In general, I do not have an issue with mixing 120V and 480V wires in the same conduit. There are some cases where this would not be acceptable, for instance it you wanted to take class 1 control wiring for motor A and run it with motor B.
I think you will find that putting more than two sets of motor wiring into the same raceway sometimes creates issues with derating that would require you to increase the ampacity of the wires.
I don't have a problem with specifications that exceed the bare minimum NEC requirements. If an end user wants to pay to run an extra conduit just for control wires, they are free to do so. It is not generally required though by the code.
Where in the code does it require AC wiring to be in a separate conduit from DC wiring? Again, there are some cases where wiring that just happens to be DC would have to be separated from wiring that just happens to be AC, but it is not a general requirement of the NEC.
It doesn't, under 600 volt's, again, it's just a habit I've learned from guy's I've worked with, especially if I'm just pulling in individual conductors. Shielded Beldens may be a different story, but then, Belden sheathing is so darned sticky, it doesn't play nice with other conductors, sometimes causing a huge cluster fustage in the raceway. So it's just easier and faster to pull them in their own pipe. If I'm running new conduit, it's just as easy to run 2, 3 or more extras at the same time. There's
always stuff that needs to be added later, extra pipes are just too handy to not install them. Like pulling in control, I always pull in an extra group, that way I've got 'em if i need 'em.
P.S. I always de-rate where needed, and never skirt the bare minimums! Anything worth doing, is worth doing right!:happyyes: As the plant electrician, I
am the end user, so I'd by far rather over-build it than not! Save myself some heartaches down the road.