Why do open knockouts matter during an inspection?

Open knockouts allow dust, moisture, and pests to enter the enclosure and expose wiring to physical damage. Inspectors flag them because they compromise the protective rating of the electrical enclosure.

How do I remove a knockout without damaging the box?

Place a flat-blade screwdriver against the knockout edge inside the box and strike the handle with a hammer to bend the disc inward. Grip it with pliers and twist it free. A knockout punch tool makes cleaner openings on larger sizes.

What is the difference between concentric and eccentric knockouts?

Concentric knockouts have rings centered on the same axis, while eccentric knockouts have offset rings. Eccentric types can compromise the grounding path through the enclosure if all rings are removed.

Do I need a grounding bushing when I remove a knockout?

If removing concentric or eccentric knockout rings disrupts the metal continuity of the enclosure, a grounding bushing with a bonding jumper is required to restore the grounding path per NEC 250.97.

Can I put a knockout cover back on after removing it?

The original stamped disc cannot be reattached. A listed knockout seal or snap-in filler plate is used to close an unused opening and restore the enclosure protection.

Knockout — Electrical Box Conduit Opening Explained

A knockout is a pre-stamped circular disc in the wall of an electrical box, panel, or enclosure that can be removed to create an opening for conduit or cable entry.

Knockout diagram — labeled parts and installation context

What It Is

A knockout is a factory-formed weakened area in the metal wall of an electrical enclosure, created during the stamping process by pressing a circular outline into the sheet metal without fully separating it. The disc remains attached by a narrow bridge of metal that holds it in place until an installer punches it out with a screwdriver and hammer or a knockout punch tool when a conduit or cable connector needs to enter the box.

Unused knockouts must remain intact to maintain the enclosure's protective integrity. An open knockout compromises the NEC-required protection of wiring from physical damage, allows dust, moisture, and pests to enter the enclosure, and can void the enclosure's listing for its intended application. NEC 110.12(A) requires unused openings to be closed with identified fittings that provide equivalent protection.

Types

Knockouts come in concentric and eccentric configurations. Concentric knockouts have multiple rings sharing the same center point, allowing progressively larger openings from the same location. Removing only the center disc creates the smallest opening, typically 1/2-inch trade size, while removing additional rings creates 3/4-inch, 1-inch, or larger openings from the same knockout location.

Eccentric knockouts have offset rings where each successive ring is centered differently from the one inside it. This offset design can compromise the grounding path through the enclosure wall when rings are removed, which is why NEC 250.97 may require a grounding bushing with a bonding jumper at eccentric knockout locations. Pryout knockouts are a third type found on some junction boxes and are removed by inserting a screwdriver and twisting the disc free rather than punching it inward. Standard trade sizes range from 1/2 inch through 4 inches.

Where It Is Used

Knockouts are found on junction boxes, outlet boxes, switch boxes, breaker panels, subpanels, meter bases, pull boxes, gutter troughs, and virtually every metallic electrical enclosure manufactured for building wiring. They appear on the top, bottom, sides, and back walls of these enclosures wherever conduit or cable connections might be needed.

The number and sizes of knockouts on a given enclosure are determined by the manufacturer based on the enclosure's intended application. A standard 4-inch square junction box typically has knockouts on all four sides and the back, while a residential breaker panel has knockouts across the top, bottom, and both sides. Non-metallic boxes used with NM cable have pre-formed cable entry slots or internal clamps rather than traditional metal knockouts.

How to Identify One

A knockout appears as a circular outline stamped into the metal wall of a box or panel. It may show concentric rings indicating multiple available sizes, with the smallest ring in the center and progressively larger rings around it. Unused knockouts remain flush with the enclosure surface and show the stamped outline clearly.

Removed knockouts leave a clean round opening that should be filled with a conduit connector, cable clamp, or listed knockout seal. An open hole with no fitting installed is an immediate inspection deficiency. Partially removed knockouts with bent or ragged metal edges are also deficient because they damage conductor insulation and fail to secure the entering cable or conduit.

Replacement

A knockout that has been accidentally removed or left open must be sealed with a listed knockout seal, snap-in filler plate, or threaded plug to restore the enclosure rating. These closures are inexpensive and available in all standard trade sizes. They press or thread into the opening and sit flush with the enclosure surface.

If the knockout opening is damaged, oversized from removing too many concentric rings, or distorted from impact, a reducing washer can step the opening down to accept a smaller conduit connector. A repair plate or flat washer may also be needed to restore the enclosure wall around a damaged knockout. All closures and repairs must be identified for the purpose, meaning listed knockout seals or reducing washers rather than tape, sheet metal scraps, or other improvised covers.