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Question DetailsAsked on 4/17/2018

replacing knock outs in breaker box

missing knock outs at sides and bottom and missing spacers

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1 Answer


All boxes are, by code, supposed to have all unused or abandoned knockouts replaced - boxes are designed to contain electrical fires caused by wiring problems or excessive demand, and having holes in them negates that protection.

You can commonly find singleton knockout plugs and breaker slot filler plates for common sizes and brands at large home improvement box stores, and at many box stores and online can buy them in 10, 25, 50, and 100 packs of any one size. Commonly run about $1-1.50 each for knockout plugs, around $2-3 for breaker slot filler plates in singletons or 10 packs, more like $0.50-0.75 and $1-2 each respectively in larger packs. Knockout holes are generally standard 1/4" increments in size so most plugs are "universal fit" for their specific hole size - breaker slot filler plates tend to be pretty specific to each brand of box, so measure actual opening and note box manufacturer name before you go in. The plastic breaker slot filler plates occasionally break their tabs off putting them in and out, so get an extra one or two of those.

For breaker boxes, where you sometimes have tight clearance from bus bars or wires, there are also fire-rated plastic non-conducting knockout plugs for about $7-10 each at electrical supply houses - rarely seen in residential use but common in industrial work. You should have minimum (for residential 110-240V service) 1/4" clearance between powered buses or wires and metal box or plugs - preferably 1/2 inch.

Also - be sure you do not have knockouts which did not drop free projecting into or out of the box - projecting in is a shorting risk, projecting out makes the plug fit poorly - be sure to fully bend down (flat against outside of box) any outside knockouts still attached so the knockout hole is fully round to receive the plug. If that box side or back is tight up against a stud it may require a bit of cold chisel work then reaming out the stud a bit with a forstner or spade bit to make room for the plug to fit in if the knockout holes are tight up against a stud on that side of the box. Be aware that there are likely a lot of wires running through (behind box) or along (at sides) the studs in that area so don't drill deeper than needed to clear the plug - that is why I use a forstner bit rather than the spade bit which tip reaches further into the stud for a given hole depth.

Of course, when doing this make sure power is off to the box (and confirm that, because main breakers have been known to fail and stay on even when they are flipped off), and be careful to check all wires afterwards to be sure you did not knock any loose working the plug in around them. A hint - getting the plugs in place and tight workings pace around wire leads can be a bit of a job - I push the plug place by hand then use a piece of scrap dowel with electrical friction tape on the end to tap into place to seat it fully with a hammer - a dowel fits through between the wires a lot better than a hammer. Electricians commonly also use a small diameter screwdriver handle for this, tapping on the screwdriver blade end with a hammer to tap the plug snugly into place. BE SURE all plugs are fully seated in the knockout hole - put only half way in so the head is not flush with the metal box, or have it come up against a stud and stop short, and the spring prongs can eject it back into the box over time, causing a nasty short.

Of course, if not an electrical DIY'er then an electrical contractor is the Search the List category for you - be sure to tell him make of panel and approximate number of knockouts and breaker slots he needs to bring plugs/fillers for. If just doing the breaker box that is likely only 5-10 minutes of work, so if he has minimum service call time left over having him check the main and distribution breaker panels for any issues, loose wires, aluminum wire connections needing tightening, etc would be a good thing to have him do at the same time. Minimum service call commonly 30 or 60 minutes work included, plus as many plugs and filler plates as are needed.

Answered 2 years ago by LCD

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