Learn how to properly close unused openings in electrical boxes according to NEC Article 314. Discover acceptable methods like silicone sealant or box replacement for code compliance. Before starting, ensure the power is turned off at the circuit breaker to avoid electrical hazards. Gather necessary tools such as a voltage tester, screwdriver, and possibly a utility knife. Begin by. A plastic electrical box —sometimes called a PVC junction box —comes with small, thin discs called knockouts. Would it be safe to cover with drywall-if so what's the official how to? box pic second pic Archived post.