Definition of Junction Box

This is an essential part of any home's electrical wiring system.

Electrical junction box with six electrical conduits.
Grigorev_Vladimir / Getty Images

Junction boxes are an essential part of the electrical wiring systems for homes and buildings alike. The purpose of these boxes, which are often made from metal or plastic, is to house and safely protect a structure's electrical connections.

This sort of electrical casing typically comes in two sizes. Junction boxes that measure 2 inches x 3 inches, with a depth of 2.5 inches, generally contain three wires. Boxes that measure 2 inches x 3 inches and have a depth of 3.5 inches are made for five or more wires.

The Junction Box in Action

The electrical wires these boxes protect are known as hot (black), white (neutral), and grounding (green or copper). You may also see some other wire colors, which are used for secondary functions and lighting. A ROMEX, or an encased wire, runs from the main electrical panel (or a subpanel) to the junction box.

Within the junction box, wires are connected to the original ROMEX wire and are distributed to other fixture boxes. All wire gauges, or the measurement of wires' diameters, 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 in order to protect against threat of a potential electrical fire.

When adding a circuit to an older home, running a new ROMEX wire from the electrical panel is often your best option. 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.

You can buy junction boxes at most any hardware or home improvement store. A junction box usually costs less than a dinner for two at McDonald's and increases the safety of your home's electrical system. While at the store buying your box, be sure to pick up a couple of extra covers for any uncovered junction boxes you might find in your home's garage or during an attic inspection.

Electricians typically secure a junction box to a strong structural location, such as a stud or joist. This is especially important if the box will be used to support a light fixture.

Other types of junction boxes have wings that fit within a cut-out drywall hole, but these usually 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, keep in mind that this term is sometimes used more generically and isn't always referring specifically to 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

Always turn off the power prior to working on a junction box. You don't want to be the unlucky person who comes across a highly dangerous hot wire.

Also, don't just guess which breaker switch controls the room you're working in. Instead, hit the main breaker so that there will be no question at all about the fact that the power is turned off.

(Really, in comparison, it's a small inconvenience to have to reset digital clocks to avoid a potentially fatal outcome.)

Finally, punch out a hole 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 securely.