Which industrial cover/box combination violates NEC when all conductors are 12 AWG?

• A. 4 × 1 1/2 square with 5.0 in3 industrial cover
• B. 4 × 11/16 square with 4.2 in3 industrial cover
• C. 4 × 21/8 square with 3.6 in3 industrial cover
• D. 4 × 2 1/4 square with 4.0 in3 industrial cover

Answer: (A)

Box-fill is the rule that makes sure there’s enough space inside a box to safely contain all the conductors and any other items that occupy volume. For copper conductors of size 12 AWG, each counted conductor requires 2.25 cubic inches of space. When figuring box fill, you count every current-carrying conductor, add the grounding conductors (together, they count as one conductor), and include any internal clamps or device yokes as required by NEC rules. The total fill must not exceed the enclosure’s actual cubic inch rating.

In this set of options, the combination described as having a 5.0 in³ industrial cover represents the smallest available cubic inch capacity. If you have the typical number of 12 AWG conductors in that box, the required fill will exceed 5.0 in³, making it noncompliant with NEC. The other options provide larger cubic inch capacities (4.2, 3.6, and 4.0 in³ in the respective cases), which are sufficient for the conductors counted in those configurations, so they do not violate NEC. Therefore, the 5.0 in³ option is the one that does not meet the NEC box-fill requirements.