Definition of Junction Box

junction box
••• © Big Stock Photo

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.

Installing the Box

Professionals usually recommend installing a junction box by bringing a new wire from the main electrical panel to the box rather than tapping off an existing electrical box. This is done to avoid circuit overload, which is important to protect against because of the threat of a potential electrical fire.

If you are working on adding an additional circuit to an older home, running new ROMEX wire from the electrical panel is often your best option because you do not want to plug your expensive big screen TV or computer with all of your important data into an older two-wire receptacle.

It's recommended to install new ROMEX circuits to safely accommodate modern-day technology in an older home. You can buy junction boxes at any hardware or big-box home improvement store. A junction box will cost less than a dinner for two at McDonald's and increase the safety of your home's electrical system. While you are there, pick up a couple of extra covers for any uncovered junction boxes you might later discover during an attic inspection or in the garage.

Electricians typically secure a junction box to a strong structural location such as a stud or joist, and it's absolutely necessary if the box will be used to support a light fixture. Other types of junction boxes have wings that fit inside a cut-out drywall hole, but these ones aren't sturdy or stable enough to support a light fixture.

People sometimes call a receptacle box a junction box and use the words interchangeably, but may use receptacle box as a more generic term, and it isn't always a junction box.

For example, a variety of other electrical boxes exist, with specific names, including the following types:

  • mounting box
  • fixture box
  • handy box
  • remodeling box
  • light switch box
  • receptacle box
  • outlet box
  • electrical box
  • ceiling fan box

Safety First When Working with a Junction Box

Before doing any work, always turn off the power prior to working on a junction box. Some experienced, professional electricians may not always turn off the power, figuring if they never cross a hot wire, they will live to see another day, and most of them do, except for that one guy. Don't be that one guy. Turn off the power first.

Also, resist the urge to guess at which breaker switch controls the room you are working in, and just hit the main breaker so that there will be no question that all of the power is turned off. It's a small inconvenience to have to reset digital clocks to avoid a potentially fatal outcome.

Punch a hole out to provide a path for the wires to enter the box. Use a cable clamp to secure the ROMEX wiring that comes into the box, and cap wires inside the box with wire nuts. If you have difficulty twisting the wires together, use needle-nose pliers to twist black to black and white to white, before attaching the wire nut, then ground the box.