Why bother either assuming or bringing this up at a production meeting? The specification for what sort of conductors can be fitted to your standard equipment should be on the prints supplied with the equipment, and there should be no reason to discuss putting this specification in.

Besides, with your standard block, the customer could bring in two sets of 3 conductor 500MCM _in the same pipe_ and meet your ampacity requirements. (2 * 380A * 0.8 = 608A), so even if the customer prefers to use a single conduit run, they should probably be encouraged not to use 1250MCM.

-Jon