How to Knock Out a Junction Box


Metal junction boxes have circular holes covered by metal tabs where wires and conduit may enter the box. The holes and tabs are formed by the metal stamping machine when the box is manufactured. These holes and tabs are called knockouts and, in order to make use of one, you have to remove the metal tab. Threaded adapters with nuts allow you to attach conduit or clamp sheathed cable where it enters the box.

Things You'll Need

  • Screwdriver
  • Hammer
  • Pliers
  • Sheathed cable clamp
  • Place the screwdriver blade against the junction box knockout. The knockout is joined to the metal box in two places by a small tab. Place the screwdriver at the edge of the knockout away from the tabs.

  • Strike the butt of the screwdriver handle lightly with the hammer. The knockout will push out of the hole, but will still be attached.

  • Grasp the knockout with the pliers and twist it off. You may have to bend it back and forth a few times to remove it.

  • Remove the nut from the sheathed cable clamp. Insert the threaded end of the clamp into the knockout hole from the outside of the junction box. Screw the nut clockwise onto the clamp on the inside of the box.

  • Place the blade of the screwdriver against one of the notches on the nut. Use the blade of the screwdriver to push on the nut and tighten it clockwise. Tap the screwdriver handle lightly with the hammer to tighten the nut.

Tips & Warnings

  • Some boxes have two or more sizes of knockout holes ranging in size from 1/2 inch diameter up to 1 1/2 inch diameter. Be sure you remove a knockout of the correct size for the clamp or other adapter you intend to put in it.
  • Always turn off power to any box with live wires in it at the circuit breaker. Test to be sure the power is off with a no-contact voltage tester.

Related Searches


  • "Wiring Simplified, 40th Edition"; Herbert P. Richter, et al.; 2002
  • Photo Credit Jupiterimages/ Images
Promoted By Zergnet



You May Also Like

  • How to Install an Electric Light Fixture Junction Box

    Electric light fixture junction boxes are enclosures mounted behind the wall or ceiling that perform a two-fold function. First, they provide a...

  • How to Replace Knockouts

    As a safety precaution and for protection against fire, all electrical panels and junction boxes are required to be fully secure. This...

  • How to Punch Holes in Electrical Panels

    Electrical conduit and connectors come in standard sizes. Common sizes of conduit used in residential construction are often larger than readily available...

  • How to Connect AWG8 Wire in Junction Box

    Electrical junction boxes aren't used just to provide convenient ways to mount receptacles, switches and light fixtures. They're actually required by the...

  • How to Add New Wiring From a Junction Box

    When a new receptacle, outlet or switch needs to be added to a room, the easiest source for providing electricity to the...

  • How to Pop Out Electrical Box Holes

    Electrical boxes are made of metal and are installed in walls and ceilings to house electrical outlets, light fixtures and other electrical...

  • How to Replace a Junction Box

    The National Electrical Code (NEC) requires certain codes to be followed when installing a junction box. The existing junction box should already...

  • How to Move a Junction Box

    When an electrical cable is not long enough to reach from one switch/receptacle to another, a junction box is placed somewhere in...

Related Searches

Check It Out

22 DIY Ways to Update Your Home on a Small Budget

Is DIY in your DNA? Become part of our maker community.
Submit Your Work!