When noncombustible surface material is used on walls or ceilings, outlet boxes must be mounted so that they will be set back not more than ____ from the face of the surface material.

• A. 1/4" (6 mm)
• B. 1/2" (12 mm)
• C. 3/8" (10 mm)
• D. 0 inch; flush

Answer: (A)

The main idea is to keep the outlet box aligned with the finished wall surface. On walls or ceilings finished with noncombustible materials (like concrete, brick, or plaster), the box should be set back no more than a small amount from the face of the surface so that the face of the box sits essentially flush with the finished surface once the wall finish is applied. The reason this distance matters is that cover plates are designed to sit flat against the wall, and finishes have thickness and small variances. If the box sits too far back, the plate won’t sit flush, creating gaps or misalignment and potentially stressing the wiring or plate. The 1/4 inch (6 mm) limit provides enough tolerance for typical finish thickness while keeping the device visible edge even with the wall. Larger setbacks would cause noticeable protrusions of the wall finish relative to the plate, while a perfectly 0 inch flush situation isn’t practical with real finishes.