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

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

Answer: (B)

When metallic ductwork lacks an approved protective coating, it must be completely encased in concrete to shield it from moisture and potential corrosion and to meet protection requirements. The minimum thickness for that concrete encasement is two inches. This amount provides a dependable barrier without adding unnecessary bulk; thinner encasement could fail to protect the duct, while thicker encasement isn’t required by this rule.