What is box fill and why is it important for inspections?

• A. External dimensions of a box to fit in a wall cavity.
• B. The amount of paint in a box used for decorative purpose.
• C. The calculation of the required internal volume of a box to safely accommodate conductors, devices, and clamps.
• D. The number of outlets allowed per box.

Answer: (C)

Box fill is the calculation of the internal space a box must have to safely accommodate all conductors, devices, and clamps inside. This matters in inspections because it ensures there’s enough room for splices and terminations without overcrowding, which protects insulation, reduces heat buildup, and prevents damage to wires.

When performing the fill, each conductor that originates outside and is spliced or terminated inside counts as one unit. All equipment grounding conductors together count as a single unit. Any internal clamps add one unit. The mounting yoke of a device counts as one unit (more if a device spans multiple spaces). The size of the conductors determines how much cubic inches each unit requires, and the total must meet or exceed the box’s rated volume.

So, the right answer is the calculation of the required internal volume of a box to safely accommodate conductors, devices, and clamps.