Q: I have several electrical boxes in the basement with open knockouts that I want to seal. The holes are ¾ of an inch across. I've found numerous plug products online and at local electric supply houses, but they are either too small or too large. Any suggestions?
Darnestown, Md.
A: At first glance, there seems to be an easy answer to your question. Google “ ¾ -inch knockout plug,” and loads of possibilities pop up.
Knockouts are the places where an electrical box has been punched through to allow for wiring, so connections for outlets, light fixtures or additional wiring can be made within the box, which is rated to resist fire if anything overheats. The boxes usually come with several possible knockouts to accommodate wiring that comes in or out in different directions. The knockout locations are formed so it’s easy for an electrician to pop out the openings that are needed. Ideally, only those get opened. But electricians sometimes change the plan for which wires go where, and homeowners sometimes make additions, so extra openings can exist. And electricians occasionally drill directly through the box, leaving openings that aren’t a standard size.
It’s wise to want to plug the extra openings in your boxes. Electrical codes require that unused openings be closed to ensure that the box can corral any fire that might start inside it. Keeping the openings closed guards against someone poking something through a hole and being electrocuted, and it also keeps out mice, which might find the confines of an electrical box a cozy place to nest.
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But as you have discovered, plugs labeled as being ¾ of an inch aren’t designed to cover holes that diameter. Instead, the size listed on the packaging is the “trade size,” which means the diameter of the electrical conduit that would fit through the hole with a little extra wiggle room to make it easy and quick to fit the parts together. A plug with a trade size of ¾ of an inch is designed to fill a hole 1.109 inches in diameter, or about ⅓ of an inch wider than what you need.
The next smaller size, ½ inch, doesn’t work, either. It is designed to fill a hole 0.875 inches in diameter. Plugs with a trade size of ⅜ of an inch come closest, but they are a bit too small, filling holes 0.718 inches in diameter. Plus, this size is hard to find, because most household wiring requires conduits of at least ½ inch.
Standard knockout plugs have crimped prongs on the back. The idea is to press or hammer a plug into place from outside the box. The prongs compress and press inward to fit through the hole, then spring back into place to keep the plug snug against the box and virtually impossible to remove. It might be possible to use a ½ -inch plug and reach inside the box (with the power cut off at the circuit breaker) with a screwdriver to press the tabs outward enough to hold the plug in place.
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But a less common type of plug, called a three-piece seal, might work without that fussing. This type of plug has a threaded strap on the back and a circular cap on the front, with a screw in the center that connects those two parts.
You loosen the screw but don’t remove it. Then you wiggle the strap through the opening. By pressing down, you can get the whole length of the strap through the hole. Then you raise the cap so it’s centered over the hole and tighten the screw, which pulls the inside and outside parts together for a tight seal. The back of the strap is shaped so it automatically centers in the hole, although it is designed to do this for a standard trade-size opening. (If you buy ½ -inch seals, the best fit for your situation, the cap probably won’t tighten exactly over the center of the hole.) An off-center fit is fine, but shine a flashlight into the box. If light comes through a gap, somewhat loosen the screw and nudge the cap into a slightly different position. If you are working overhead, try removing the screw and reversing the direction. Then press the strap through from the inside of the box, and tighten the screw from the outside.
Several companies make three-piece knockout seals. Platt Electric Supply carries ½ -inch seals made by LH Dottie for $1.71 each. Home Depot offers Halex seals in the same size, with a package of four listed at $2.24. But you might need to special-order them.
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Whatever solution you try, always cut off power at the circuit breaker box before you start. For work in a basement, you’ll probably have to use a headlamp or other battery-powered light to clearly see what you’re doing.
Match the plug material to the material of the box. The pictures you sent show a galvanized steel box, so the parts mentioned in this column are made of steel coated in zinc. Plastic plugs also exist, for use with plastic boxes.
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