planned for the neutral.
As a minimum, use a large enough pipe.
