300.3(C)(1)
Conductors of circuits rated 600v, nominal, or less, ac circuits, or dc circuits, shall be permitted to occupy the same equipment wiring enclosure, cable, or raceway. All conductors shall have an insulation rating equal to at least the maximum circuit voltage applied to any conductor within the enclosure, cable, or raceway.
As outlined in 725.55(A), this is not permitted with circuits considered class 2 or class 3 circuits.
BUT...I've been on several projects where the project specs did not allow the mixing of different voltages, or different systems, in the same raceways.
[This message has been edited by XtheEdgeX (edited 01-22-2006).]