No, I literally mean in the same cable. Meaning, I have a multi-conductor cable with several conductors going to supply an AC load and 2 to supply a DC load. Is there some reason (NEC or otherwise) that this can't be done?
Sorry..I don't have the 2005 NEC at home as I type this, but the 1999 code says:
300-3(c)(1): Conductors of circuits rated 600V, nominal or less, ac circuits and 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.
Depends on what you are trying to do. For example, if the DC power is to supply the back up lube oil pump on a turbine which needs to operate if the AC fails, the DC wiring must be separated from the AC wiring to prevent a single point failure.
If the DC is just powering a miscellaneous device, and the voltages are all within the cable insulation rating and there is not a sensitive signal circuit that could be affected by noise, there is no problem having the AC & DC in the cable.
Are the DC wires being used for status indications or control operations? 'DC' wires in close proximity to AC over long distances have been thought to cause 'false positives' and even 'uncommanded operations' under certain conditions, due to 'inductive pick-up'.
Our utility requires that DC conductors used for control and/or indication be in separate cables from AC, preferably in different conduits (although we will share a conduit if there is only one present). DC wires used to supply light bulbs, cabinet heaters or other 'dumb' loads ARE shared with AC.
We are NEC/CEC exempt for our in-house wiring, so I cannot comment on areas that are subject to it.
In our case we have a substation with both AC and DC loads. Though we will likely use separate cables within one conduit, the loads will involve short distance connections to some microprocessor based relays and other connections to motors, heaters, and lights. Thanks to all for your input, much appreciated.
AC conductors can impose 'noise' on cables carrying DC. If the microprocessors that the DC wires are serving are sensitive to noise, you might have a problem. If the microprocessors have adequate filtering, you might be OK.
Are your DC cables shielded? Also, another thing I have seen done when DC had to run in cable trays with AC is to take an unused conductor and ground it on end, making it act as a shield for the active DC conductors.
