My book is at home now and I have this question for the industrial control people here.

Is there anything in the NEC that dictates a defined separation between control wiring and power wiring?

I.E. 14 inches per kilovolt ....

so long as the wiring is separated by an enclosed wiring method, (NM cable, conduit, etc.) no.

1/4 inch for class two and three. Or a suitable barrier.

Need to know what the control wiring is controling. If it is Class 1 wiring and it is functionally associated with the power wiring there is no problem having it in the same raceway as long as the insulation on all the conductors has a high enough rating. If the control wiring is Class 2 then it can not be in the same raceway with the power wiring without a barrier. 725.26(B)(1), 725.55(A)