IMC 308 matters when equipment, chimneys, vents, kitchen exhaust equipment, or mechanical devices are close to combustible construction. It gives a path to reduce required clearances, but only when the protection method is real, approved, and documented.
Plain-English Summary
Start with the listed clearance. If the appliance or equipment listing prohibits reduction, the discussion is over. If reduction is allowed, the protective assembly must comply with IMC 308 and cannot be placed less than 1 inch from the equipment regardless of the table result.
Protective Assembly Requirements
Reduced-clearance protective assemblies and their supports must be noncombustible. If the method depends on an airspace, that airspace must be maintained around the edges and the spacers must allow convection. Blocking the airspace defeats the protection.
Two Approval Paths
Path 1 is an approved reduced-clearance protective assembly listed and labeled to UL 1618. Path 2 is a method from Table 308.4.2, applied within the table limits. Interpolation is allowed where required clearances fall between table values; extrapolating below the table range is not.
Hard Limits
Table methods cannot be used to reduce some clearances: solid-fuel appliances with small listed clearances, masonry chimneys, chimney connector pass-throughs, masonry fireplaces, and kitchen exhaust ducts enclosed in shafts have specific limitations.
Permit Proof Chain
Heat source or vent type -> listed clearance -> whether reduction is allowed by listing -> selected protective assembly -> airspace and support detail -> table or UL 1618 basis -> limitation check -> combustible surface coordination -> field-inspection access.
Masterbuild QA Lens
A clearance reduction is a detail, not a note. If the drawings only say provide clearance reduction per code, the reviewer cannot verify the assembly, airspace, material, or limitation.
Drawing / Submittal Check
On M-501 and architectural details, show the original required clearance, proposed reduced clearance, protective assembly material, airspace, support method, UL/listing or table basis, and the combustible construction being protected.
Field Tip
Walk the actual installed clearance before startup. If the protective board, sheet metal, spacer, or airspace changed in the field, the calculated reduction may no longer exist.