Missing Knockouts

Find out why missing knockouts are dangerous and learn this easy, inexpensive way to fix them.

One of the most common items I find myself repairing is an unfilled opening in an electrical box or panelboard.  If you walk through the garage or basement of just about any old Minneapolis or Saint Paul home, you’ll probably find a few of these.

Missing knockout plug | Construction Pro Tips
 
Opening in panel cover | Construction Pro Tips

The national electric code says that unused openings in such equipment need to be closed.  Covering these openings helps to prevent accidental shocks (think of kids), helps to prevent hot sparks from escaping if something goes wrong inside the enclosure, and helps to prevent pests from entering.  It happens.

Dead mouse in electrical panel | Construction Pro Tips

How to fix a missing knockout plug

First, determine the size of the missing knockout hole and subtract 3/8″ to figure out what size knockout plug you’ll need.  For example, if you have a 7/8″ hole, you’ll need a 1/2″ knockout plug.

Next, go buy some knockout plugs.  They’re sold in small quantities at hardware stores and home improvement stores for about a quarter each.

Knockout plugs

Next, kill the power to any circuits going through the box you’re working on.

Finally, put the plug in the hole.  If you bought the right size, it will be nearly impossible to push the plug into place with just your hands – you’ll need to tap the plug into place using a hammer.

Knockout plug missing
 
Knockout plug installed

If you want to make the plug super tight and impossible to remove, you could bend the tabs inside the box with a screwdriver.  I’m sure some electricians would say this should always be done.  The photo below left shows the tabs in their normal position, and the photo below right shows the tabs bent out.

Knckout plug inside view
 
Knockout plug inside view with tabs bent

How to fix a missing twist-out at the front of a panel

First, determine the brand and type of panel.  It will probably say so right on the door.  If it helps, take a photo of the label.

Next, go buy some filler plates at a home improvement store or hardware store.  You’ll find them next to the circuit breakers.

Filler Plates

Finally, shut off the power to the panel and pop the appropriate filler into place.  This can typically be done without even removing the cover.

Now go forth and fill your holes with impunity.