In existing hollow walls or ceilings where branch‑circuit wiring is fished, device boxes must be secured to which surface?

• A. finished surface
• B. framing member
• C. stud
• D. ceiling joist

Answer: (A)

When wiring is fished through existing hollow walls or ceilings, the box needs a reliable place to mount without relying on a backer in the cavity. Securing the device box to the finished surface provides that stable anchor: the mounting hardware grabs the wall finish and the substrate behind it, keeping the box firmly in place and the faceplate flush. This retrofit approach uses the finished surface as the attachment point because there isn’t guaranteed access to a stud or joist behind the hollow space to support the box.

If you could consistently hit a framing member, you would attach there for solid support, but in many retrofit scenarios the only practical and code-appropriate attachment is to the finished surface itself, using the box’s mounting ears or brackets that fasten through the wall finish into the structure.