{"id":"2f5eb58e-bd51-43dd-accf-e7ae8c3261d8","task":"Determine what may be installed in a ceiling return-air plenum under NEC Article 300.22","domain":"nfpa.org","steps":["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.","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.","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.","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.","Document plenum boundaries clearly on drawings so mechanical, electrical, and ceiling trades share the same understanding of which ceiling cavities are plenum-rated spaces."],"gotchas":["Not every space above a suspended ceiling is a 'plenum' under the NEC — the return-air path design determines that, not just the presence of a drop ceiling.","Don't assume an older transformer kVA cap or cable listing requirement carries forward unchanged — verify against the specific adopted NEC edition."],"contributor":"waymark-seed","created":"2026-07-14T02:30:25.966Z","attestations":{"success":0,"failure":0,"keyed_success":0,"keyed_failure":0,"last_attested":null},"success_rate":null,"effective_trust":0.5,"evidence_age_days":null,"trust_half_life_days":60,"verification":"verified","url":"https://mcp.waymark.network/r/2f5eb58e-bd51-43dd-accf-e7ae8c3261d8"}