7 Best Knockout Seals for Unused Box Openings

Leaving an unused knockout hole in an electrical box is an open invitation for moisture, dust, and rodents to compromise a wiring system. Proper sealing isn’t just about meeting local electrical codes; it is a fundamental step in ensuring the longevity of connections and the safety of the entire structure. Choosing the right closure depends heavily on the environment, the material of the enclosure, and the ease of access during the installation phase. This guide evaluates the most reliable options available to ensure these critical gaps are sealed effectively and permanently.

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Arlington NM900: Best Snap-In Knockout Seal

The Arlington NM900 is the industry standard for fast, non-metallic installations. Designed for quick snap-in application, these plugs save significant time on large commercial or residential rough-ins where dozens of boxes require termination.

Because they are made from high-impact plastic, they won’t corrode or rust when exposed to standard atmospheric humidity. This makes them ideal for interior applications like basement walls or garage framing where metallic oxidation might be a long-term concern.

Avoid these in environments where high mechanical impact or extreme heat is a factor. They are intended for standard electrical boxes and provide a clean, flush finish that satisfies inspector requirements for closing unused openings.

Heyco Liquid Tight Plugs: Best for Wet Locations

When working on exterior installations, such as junction boxes mounted on a masonry wall or near a roofline, moisture intrusion is the primary enemy. Heyco Liquid Tight plugs are engineered specifically to maintain a NEMA 6P rating, ensuring that water cannot penetrate the box even under pressurized spray.

The design utilizes a sealing ring or gasket that compresses against the face of the knockout. This creates a physical barrier that holds up far better than standard plastic buttons under heavy rainfall or seasonal freeze-thaw cycles.

For projects where reliability in wet conditions is non-negotiable, this is the superior choice. Always verify that the plug size matches the specific knockout diameter, as these require a precise fit to maintain their liquid-tight seal.

Halex 90510 Steel Plug: Top Choice for Metal Boxes

The Halex 90510 is the go-to solution for heavy-duty metal enclosures where structural integrity is key. These steel plugs are rugged, vibration-resistant, and provide a secure, locking fit that won’t vibrate loose in high-traffic industrial or commercial environments.

Installation requires a simple tap with a hammer or screwdriver to seat the locking tabs. Because they are zinc-plated, they offer excellent resistance to surface rust, making them suitable for both indoor and semi-protected outdoor applications.

These plugs provide the clean, professional look expected in exposed conduit work. If the box is subject to frequent movement or seismic activity, a steel plug like the Halex 90510 is significantly more reliable than its plastic counterparts.

Sealproof Push-In Blanks: Best Non-Metallic Seal

Sealproof push-in blanks offer a versatile, low-cost solution for non-metallic boxes used in residential renovation work. Their primary advantage lies in the simplicity of the “push-in” design, which eliminates the need for specialized tools or hardware.

These are particularly useful when working in tight, cramped wall cavities where maneuvering a screwdriver to tighten a metal nut is difficult. They sit flush against the box wall, preventing debris from falling into the enclosure during drywall installation.

While effective for most interior residential needs, they lack the mechanical strength of steel. Use these primarily in low-voltage or standard branch circuit enclosures where high-impact resistance is not a primary design requirement.

Raco 1701 Steel Seal: The Go-To Budget Option

The Raco 1701 is a classic “button” plug that serves as the workhorse for electrical contractors. It is an affordable, straightforward solution for sealing 1/2-inch knockouts in standard steel electrical boxes, ensuring compliance with code without inflating the material budget.

The design is straightforward: push it in and it stays put. It effectively blocks dust and vermin, which are common culprits for short circuits in older electrical systems.

Keep a handful of these in the tool bag at all times, as they are universal for standard steel boxes. They are not intended for wet locations, but for dry-wall installations or attic junctions, they offer the best value for the money.

Sigma 49831 Blank Cover: Most Durable Steel Seal

The Sigma 49831 is not just a plug, but a cover plate designed to bridge over a knockout hole with high-durability steel. This is the preferred choice when an enclosure has been damaged or the knockout hole has been enlarged beyond the reach of a standard button plug.

Because it includes its own gasket and mounting hardware, it provides a superior level of weather protection compared to simple push-in plugs. It stands up to the rigors of exterior exposure, including harsh sunlight and extreme temperature fluctuations.

Consider this option when the knockout area shows signs of deformation or when a more permanent, heavy-duty seal is required. It provides peace of mind in high-exposure areas where standard plugs might eventually fail or fall out.

Hubbell-Raco 1751: Best Gasketed Knockout Seal

The Hubbell-Raco 1751 stands out by incorporating a dedicated sealing gasket into the construction of the plug. This is the professional’s choice for sealing boxes that need to meet stricter NEMA standards for dust and water entry.

The gasket acts as a cushion, ensuring that even if the box surface is slightly uneven, the seal remains tight. This is especially useful in retrofit scenarios where the original knockout might have been bent during the initial rough-in.

It provides a level of security that standard steel plugs cannot match without additional sealants. When the task involves high-performance requirements or sensitive electrical equipment, relying on the integrated gasket of the 1751 is a proactive measure.

Metal vs. Plastic Seals: Which One Do You Need?

• Metal Seals: Choose these for heavy-duty, high-vibration, or exposed commercial environments. They offer better mechanical protection against impacts.
• Plastic Seals: Best for residential, low-voltage, or interior applications. They are non-conductive, easy to install, and immune to rust.
• Environmental Tradeoffs: Always use liquid-tight or gasketed seals for exterior locations to prevent corrosion from moisture and salt air.

Selecting the right material comes down to the environment. Metal holds up better in industrial settings, whereas plastic is often sufficient for home use. Do not use plastic plugs in areas where they might be hit by heavy equipment or tools, as they can crack under impact.

Sizing Your Knockout: A Quick How-To Guide

Standard knockouts are sized by the conduit diameter they are intended to accept, not necessarily the physical diameter of the hole. A “half-inch” knockout hole is actually closer to 7/8 inches in diameter.

Always measure the hole diameter with a tape measure or calipers before purchasing your seals. Relying on nominal trade sizes without verification often leads to purchasing plugs that are either too loose to stay or too large to fit.

If the knockout has been damaged or enlarged with a knockout punch, you may need a larger cover plate rather than a plug. Never use tape or non-approved materials to fill an oversized gap; it is a fire hazard and a code violation.

NEMA & IP Ratings for Seals: What They Mean

NEMA ratings define the environmental protection of an electrical enclosure, and the seals must match these ratings. For example, a NEMA 3R box requires a weather-tight plug to maintain its rain-proof status.

IP ratings (Ingress Protection) operate similarly, using numbers to indicate protection against solid objects and liquids. An IP65 rating, for instance, means the enclosure is dust-tight and protected against water jets.

When in doubt, check the label on the box. If the box is rated for outdoor use, ensure the seal you select has a corresponding gasket or liquid-tight designation. Never compromise the integrity of a rated enclosure with a cheap, non-rated plug.

Properly sealing an unused knockout hole is a hallmark of professional work that protects the electrical system from the elements and code inspectors alike. Choose your seal based on the specific environmental demands of the job—prioritizing gaskets and metal for the outdoors, and quick-set plastics for interior efficiency—to ensure a safe and lasting result.