There are many types of electrical metal boxes that you will find at the hardware store. They come in many shapes and sizes, making it hard for someone new to electrical projects choose between them. You'll find octagon shaped metal boxes, round ones, square or rectangular. They will also vary in size; some will be deeper than others. But all these metal boxes are standard and manufactured to code.
- Remove the knockouts. Inspect the metal box and you'll notice that there are round shapes embedded on the surface. These are the knockouts that you can remove and have an opening that you can send the NM cable through. There will be several of these knockouts around the metal box, look for them on the side and rear panels. Some metal boxes will be limited to just two knockouts. Decide which type of metal box you need for your project and look for the knockout positions. Some of these knockouts on metal boxes are hard to remove and you'll need some tools to aid you. You can use a hammer and give it a few taps to clear the knockout but if it won't come off easily, give it heavier blows from the hammer. If you get a section of the knockout open, use a pair of pliers to pull the knockout free from the metal box. You may have to twist it a bit to loosen sections of the knockout. On other metal boxes there are slots that you can send the end of a flathead screwdriver through and pry the knockout from the metal box. When you clear the metal box of a knockout you may leave some sharp metal edges and could possibly damage the NM cable. Use a metal file to file down these sharp edges and smooth it out.
- Secure the NM cable with the cable clamp. Some metal boxes will have a cable clamp ready to hold the NM cable in place. If a clamp is not available you will have to add one in the metal box. There are cable clamps you can purchase at the hardware store; these clamps are often called ROMEX connectors. When you install a cable clamp on the metal box you can now send the NM cable through the opening then secure the cable with the cable clamp. You need a pair of pliers to tighten the lock nut to hold the cable in place. Do not over tighten the clamp at the risk of damaging the cable.
Plan your project carefully to aid you in looking for the right metal box with the specifications you need.