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In my finished basement, the space between one section of studs is being used as a return air duct for my furnace and central air.

Here is a picture, looking up from the vent at the floor level:

enter image description here

On the left is a standard outlet box.

I would like to install a sconce light on the wall in this section. I will use a remodel electrical box and the sconce light is of metal construction. I know that wiring that passes through air ducts must be non-metallic, but beyond that I don't know if this is a safe idea. I think it should be, since there is already an electrical box in this space, but I'm not sure how to know for sure. Any advice?

The second question is the hole for the wire that will be drilled in the stud on the right. Will that hole need to be sealed once the wire is through?

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up vote 6 down vote accepted

This is what 2008 NEC has to say...

300.22 Wiring in Ducts, Plenums, and Other Air Handling Spaces. The provisions of this section apply to the installation and uses of electrical wiring and equipment in ducts, plenums, and other air-handling spaces

(A) Ducts for Dust, Loose Stock, or Vapor Removal. No wiring systems of any type shall be installed in ducts used to transport dust, loose stock, or flammable vapors. No wiring system of any type shall be installed in any duct, or shaft containing only such ducts, used for vapor removal or for ventilation of commercial-type cooking equipment.

(B) Ducts or Plenums Used for Environmental Air. Only wiring methods consisting of Type MI cable, Type MC cable employing a smooth or corrugated impervious metal sheath without an overall nonmetallic covering, electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, or rigid metal conduit without an overall nonmetallic covering shall be installed in ducts or plenums specifically fabricated to transport environmental air. Flexible metal conduit shall be permitted, in lengths not to exceed 1.2 m (4 ft), to connect physically adjustable equipment and devices permitted to be in these ducts and plenum chambers. The connectors used with flexible metal conduit shall effectively close any openings in the connection. Equipment and devices shall be permitted within such ducts or plenum chambers only if necessary for their direct action upon, or sensing of, the contained air. Where equipment or devices are installed and illumination is necessary to facilitate maintenance and repair, enclosed gasketed-type luminaires shall be permitted.

(C) Other Space Used for Environmental Air. This section applies to space used for environmental air-handling purposes other than ducts and plenums as specified in 300.22(A) and (B). It does not include habitable rooms or areas of buildings, the prime purpose of which is not air handling.

FPN: The space over a hung ceiling used for environmental air-handling purposes is an example of the type of other space to which this section applies.

Exception: This section shall not apply to the joist or stud spaces of dwelling units where the wiring passes through such spaces perpendicular to the long dimension of such spaces.

(1) Wiring Methods. The wiring methods for such other space shall be limited to totally enclosed, nonventilated, insulated busway having no provisions for plug-in connections, Type MI cable, Type MC cable without an overall nonmetallic covering, Type AC cable, or other factory assembled multiconductor control or power cable that is specifically listed for the use, or listed prefabricated cable assemblies of metallic manufactured wiring systems without nonmetallic sheath. Other types of cables, conductors, and raceways shall be permitted to be installed in electrical metallic tubing, flexible metallic tubing, intermediate metal conduit, rigid metal conduit without an overall nonmetallic covering, flexible metal conduit, or, where accessible, surface metal raceway or metal wireway with metal covers or solid bottom metal cable tray with solid metal covers.

(2) Equipment. Electrical equipment with a metal enclosure, or with a nonmetallic enclosure listed for the use and having adequate fire-resistant and low-smoke-producing characteristics, and associated wiring material suitable for the ambient temperature shall be permitted to be installed in such other space unless prohibited elsewhere in this Code.

Exception: Integral fan systems shall be permitted where specifically identified for such use.

After reading subsection (C), it looks like nonmetallic cable is not allowed in ducts unless it's in conduit.

Equipment and devices shall be permitted within such ducts or plenum chambers only if necessary for their direct action upon, or sensing of, the contained air.

Which means you can't have equipment in the ducts that is not part of the HVAC system itself. So a junction box for a light, would not be allowed (unless the light is inside the duct for maintenance purposes).

If your local government follows National Electrical Code (NEC), then you should not install a junction box for the light in the duct. If you don't follow NEC, it's still probably a bad idea. The boxes and cable are a good place for dust, dirt, and debris to collect. And the cables and equipment could be damaged during duct cleaning.

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this implies that the current electrical box there is out of code. did i read it right? – longneck Jan 23 '12 at 17:07
@longneck That is correct. – Tester101 Jan 23 '12 at 17:10
300.22(B) does not apply to a non-fabricated duct. A fabricated duct is a metal tube of some sort, with various cross-sections. A stud or joist cavity is not a fabricated duct! It's a piece of building structure -- a.k.a. other space used for environmental air, subject to 300.22(C). – Kuba Ober Mar 27 '13 at 3:02
@KubaOber I'm not sure that's exactly right, but I guess it would depend on the AHJ. In either case, NM cable and nonmetallic boxes are not allowed. – Tester101 Aug 3 '13 at 14:00
@Tester101: See my other answer, especially the "definitely OK" link. – Kuba Ober Aug 5 '13 at 19:35

First of all, there may be other codes besides NEC that cover this scenario. I don't know much about them, so I'll deal with NEC only.

There is a lot of confusion on the forums, but let's be clear about it: Per 2011 NEC, if it's a fabricated duct for environmental air, then NEC 300.22(B) applies. If it's not a fabricated duct, then it's "other space used for environmental air" and 300.22(C) applies. Spaces between studs and joists are not fabricated ducts, they are a part of the structure of the house. It just so happens that the air is pushed through them.

So yes, it's definitely OK to install anything that is allowed in NEC 300.22(C). That means that any sort of exposed Romex or NM cable is a NO-NO unless it goes across (perpendicular) to the airflow only (300.22(C) Exception). Similarly, nonmetallic boxes are a NO-NO. Even though the NEC merely insists on metallic boxes, I'd personally use boxes with no holes, so handyboxes etc. are out. The only thing left is cast metal boxes with threaded holes, normally used for exterior applications -- and of course rigid metal conduit (RMC). Per NEC, it'd be OK to use, say, an octagon box and EMT. Myself, I wouldn't. You can put whatever you want inside the metal conduit, so Romex/NM etc. are fine.

You of course need to properly support the conduit, so there will be some hole-poking involved, but it's easy to fix.

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