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I want to run an in-wall HVAC supply duct up a wall and I would also like to put a electrical box for a light switch in the same wall stud cavity. Assuming the stud cavity is wide enough for both, can I put them both in there together as long as each one is anchored to the studs independently? Is it ok for them to touch or do they have to be physically separated somehow? (United States-Ohio if that matters). I am wiring it up using NM wire which would be stapled/anchored to the stud within 8 inches of exiting the switch box and every 4 feet from there up to the ceiling joist cavity.

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If they are going to touch, I'd recommend a nonmetallic box. It's quite possible that the ducts are grounded through the HVAC system. If the grounded metal box touches the grounded metal ducts, you could end up with a ground loop. –  Tester101 Apr 30 at 13:09
    
Isn't everything being grounded together being a good thing? Just curious... I have seen metal boxes in place where subs mashed (not terribly, but slightly deformed)the duct to get them in, wire ran around the ducting to get to point to point, etc. etc... –  Jack Apr 30 at 13:23
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In addition to what @Tester101 siad, metallic ducts can potentially cause condensation which in significant amounts could drip along the wire. While I doubt any code requires mitigation for this, you could simply loop the wire before it enters the box to make sure no water could run into it. –  David Wilkins Apr 30 at 15:52
    
NO! Ground loops are how induction stoves work and high wattage soldering guns work. One soldering gun I took apart had a single copper strap as the secondary that was shorted through the tip. The tip being high resistance in comparison to the strap, of course, heats. Magnetic fields crossing a loop develop a current. There might not be much voltage in the loop, but there's corrosion and electrical erosion wherever there's bad contact. And in industry where you're working with 440VAC, the ground loop secondary voltage can be quite high. –  Fiasco Labs Apr 30 at 15:55
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And on the non-dangerous side of things, ground loops cause 60hz hum in most audio devices and TVs. It's really really annoying, and some people go to great, even dangerous lengths to avoid it. –  Grant May 1 at 1:57
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I see no problem with that at all. Both are independent of each other. If they touch, it sounds as if you need to provide some sort of non conductive material between the duct and box,, or use a plastic box.

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They may not be independent of each other, if it's a metal box and the ducting is grounded. –  Tester101 Apr 30 at 16:17
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