Once materials or devices are exposed in the airstream, the question is no longer "can it fit above the ceiling?" It becomes "is it noncombustible, enclosed, or specifically listed for plenum use?"
Plain English: Plain-English Highlights
602.3 Materials Within Plenums
Materials within plenums must be noncombustible unless they meet a specific allowance in 602.3.1 through 602.3.10.
Key exceptions include one- and two-family dwellings, fully enclosed combustible materials, listed plenum materials, and certain Group H Division 5 fabrication areas.
602.3.1 Ducts, Connectors, Duct Coverings, Linings and Tape
Ducts and connectors must comply with 603.
Coverings, linings, tape, and connectors must comply with 603 and 604.
602.3.2 Smoke Detectors
Smoke detectors in plenums must be listed and labeled.
602.3.3 Wiring
Exposed combustible wiring and optical fiber in plenums must be plenum rated or installed in metal raceways / metal-sheathed cable.
Only plenum-rated wires and cables can be installed in plenum-rated raceways.
602.3.4 Fire Sprinkler Piping
Plastic fire sprinkler piping exposed in plenums is limited to wet-pipe systems and must be properly listed and labeled.
602.3.5 Pneumatic Tubing
Combustible pneumatic tubing exposed in plenums must be properly listed and labeled.
602.3.6 Discrete Electrical, Plumbing and Mechanical Products in Plenums
Discrete products with exposed combustible material in a plenum must be listed and labeled for that use per UL 2043.
Exception: electrical equipment with metallic enclosures.
On Plans: Why it matters
This is one of the easiest places for coordination to fail. Mechanical may show the plenum, but electrical, FP, plumbing, and controls may still place noncompliant cable, piping, tubing, or devices above the ceiling without showing the required listing basis.
Code Path: Where to show it
M-001: IMC 602.3 plenum-material note and exceptions used
M-101 / coordination plans: plenum areas and exposed MEP components
M-501: enclosure details, duct coverings, and plenum transitions
Check: Do
Identify what is exposed to the airstream
Call out plenum listings where combustible materials remain exposed
Coordinate E, FP, plumbing, controls, and mechanical sheets
Review Risk: Don't
Don't assume "above ceiling" means plenum-compliant
Don't mix non-plenum cable into plenum-rated raceways
Don't show plastic sprinkler piping in dry systems within plenums
Don't place combustible discrete products in plenums without UL 2043 basis
Field Tip: Field tip
Do one review pass just for exposed combustible materials and devices. Trace only cable, tubing, piping, duct liner, tape, detectors, and appurtenances exposed to airflow. That is usually where the fastest IMC 602.3 comments come from.
Comment "IMC602" if you want a paste-ready plenum review checklist + M-001 note set.
Masterbuild QA Lens
Duct-system sections are coordination sections. The question is not only whether air moves, but whether materials, insulation, plenums, dampers, access, and penetrations are correct for the location.
Drawing / Submittal Check
Trace the air path across plans, risers, details, schedules, specifications, and reflected ceiling constraints. Confirm duct material, insulation, vapor control, fire/smoke dampers, access, and exposed conditions.
Common Review Risk
Small duct notes create large field cost when they miss rated assemblies, plenum limitations, weather exposure, internal liner restrictions, damper access, or condensation control.
When To Escalate
Escalate when ducts cross rated construction, run outdoors, serve healthcare spaces, use internal lining, connect to smoke control, or pass through congested existing-building conditions.
Rated Assembly Coordination
When rated construction is involved, the drawings should identify the assembly, damper type, access location, actuator/control basis, fire alarm interface if applicable, and who coordinates the opening.
Duct System Coordination
For duct and plenum items, check material limits, insulation continuity, vapor control, access, listed products, and whether the surrounding space changes the requirement.