If you're using a metal box, remove a knockout on the box for each cable that will enter the box. Use a screwdriver and hammer to break out each knockout (metal disc), then twist off the knockout with pliers.
The National Electrical Code (NEC) dictates that no wiring splices are allowed outside of an approved enclosure. One type of approved enclosure is a junction box. A junction box is simply a standard electrical box that is mounted securely (to house framing or another structure) and contains the splice—the wiring connection—of two or more circuit cables or wires. The cables are secured to the box with cable clamps (or conduit connectors, if the circuit includes conduit), and it must have a removable cover to create a complete enclosure. Junction box covers must remain accessible; they cannot be covered with drywall or other surface material.
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