Mlhezy 20PCS Seal Electrical Box Knockout in 2 Sizes, Removable Round Steel Instant Knockout Plugs for Closure Unused Electric Box Outlets Holes, Snug Pocket Hole Plugs

What this set is and why I keep it in my kit

Every electrician (and plenty of DIYers) has made the same mistake: you knock out the wrong hole in a box or change the plan after the fact. That’s where a simple, reliable knockout plug saves the day. The Mlhezy knockout plug set is exactly that—20 press‑fit steel discs that close off unused openings in metal electrical boxes. The kit includes ten 1/2‑inch and ten 3/4‑inch trade sizes, which covers the two most common knockouts you’ll encounter in residential and light commercial work.

I put this set through a few weeks of real use across several jobs—cleaning up 4-inch square boxes, a couple of 1‑gang handy boxes, and an outdoor weatherproof enclosure—and was mostly impressed with how straightforward and dependable they were.

Build and finish

These plugs are nickel‑plated carbon steel. The finish is smooth and bright—shinier than the galvanized look on most boxes, but not gaudy. Edges on my set were clean with no burrs, and the spring tabs were consistent from piece to piece. A quick magnet check confirmed they’re steel, not aluminum, and the plating seemed uniform.

They’re a touch thicker than the bargain-bin thin tin you sometimes see, which helps them feel more secure once seated. The plating will resist surface rust, though if you’re working in damp environments, galvanized or stainless will always win the long game. After a week on an exterior service wall during rainy weather, my test plugs showed no discoloration.

Sizes and compatibility

“1/2‑inch” and “3/4‑inch” here refer to conduit trade sizes, not literal hole diameters. In practice:

• 1/2‑inch plugs fit the typical ~7/8-inch knockout
• 3/4‑inch plugs fit the typical ~1-1/8-inch knockout

They’re made for metal boxes and enclosures. They’re not useful on plastic boxes, which generally don’t use removable knockouts. I used them on standard steel junction boxes from familiar brands (Raco, Steel City) and a weatherproof cast box. Fit was consistent across all.

Installation and removal

These are press‑fit. No screws, no tools required—at least in theory. On newer boxes with clean, round knockouts, I could push them in by hand with firm thumb pressure. On older boxes with a bit of distortion, I needed a light tap with a rubber mallet or the plastic handle of a screwdriver to seat them fully. Once seated, they sat flush and didn’t rattle.

If a plug feels a hair loose going in, a simple trick helps: very slightly pinch the spring legs together before insertion. Don’t overdo it—just enough to gain a bit more bite. In one case, that snugged up a 3/4‑inch plug nicely in a weathered box.

Removal is simple. Slip a small flat screwdriver under the lip, twist, and it pops free without mangling the box. Because they’re removable and reusable, I didn’t hesitate to test‑fit a couple during layout.

How they held up in the field

• 4-inch square box above a drop ceiling: Ten minutes saved. I closed two accidental knockouts and one orphaned opening after rerouting EMT. Both sizes locked in firmly and didn’t budge when I tugged on the conduit bushings nearby.

• Handy box retrofit: A 1/2‑inch plug went in by hand; the 3/4‑inch needed a tap. No deforming, no finish chipping. That nickel plating cleans up fingerprints easily if you’re leaving the box exposed.

• Exterior weatherproof enclosure (NEMA 3R): I used a 3/4‑inch plug on the bottom knockout while relocating a hub. After a week of steady fall rain, there was no visible water intrusion. That said, these are not gasketed. For demanding exterior conditions or washdown areas, a listed raintight knockout seal with a gasket or screw clamp is the safer bet.

Code and safety notes

Unused openings must be closed (NEC 110.12). These plugs do that cleanly. However, two caveats:

• Listing/marking: My plugs did not have a visible UL or ETL mark. Many jurisdictions and commercial specs require listed closures. If that applies to your job, verify acceptability with your AHJ or use a listed alternative.

• Wet locations: These press‑fit plugs aren’t watertight. They’re fine in damp basements and protected exteriors, but for true wet locations, use a gasketed, listed KO seal.

Also, don’t treat knockout plugs as bonding devices; they’re just closures.

The small stuff that matters

• Packaging: All 20 came in a single bag with sizes mixed. Not a deal‑breaker, but it adds a few seconds of rummaging. I tossed mine into two labeled bins to speed things up.

• Appearance: The shiny nickel can stand out on raw galvanized or painted boxes. Functionally it doesn’t matter, but if you’re working on exposed industrial gear and want a uniform look, you may prefer a matte or galvanized plug.

• Tolerances: On well-formed knockouts, the fit is excellent. On older or slightly stretched holes, a couple of plugs felt a tad loose until fully seated. The spring legs have enough compliance to bridge minor variations, and a light tap solved all of my fit issues.

How they compare to other options

• Press‑fit knockouts from big‑box stores: Performance was comparable, and these felt slightly thicker. The main difference is value; buying 20 at once costs far less than grabbing two‑packs repeatedly.

• Screw‑type or gasketed KO seals: Those are more secure and can be watertight, but they’re slower to install and cost more per opening. I reserve them for outdoor or critical enclosures.

• Plastic plugs: Cheaper, but they don’t hold as well, aren’t durable, and can look out of place on metal boxes. I avoid them.

Who this set is for

• Electricians and maintenance techs who want a small, reliable stash of the two most common sizes.
• DIYers doing light remodels who want to correct an open knockout without replacing a box.
• Facilities teams who periodically reconfigure conduit runs and need reusable, quick closures.

If you’re doing spec‑driven commercial work where listing and documentation are strictly enforced, verify markings first. If you’re buttoning up a garage junction box, adding circuits in a shop, or tidying up a basement run, these plugs are a simple, effective fix.

Value

Buying knockout plugs piecemeal adds up fast. This 20‑piece set means you won’t last‑minute a trip to the store, and you’ll have both 1/2‑ and 3/4‑inch on hand. For the price, thickness, and overall consistency, the value proposition is strong.

Wish list

• Separate, labeled bags for each size would make life easier.
• A matte or galvanized finish option would blend better on exposed boxes.
• A clearly marked listing stamp (if applicable) would broaden where I can use them without questions.

Recommendation

I recommend the Mlhezy knockout plug set for anyone who needs a dependable, budget‑friendly way to close unused openings in metal electrical boxes. The plugs are well‑made, install quickly, and hold securely across common box types. They’re not gasketed or screw‑secured, so I wouldn’t choose them for harsh outdoor environments or projects requiring listed components with visible markings. But for everyday residential and light commercial work, they deliver exactly what you need—clean, secure closures at a price that makes sense—and they earn a permanent spot in my kit.