Physically Constructed Ducts or Plenums
This would be the actual tube where HVAC air is moved through.
In a physically constructed duct or plenum, there is no wiring allowed unless it is necessary for the direct action upon, or sensing of, the contained air. Even then, nonmetallic boxes are not allowed.
Spaces Used for Environmental Air, Other Than Ducts and Plenums
This might be the area above a drop ceiling, or under a raised floor.
In this case, only specific things are allowed.
- Type MI cable.
- Type MC cable without an overall nonmetallic covering.
- Type AC cable.
- Factory assembled multiconductor control or power cable that is specifically listed for the use.
- Listed prefabricated cable assemblies of metallic manufactured wiring systems without nonmetallic sheath.
- Electrical Metallic Tubing (EMT).
- Flexible Metallic Conduit (FMC).
- Intermediate Metal Conduit (IMC).
- Rigid Metal Conduit (RMC) without an overall nonmetallic covering.
- Surface metal raceway, or metal wireway with metal covers, or solid bottom metal cable tray with solid metal covers.
Again, no nonmetallic boxes.
This section does not apply, since a junction box is likely not considered "Electrical Equipment" as far as this code section is concerned.
If you'd like to read the actual code verbiage, it can be found in my answer to this question.
This YouTube Video from Mike Holt on the subject, might also be worth watching.