In walls or ceilings of combustible material, box set back must be ______ inches with the finished surface.

• A. 0
• B. 1
• C. 2
• D. 3

Answer: (A)

In combustible walls or ceilings, the electrical box should be mounted so that its front edge sits in the same plane as the finished surface—zero inches of set back. This flush alignment lets the cover plate sit flat against the wall and keeps the device assembly securely supported by the wall, without creating a recessed pocket behind the finish. A backset away from the surface can leave a gap that is harder to seal and may complicate finishing, while also potentially allowing heat or moisture to accumulate behind the finish. For safe, clean, and code-compliant installation in wood-based or other combustible materials, the box front must be flush with the finished surface.