Section 602.1/Plenums
IMC 602 covers requirements for using enclosed spaces as air plenums including material restrictions and wiring limitations.
Spaces used as air plenums (return air paths) must comply with material restrictions to limit smoke and toxic fume generation. The space above a suspended ceiling used as a return air plenum is classified as an "other space used for environmental air." Materials within the plenum must have a flame spread index of 25 or less and a smoke developed index of 50 or less. Combustible piping, wiring, and cable must be plenum-rated (limited combustible materials). Duct systems within plenums must be metal or listed plenum-rated material.
Why this section exists
When the space above a ceiling is used as a return air path, any fire in that space distributes smoke and toxic fumes throughout the building via the air handling system. Standard materials (PVC pipe, NM cable, foam insulation) generate dense toxic smoke when burning. Restricting materials in the plenum to those with low flame spread and smoke development ensures that a fire in the plenum produces minimal smoke and toxins before the duct smoke detector shuts down the system.
What plan reviewers look for
Plan reviewers check the mechanical drawings to identify which ceiling spaces are used as return air plenums. They then verify that the architectural specifications restrict materials in those plenums per IMC 602. They check the electrical specifications for plenum-rated cable (NEC 300.22). They verify that plumbing and fire protection piping in the plenum complies with the material restrictions.
Common violations
Related IMC requirements
NFPA 90A Section 4.3 covers the fire protection requirements for plenum materials. NEC 300.22 covers the wiring methods permitted in plenums. Section 601 covers duct system design requirements. Section 401.2 covers the general ventilation requirements.