A junction box is used in building construction and consists of a square box or housing that provides protection and a safety barrier for electrical connections. These boxes are made from metal or plastic and form part of your home or other building's electrical wiring system.
They come in two sizes, and junction boxes that measure 2 inches x 3 inches and 2 1/2 inches deep generally contain three wires. Boxes that measure 2 inches x 3 inches and 3 1/2 inches deep are made for five or more wires.
The Junction Box in Action
The purpose of a junction box is to provide a secure environment for electrical wires, known as hot (black), white (neutral) and grounding (green or copper). You might also see other colors of wires, which are used for secondary functions and for lighting. A ROMEX, or encased, wire runs from the main electrical panel (or a subpanel) to the junction box.
At the junction box, wires are connected to the original ROMEX wire and distributed to other fixture boxes. All wire gauges should be the same.
The junction box serves as the communal meeting spot for electrical wires, where they connect before moving on. All junction boxes must be covered, installed correctly and in compliance with the applicable building codes. The cover protects the wires, keeps out dirt and dust, and prevents moisture from getting inside the box.