Metallic ducts not having an approved protective coating shall be completely encased in a minimum of how many inches of concrete?

• A. 1 inch
• B. 3 inches
• C. 4 inches
• D. 2 inches

Answer: (D)

When metallic ducts don’t have an approved protective coating, they need to be fully surrounded by concrete to shield them from moisture, corrosion, and potential damage, and to help with fire resistance where they’re embedded. The required minimum thickness for this encasement is two inches of concrete. This provides adequate protection without adding unnecessary cost, since a thinner layer wouldn’t reliably protect the duct, while thicker layers aren’t typically required for standard installations. If you see options like one inch, three inches, or four inches, two inches is the stated minimum for uncoated ducts.