A rigid coduit connects a control panel with a metering device. The device uses 120VAC for control power and has a contact that will have 24vdc going through it and used back at the control panel as an input to a PLC.
Since these circuits are functionally related, even if the 24 vdc is class 2 or class 3, you could take advantage of exception #2 to 725.130(A) and also apply 725.48(B)(1) and run the 24 vdc as a class 1 circuit.
Few things are harder to put up with than the annoyance of a good example.
#187261 - 06/18/0902:28 PMRe: 120 vac and 24vdc in same conduit
Shop_Test - As I understand it the two could in certain applications be serving the same device and be combined, but the NEC calls out certain separation devices and or distance that must be maintained between the conductors?
Tom- In the case of wiring as a Class 1 circuit (upgrade the wiring), NEC2005 725.27(A) would require 18 or 16 AWG and 725.27(B) would require 600 volt rated insulation of specific types?