Knockout Plug

What Is a Knockout Plug?

A knockout plug is a simple component that pairs with the pre-scored, removable circles you see on electrical boxes, panels, and some plumbing fixtures. In electrical work, these plugs snap into unused knockout holes to close them off. In plumbing and appliances, you may encounter a molded “knockout” inside a port (like a dishwasher inlet on a garbage disposal) that you remove to enable a connection. In short, a knockout plug either fills a hole or becomes the piece you remove to open one.

Where You’ll Encounter Knockout Plugs

Electrical junction boxes and load centers: Unused knockouts are sealed with listed knockout seals (also called hole plugs) to keep out dust, prevent accidental contact with live parts, and maintain the enclosure’s rating.
Weatherproof enclosures and outdoor boxes: Gasketed knockout plugs keep out water, insects, and debris.
Garbage disposals: A small plastic knockout plug blocks the dishwasher inlet from the factory; you remove it if you’re connecting a dishwasher drain hose.
Sinks and faucets: Some sinks have accessory holes (for soap dispensers or sprayers) covered by a finish-matched hole cover, sometimes informally called a knockout plug.

Types and Variations

Snap‑in electrical knockout seals (metal): Thin steel or zinc-plated discs with spring tabs that snap into the hole from the outside. Common trade sizes: 1/2 in., 3/4 in., 1 in., 1‑1/4 in., and larger. Used indoors in dry locations.
Two‑piece or clamp‑style KO seals: A front and back plate clamp together through the hole with a screw. These offer a stronger hold and can include gaskets.
Weatherproof/gasketed KO plugs: Designed for outdoor or damp locations; typically include a rubber or neoprene gasket and carry ratings like NEMA 3R or 4X.
Nonmetallic hole plugs: Plastic versions used in nonmetallic enclosures or for cosmetic hole covers on sinks.
Appliance knockouts (removable): Molded plastic plugs you knock out in a garbage disposal’s dishwasher inlet. The removed piece must be retrieved to prevent jams.

Choosing the Right Knockout Plug

Size correctly: Electrical knockout sizes refer to “trade size,” not the exact measurement of the hole. Match the plug’s trade size (e.g., 1/2 in. KO) to the stamped knockout size on the box or enclosure. If unsure, check the box’s labeling or measure the hole and cross‑reference the manufacturer’s chart.
Match the environment: For outdoor boxes or damp locations, choose a gasketed, listed weatherproof plug. For corrosive environments (near pools or coastal areas), look for stainless steel or 4X-rated options.
Material matters: Use metal plugs in metal enclosures where grounding continuity and durability are desired; choose nonmetallic plugs for plastic enclosures or where corrosion is a concern.
Look for listings: UL/ETL listings indicate the plug is tested for safety and enclosure integrity. This helps meet electrical code requirements for closing unused openings.
Aesthetics and finish: For sink hole covers, choose a finish that matches the faucet (chrome, brushed nickel, matte black) and the correct diameter for a snug, rattle-free fit.

How to Use Knockout Plugs

Sealing an Unused Electrical Knockout

Turn off power to the circuit or panel you’ll be working on. Verify with a non-contact voltage tester.
Identify the unused knockout hole. Remove any sharp burrs with a deburring tool if needed.
Select a listed knockout plug of the matching trade size. For outdoor boxes, use a gasketed plug.
Snap-in type: Align the tabs with the edge of the hole and press firmly until it clicks into place. Tug lightly to confirm it’s secure.
Two‑piece type: Insert the front piece, hold the back plate inside the box, and tighten the screw until snug. Do not overtighten and crush the gasket.

Removing the Dishwasher Knockout in a Garbage Disposal

Unplug the disposal and turn off the breaker. Remove the dishwasher hose if preinstalled.
Locate the dishwasher inlet nipple on the side of the disposal.
Insert a flat screwdriver into the inlet and gently tap with a hammer to knock out the internal plastic plug.
Reach inside the disposal’s grinding chamber and remove the loose plug piece so it doesn’t jam the impellers.
Attach the dishwasher hose with the proper clamp and check for leaks during a test run.

Creating an Opening Instead of Using an Existing Knockout

If you need a new hole, use a knockout punch set (also called a chassis punch) or a step bit sized for your connector. Always install a proper listed connector and bushing; never run cable through a raw hole.

Maintenance and Safety Tips

Periodic checks: In garages, attics, and outdoor boxes, inspect plugs for rust, looseness, or gasket deterioration. Replace if compromised.
Keep rodents and insects out: Missing plugs invite pests. Seal every unused opening.
Preserve ratings: Any plug you install should maintain the enclosure’s rating (rain‑tight, dust‑tight, etc.).
Power and PPE: De‑energize circuits before opening boxes. Wear gloves and safety glasses to avoid cuts from sharp edges.

Common Mistakes to Avoid

Leaving unused holes open: This can expose live parts and violates electrical code. Always close them with listed plugs.
Using tape or makeshift caps: Only listed knockout plugs provide a secure, code‑compliant seal.
Choosing the wrong size: A loose plug can fall out; an oversized plug won’t fit. Match trade sizes.
Forgetting to retrieve the disposal knockout: The loose piece can damage the disposal if left inside.
Ignoring outdoor ratings: Indoor plugs often leak in rain. Use gasketed plugs for exterior enclosures.
Running cable through a plugged hole: If a cable needs to enter a box, remove the plug and install a proper connector with strain relief and a bushing.

Related Terms

Knockout (KO): The pre-scored disc in a box that can be removed to create an opening.
Conduit connector and locknut: Fitting that secures conduit to the box through the KO.
Bushing: Smooth insert that protects wire insulation from sharp edges.
Filler plate: Covers unused breaker spaces in a panel (different from KO plugs).
Chassis/knockout punch (slug buster): Tool used to create clean, round holes in metal.
Strain relief/cord grip: Connector that clamps flexible cords entering an enclosure.

Practical Examples

Adding a light on a metal junction box: After moving cables, you’re left with a 1/2 in. unused KO. Snap in a 1/2 in. steel knockout plug to seal it and keep the box code‑compliant.
Installing a dishwasher: Your new dishwasher needs to drain into the disposal. Knock out the disposal’s inlet plug, fish out the piece, attach the hose with a clamp, and test for leaks.
Outdoor receptacle upgrade: You replace a surface‑mount box and end up with an unused side opening. Install a 3/4 in. gasketed KO plug rated NEMA 3R to keep water out.

A knockout plug is a small part that makes a big difference in safety, cleanliness, and compliance. Choosing the right size and type—and installing it correctly—keeps your projects tidy and trouble‑free.