Spot the Violation

ElectricalBoxes, enclosures & coversLevel 22023 NEC

Junction box in a mechanical room

Close view, from slightly below, of a 4-inch square steel junction box with a flat blank cover mounted on a painted concrete block wall. One EMT run drops into the top of the box and another enters from the right side with set-screw connectors. The underside and left side of the box are visible, each with knockouts; red and black conductor insulation can be seen at one of them. An insulated pipe runs up the wall at left and a black-iron gas line with a yellow-handled valve runs along the lower wall.
Commercial mechanical room, junction box on a block wall

What's wrong here?

  1. A knockout in the box is open with nothing closing it
  2. The box must be sized at six times the largest raceway entering it
  3. A green grounding wire must be run to the box along with the EMT
  4. Nothing. This installation is compliant
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Show the answer and the code reference

The answer

An unused knockout in the junction box was left open; unused openings must be closed.

Correct choice: 1. A knockout in the box is open with nothing closing it

110.12(A) requires unused openings in boxes and other enclosures to be closed so the protection is substantially equivalent to the wall of the enclosure. The only openings allowed to stay open are those intended for operating the equipment, for mounting, or permitted as part of the design of listed equipment, and an empty knockout is none of those. An open knockout lets fingers, tools, debris, and rodents reach energized conductors, and in a fault it gives hot gases and sparks a way out of the box. It usually happens when a knockout is popped for a raceway that was later rerouted.

How to fix it

Close the hole with a listed knockout closure (snap-in or two-piece seal) sized for the opening, or use the opening for a raceway with a proper connector.

Code references

  • NEC 2023 110.12(A) — Unused Openings

    Unused openings, other than those for operation of equipment, for mounting, or permitted as part of the design of listed equipment, shall be closed to give protection substantially equivalent to the wall of the equipment. Metal plugs or plates used with nonmetallic enclosures shall be recessed at least 1/4 in. from the outer surface.

    Drills citing 110.12(A)

  • NEC 2023 250.118 — Types of Equipment Grounding Conductors

    Lists the permitted equipment grounding conductors, including electrical metallic tubing.

    Drills citing 250.118

2026 NEC:same rule and number. Article 110 was heavily revised in 2026 (110.12 now says "professional and skillful manner" instead of "neat and workmanlike"), but sources say the unused-openings rule in 110.12(A) did not change. Based on search snippets only. Confidence: medium

Minnesota

MN adopts the NEC rule as writtenConfidence: high

Minnesota adopts the NEC by reference without amendment (Minn. Rules 1315.0200; Minn. Stat. 326B.35), so 110.12 applies as published. The 2023 NEC governs permits filed July 1, 2023 through Aug. 16, 2026; the 2026 NEC governs permits filed on or after Aug. 17, 2026 (exams moved to the 2026 NEC on Sept. 8, 2026). See the 2026 NEC note on this drill.

How Minnesota adopts the NEC

Why the other choices are wrong

  • The box must be sized at six times the largest raceway entering it. The 314.28(A) pull-box sizing rules apply where conductors are 4 AWG or larger; a box with small branch-circuit conductors is sized by 314.16.
  • A green grounding wire must be run to the box along with the EMT. EMT is listed in 250.118 as an equipment grounding conductor; with listed fittings made up tight, no separate wire is required for the box.
  • Nothing. This installation is compliant. The open knockout leaves a hole into an enclosure with energized conductors, which 110.12(A) does not allow.
Safety hazardLevel 2 · Journeyman#knockout#unused opening#junction box#EMT#commercial

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