Unused knockouts in electrical boxes and cabinets must be closed according to which NEC provisions?

• A. NEC 110.12A
• B. NEC 314.17A
• C. Both NEC 110.12A & NEC 314.17A
• D. NEC 300.5

Answer: (C)

Keeping unused knockouts closed is about preserving the safety and integrity of electrical enclosures. An unused opening in a box is a potential path for moisture, dust, debris, or accidental contact with live parts, which can compromise insulation and safety.

Two NEC provisions address this together. One sets general requirements for electrical equipment to remain safe and protected in its installed condition, which includes maintaining the enclosure’s integrity. The other specifically targets boxes, conduit bodies, and similar enclosures and requires that unused openings be closed with a listed closure (such as a knockout plug or a blank cover). Because both rules apply—the general safety/installation requirement and the specific rule for closing unused openings—you need to follow both, hence the correct answer is that both provisions apply. In practice, use a listed knockout plug or closure to seal any unused knockout.