Are metallic device boxes and cable allowed in dwelling air return?

[emf10]

The space I'm talking about is the typical joist space cold air return found in dwellings. I thought metallic device boxes and metallic cable such as MC or AC are allowed in this space but reading the NEC 2005 handbook I have found a comment on this:

"The exception to 300.22(C) permits cable to pass through joist or stud spaces of a dwelling unit, as illustrated in Exhibit 300.21. The joist space is covered with sheet metal and used as a cold-air return for a forced warm-air central heating system. [highlight]Equipment such as junction boxes or device enclosures is not permitted in this location.[/highlight]"

but 300.22(C)(2) states:

"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 [highlight]shall be permitted to be installed in such other space unless prohibited elsewhere in this Code.[/highlight]"

btw what do they mean by the joist space being covered with sheet metal? Around here it's just drywall and studs or joists.

 

[mdshunk]

We typically build a little box around the device box with scrap pieces of 2x4 if the device must be in a stud bay cold air return. Boxing in the box effectively removes it from the environmental air space.

 

[emf10]

We typically build a little box around the device box with scrap pieces of 2x4 if the device must be in a stud bay cold air return. Boxing in the box effectively removes it from the environmental air space.

I wonder if that violates any HVAC codes by cutting down the airflow. Sometimes, though, I'm in an old work situation where I wouldn't be able to box it in.

 

[mdshunk]

I wonder if that violates any HVAC codes by cutting down the airflow.

They do plenty of calculations, but they're not so tight that you can't have a boxed in device in a stud bay return or two. No big deal. Just don't go too crazy. It's normally only when the device layout must be a certain way, like to jive with built in furniture and cabinets, where you would do this. In most cases, you can figure out a way to just mount the box one bay over, left or right.

Sometimes, though, I'm in an old work situation where I wouldn't be able to box it in.

Simple answer; you can't really put a box in there then if you don't want to do some collateral damage.