Determine what may be installed in a ceiling return-air plenum under NEC Article 300.22

Steps

1. Confirm whether the ceiling cavity actually functions as an 'other space used for environmental air' (a return-air plenum) under NEC 300.22(C) — restrictions apply only to spaces meeting that definition; a ducted-return ceiling cavity not itself part of the air path doesn't trigger the same rules.
2. Within a qualifying plenum, use only wiring methods and cables specifically listed for plenum use (low smoke/heat-release rated), or route non-plenum-rated cable and Class 2/3 cable inside a metal raceway instead.
3. Confirm equipment placed in the plenum (junction boxes, control equipment) uses metal enclosures where required, and confirm any dry-type transformer located in the plenum does not exceed the maximum kVA rating specified in the currently adopted NEC edition, since this limit is edition-specific.
4. Confirm which NEC edition (NFPA 70) is adopted by the local jurisdiction with the electrical engineer/AHJ before finalizing cable and equipment selection, since plenum-related listing requirements are revised across code cycles.
5. Document plenum boundaries clearly on drawings so mechanical, electrical, and ceiling trades share the same understanding of which ceiling cavities are plenum-rated spaces.