/ Field knowledge · IMC 308
“Per code” is not a clearance reduction.
IMC 308: start with the listed clearance. If the listing prohibits reduction, the discussion is over. A note that says “clearance reduction per code” is not an assembly.
Educational 2024 IMC 308. Confirm adopted edition, listing, and manufacturer instructions. This is not a reduced-clearance design.
- If reduction is allowed, the protective assembly must comply with 308 and cannot be placed less than 1 inch from the equipment regardless of the table result.
- Protective assemblies and supports: noncombustible. If the method depends on an airspace, keep the airspace around the edges; spacers must allow convection. Blocking the airspace defeats the protection.
- Two paths: (1) approved assembly listed and labeled to UL 1618, or (2) Table 308.4.2 within table limits. Interpolation between table values is allowed; extrapolating below the table range is not.
- Table methods cannot 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.
Detail, not a note
Show original required clearance, proposed reduced clearance, material, airspace, support, UL 1618 or table basis, and the combustible being protected. Walk the installed clearance before startup — field substitutions kill the reduction.
Source: DCT 308 · chimneys: 805 · kitchen: 506–508 · tool: clearance check.
What to send
The listing clearance, a photo or section of the tight side, and whether anyone already “reduced it in the field.”
If the unit is closer than the listing
Send the listing and the tight dimension. Masterbuild will say whether 308 is even on the path — or the listing already closed it.
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.