When a device box is installed in a wall or ceiling finished with gypsum wallboard, what is the maximum setback of the box face from the finished surface?

• A. 3 mm (1/8 in)
• B. 6 mm (1/4 in)
• C. 9 mm (3/8 in)
• D. 12 mm (1/2 in)

Answer: (B)

When installing a device box in a wall finished with gypsum wallboard, the front of the box needs to sit close enough to the finished surface so a standard faceplate can sit flush. The upper limit for how far back the box face can be set from the finished surface is a quarter of an inch (six millimeters). This allowance accounts for drywall thickness, mud, and finishing tolerances so that a normal cover plate can be mounted smoothly without gaps or misalignment. If the box is recessed more than that, the faceplate may not sit flush, which can look odd, be harder to install securely, and might not meet installation standards. The smaller option would still satisfy the maximum, but it does not state the maximum; the larger options would exceed the limit and thus would not be acceptable.