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Knockouts are partially stamped, quarter-size openings in the electrical boxes that service such things as outlets and switches. Once removed, a knockout allows electrical wires to be run in and out of the box.
Listen to BOB VILA ON REMOVING KNOCKOUTS or read the text below:
In the average electrical box, there are several knockouts, each with one or two attachment points resembling small bumps. Remove the knockout that would give the wires the straightest shot—that is, the knockout that would require the least amount of bending on the part of the wires.
To remove a knockout, position the tip of a regular, blade-tip screwdriver just inside the edge, on the side opposite the attachment points. That ought to partially dislodge the knockout, allowing you to remove it the rest of the way by twisting back and forth until the attachment points snap.
When you’re running wire in or out of a knockout, be sure to install a strain-relief clamp, an add-on that not only holds wires in place, but also keeps them from chafing against the edges of the electrical box. Multiple strain-relief options exist; check your local home improvement center.
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