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:
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:
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.
Masterbuild QA Lens
Plenum penetrations by wiring systems (602.3.3) require plenum-rated wiring (CMP designation). Confirm the electrical drawings specify plenum-rated cable in the mechanical plans plenum zones, that the electrical engineer of record has confirmed the routing, and that the plenum boundary is consistent between the mechanical and electrical drawing sets.
Drawing / Submittal Check
For 602.3-602.3.6 (materials in plenums): the drawings or spec must confirm that every material in the plenum has been evaluated for IMC 602 compliance - insulation, wiring, conduit, pipe, and mechanical equipment supports. A spec-only call-out without a drawing note is frequently missed by the reviewer. Add a plenum materials schedule to the mechanical drawings.
Common Review Risk
Plenum material compliance on tenant improvement projects is particularly complex when the existing above-ceiling space has unverified materials from previous tenants. A pre-permit plenum inspection to identify non-compliant materials is the best risk mitigation. Non-compliant materials discovered after the permit is issued but before final inspection can delay occupancy.
When To Escalate
Escalate when the building owner or the general contractor insists on using non-plenum-rated materials in the above-ceiling return air space because the plenum-rated equivalent is not available in the required size or configuration. Non-plenum materials in a plenum are a code violation regardless of availability - escalate to find a code-compliant solution, not to argue for an exception.
Rated Assembly Coordination
Sections 602.3-602.3.6 govern items passing through or into the plenum, several of which also penetrate rated assemblies. Every penetration through a rated floor-ceiling into the plenum must have a compliant detail for both the plenum and the rated assembly.
Duct System Coordination
Sections 602.3-602.3.6 limit what penetrates the plenum and how it must be handled. Show all penetrations through the plenum boundary and confirm each is either contained in a duct or meets the listed-product requirement.