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Old 10-17-2010, 02:25 PM   #1
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AC & DC in same run?

Someone on this board mentioned that he had attempted to run AC & DC wires in the same chase and that the mix had caused problems with the AC/DC converter. I have come up with a conversion floorplan that has me running all my wiring in one chaseway and if mixing ac/dc together is a problem I need to make some changes.

So, is it true that mixing the two a problem and if so, how far apart do I have to keep the two seperated?

Thanx all - Rick
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Old 10-17-2010, 03:07 PM   #2
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Re: AC & DC in same run?

Running AC and low voltage both in the same raceway is a no-no, even though some may have done it and lived. The biggest issue is that the insulation requirement for devices working at low voltage is less stringent than at high voltage. If the AC was to have a short that melted insulation, you would not want to have AC on your DC lines. Imagine grabbing metal knobs on a 'car' stereo with AC on them. Or maybe just dealing with a fire caused by the DC devices melting.

If the AC is in a metal chase, then the DC can probably be attached right to the outside, the way low voltage wiring is often attached to EMT conduit with zip ties. Two metal raceways could be run side-by-side. I have worked on municipal buildings where there was one raceway with a metal divider that kept the AC and low voltage 100% isolated inside, so that must be OK, too.

The other issue is in coupling. Long wires running side-by-side act as somewhat of a transformer, and changing (alternating) currents can be induced by magnetic effect onto the DC lines. Separating the lines can reduce the coupling. I think two inches between unshielded DC wires and unshielded AC wires (Romex, extension cords) might be the code. That's what my old boss told me to do when running telephone lines in offices. If you were using two plastic conduits, I would maintain the spacing.

Magnetic coupling can also be reduced by twisting the pairs for each circuit. If every inch, half-inch, or whatever, the opposite wire lies alongside the source "transmitting" the magnetic field, all the little induced currents buck each other rather than adding up. That's why telephone lines use "twisted pairs" when running a few feet below power lines for miles and miles. It is also why the Cat 5/Cat 6 computer cables running Megahertz and Gigahertz inside have tight twists on the pairs to reduce interference both coming in and going out.
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Old 10-17-2010, 03:48 PM   #3
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Re: AC & DC in same run?

Thanx for the confirmation. I was thinking that the AC lines might be capable of inducing voltages in the DC lines. Two inches separation is very do-able and I'm going to take your suggestion and use two plastic conduits spaced several inches apart instead of running all of them in the same chaseway.

My "mixed" wire runs would only be 6 feet or so and I expect that would cause minimal issues but experience has shown me that it's way better to design out potential problems than to try to fix them later.
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Old 10-18-2010, 06:55 AM   #4
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Re: AC & DC in same run?

Quote:
Originally Posted by Dr LuLu
. . . Can AC and DC lines cross each other at a 90 degree angle? . . .
I can't quote chapter and verse from the electrical code, but I think I heard it's OK. Knowing that "cross-polarization" de-couples antenna power by over 99%, I'm sure the magnetic coupling between wires would not be an issue.

Smitty/Angus: Thanks. That gave me a smile.
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