How to Remove a Knockout From an Electrical Box

A screwdriver and hammer will get the job done!

I first came across knockouts in electrical boxes when installing new ceiling fans in our house. The ceiling fan brace had a metal electrical box that I needed to run 14/2 cable in from a light switch, and to do that I needed to remove a knockout from the electrical box. So if you’re in a similar situation, here’s a quick guide on what knockouts are and how to remove them from electrical boxes.

What is a Knockout?

Knockouts are small openings in electrical boxes that provide space for electrical cables/wiring to enter and exit the electrical box. In electrical boxes made out of metal, knockouts are circular and approximately quarter-size. Most electrical boxes have several knockouts, allowing you to remove the knockout that gives the electrical cables/wiring the most direct path to the electrical box (and to minimize the amount of bending of the electrical cables/wiring).

The following picture shows an electrical box with five knockouts – three on the face of the box (blue arrows) and two on the sides of the box (yellow arrows):

Silver-colored electrical box with bolts and nuts. Blue and yellow arrows indicate the positions of knockouts
The interior of a metal electrical box showing several knockouts.

Knockouts in electrical boxes come partially stamped out, so in order to remove a knockout all you need to do is apply a little extra force to dislodge the knockout. Here’s what to do:

Step 1: Locate the knockout tab/attachment point

While most of a knockout is partially stamped out, there is a little tab that is not stamped out and keeps the knockout attached to the electrical box. Locate this tab. Here is an example of what a knockout tab looks like (circled in yellow):

The knockout tab (circled in yellow) that attaches the knockout to the electrical box.

Step 2: Use a flathead screwdriver and hammer to detach the knockout

You’ll need something pointed and durable. I like to use a flathead screwdriver, but any sort of screwdriver, chisel, or similar tool will work. Place your tool of choice opposite the knockout tab/attachment point, just inside the edge of the knockout. Grab a hammer, and hammer the screwdriver several times until the knockout starts to dislodge – like this:

Silver-colored flathead screwdriver on a silver-colored metal electrical box.
Hammering a screwdriver provides enough force to dislodge a knockout from an electrical box.

Step 3: Twist the knockout back and forth until it breaks off from the electrical box

Once the knockout is mostly detached from the electrical box, grasp the knockout between your fingers and start twisting it back and forth (like opening a soda can). The knockout will pretty quickly snap off, leaving you a nice hole for electrical cables/wiring:

Silver-colored electrical box with a small, circular hole in it.
The hole that remains in an electrical box after having removed a knockout.

(Optional) Step 4: Insert a strain-relief clamp

Strain-relief clamps can now be inserted into the knockout hole. While optional, they really help to hold electrical cables/wires in place and prevent them from rubbing against the edges of the electrical box. If you choose to install one, it will look something like this:

Gray plastic strain-relief clamp in a silver-colored electrical box
Strain-relief clamp in an electrical box (view 1/2).

From the other side of the electrical box, the strain-relief clamp looks like this:

Gray plastic strain-relief clamp in a silver-colored electrical box
Strain-relief clamp in an electrical box (view 2/2).

Congratulations, you’ve successfully removed a knockout form an electrical box!

Did this guide help you remove a knockout from an electrical box? Have any other questions about electrical boxes? Let me know in the comments below!

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