/ Field knowledge · IMC 602.3
CM cable and dry-pipe CPVC do not belong in a return plenum.
Once materials or devices are exposed in the airstream, the question is no longer “can it fit above the ceiling?” It is “is it noncombustible, enclosed, or specifically listed for plenum use?” IMC 602.3: materials within plenums must be noncombustible unless they meet a specific 602.3.1–602.3.10 allowance. Exposed combustible wiring and optical fiber: plenum-rated, or in metal raceways / metal-sheathed cable. Plastic fire-sprinkler piping exposed in plenums is limited to wet-pipe systems and must be listed. Discrete products: UL 2043 (metallic electrical enclosures excepted).
Educational 2024 International Mechanical Code (IMC) 602.3–602.3.6 from Daily Code Talk. Confirm the adopted edition. Distinct from 602.2 construction and 602.3.7 foam / flood. This is not a listing. Packet: 602.3 PDF · ES.
- 602.3: noncombustible unless a specific 602.3.1–602.3.10 allowance applies. Key exceptions: one- and two-family dwellings, fully enclosed combustibles, listed plenum materials, and certain Group H Division 5 fabrication areas.
- 602.3.1: ducts and connectors per 603; coverings, linings, tape per 603 and 604.
- 602.3.2: smoke detectors in plenums listed and labeled. Pair with 606.3.
- 602.3.3 wiring: exposed combustible wiring and optical fiber plenum-rated or in metal raceways / metal-sheathed cable. Only plenum-rated wires and cables may be installed in plenum-rated raceways.
- 602.3.4: plastic fire-sprinkler piping exposed in plenums — wet-pipe only, listed and labeled. Dry and preaction plastic in a return plenum is the usual miss.
- 602.3.5: combustible pneumatic tubing exposed in plenums listed and labeled. 602.3.6 discrete products: UL 2043 (metallic electrical enclosures excepted).
This is where coordination fails. Mechanical shows the plenum. Electrical, fire protection, plumbing, and controls still drop noncompliant cable, piping, tubing, or devices above the ceiling with no listing basis. Put the 602.3 note on M-001 and make the other trades see it.
CM cable and dry-pipe CPVC in the return plenum
Or a plastic control box not listed UL 2043. Those fail inspection after the ceiling is closed — demolition, not a punch item.
Source: Daily Code Talk — IMC 602.3 · construction: 602.2 · foam/flood: 602.3.7–602.4.
What to send
Whether the ceiling is a return-air plenum, what cable / sprinkler pipe / tubing / devices are exposed in it, and any plenum listing (Communications Plenum Cable / CMP, wet-pipe listing, UL 2043) if one exists.
If CM cable and plastic sprinkler pipe are in the plenum
Send the RCP and the product types. MasterBuild will say whether 602.3.3 still has non-plenum cable — or 602.3.4 still has dry-pipe plastic in the airstream.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
MasterBuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. Plenum-rated cable, wet-pipe-only plastic sprinkler, listed pneumatic tubing, and UL 2043 are 2024 IMC 602.3 education from Daily Code Talk — not a listing. This page is not legal advice.