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I have two circuits right now that both terminate in the outlet box for the range hood/microwave. The one that goes into the box from above up in the second floor joists is a currently unused circuit (no devices on it). So, I'd like to run that unused circuit over to install a new outlet over what will be a narrow countertop to the left of our range (two studs to the left from the microwave outlet box where it currently terminates). I have two problems I'm trying to navigate: 1) The location of the new outlet box will be inside a cold air return and 2) all the studs on the wall already have wires running down them, so I'm not sure if I can just bore through the 2 studs in my way.

My inspector said the only way he can approve an outlet inside a cold air return is if the outlet box is metal and the portion of the wiring inside the cold air return is housed in flexible metal conduit.

Below is a photo of my situation. I'm not sure what will be the best path to run the romex and the conduit. Any suggestions?

wall photo

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Your inspector is on the right track -- burning NM in an air return is no fun!

A smoldering cable in an air return will cause the HVAC system to blithely send all that smoke throughout the house. Hence, the NEC restricts the wiring methods usable in ductwork and plenum spaces in 300.22; since your panned-stud air return is a kind of plenum space, 300.22(C) is the specific Code citation that applies:

(C) Other Spaces Used for Environmental Air (Plenums). This section shall apply to spaces not specifically fabricated for environmental air-handling purposes but used for air-handling purposes as a plenum. This section shall not apply to habitable rooms or areas of buildings, the prime purpose of which is not air handling.

Informational Note No. 1: 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.

Informational Note No. 2: The phrase “Other Spaces Used for Environmental Air (Plenum)” as used in this section correlates with the use of the term “plenum” in NFPA 90A-2015, Standard for the Installation of Air-Conditioning and Ventilating Systems, and other mechanical codes where the plenum is used for return air purposes, as well as some other air-handling spaces.

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 without an overall nonmetallic covering, 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 use within an air-handling space, 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.

Nonmetallic cable ties and other nonmetallic cable accessories used to secure and support cables shall be listed as having low smoke and heat release properties.

Informational Note: One method to determine low smoke and heat release properties is that the nonmetallic cable ties and other nonmetallic cable accessories exhibit a maximum peak optical density of 0.50 or less, an average optical density of 0.15 or less, and a peak heat release rate of 100 kW or less when tested in accordance with ANSI/UL 2043-2008, Fire Test for Heat and Visible Smoke Release for Discrete Products and Their Accessories Installed in Air-Handling Spaces.

(2) Cable Tray Systems. The provisions in (a) or (b) shall apply to the use of metallic cable tray systems in other spaces used for environmental air (plenums), where accessible, as follows:

(a) Metal Cable Tray Systems. Metal cable tray systems shall be permitted to support the wiring methods in 300.22(C)(1).

(b) Solid Side and Bottom Metal Cable Tray Systems. Solid side and bottom metal cable tray systems with solid metal covers shall be permitted to enclose wiring methods and cables, not already covered in 300.22(C)(1), in accordance with 392.10(A) and (B).

(3) Equipment. Electrical equipment with a metal enclosure, or electrical equipment with a nonmetallic enclosure listed for use within an air-handling space and having low smoke and heat release properties, 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.

Informational Note: One method to determine low smoke and heat release properties is that the equipment exhibits a maxi‐ mum peak optical density of 0.50 or less, an average optical density of 0.15 or less, and a peak heat release rate of 100kW or less when tested in accordance with ANSI/UL 2043-2013, Fire Test for Heat and Visible Smoke Release for Discrete Products and Their Accessories Installed in Air-Handling Spaces.

Exception: Integral fan systems shall be permitted where specifically identified for use within an air-handling space.

As a result, you will need to use an unjacketed, metal-clad wiring method (type AC or MC cable is probably the easiest thing to use for this) with a metal box and faceplate if you want to mount a receptacle in this air-handling stud space.

  • Thanks, so now i just need to confirm the path to run the new wire. I think I need to pull that wire back up into the joists, put in a junction box, and run the new wire over above the stud where the new outlet will go. Then, down through the top plate there. The wiring inside the cold air return will be housed in flexible conduit – Eaton Editing Mar 16 at 2:44
  • @EatonEditing -- if you are going to use FMC, then I would transition from NM to THHN/FMC at the junction box in the ceiling -- pulling NM through conduit is a whole bunch of ado for nothing. – ThreePhaseEel Mar 16 at 14:46

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