Definition of Junction Box

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

junction box outdoors

The Spruce / Ana Cadena

Junction boxes are an essential part of the electrical wiring systems for homes and buildings alike. These metal or plastic boxes house and safely protect a structure's electrical connections. These electrical casings typically come in two sizes: three-wire boxes measuring 2 inches by 3 inches with a depth of 2.5 inches, and boxes with five or more wires that measure 2 inches by 3 inches and have a depth of 3.5 inches.

Junction Box in Action

The junction box serves as the communal meeting spot for electrical wires, where they connect before moving on. These boxes protect hot (black), white (neutral), and grounding (green or copper) electrical wires and may contain some other wire colors for secondary functions and lighting.

An encased Romex wire runs from the main electrical panel (or a subpanel) to the junction box. Romex is the trade name for a non-metallic sheathed electrical wire that is most commonly used for residential branch wiring. Wires connect to the original Romex wire and get distributed to other fixture boxes. All wire gauges (size of wire diameters) should be the same.

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.

Safety First When Working With a Junction Box

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

Warning

Since it can sometimes be a guessing game which breaker switch controls the room you're working on, instead turn off the main breaker. Knowing all power is turned off eliminates any confusion and removes the risk of a potential fatal situation. It's a small inconvenience to have to reset digital clocks to prevent injury or death.

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 wrap black to black and white to white, before attaching the wire nut. Then, ground the box securely.

Install 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. You do this to avoid circuit overload, which helps protect against a potential electrical fire.

When adding a circuit to an older home, run a new Romex wire from the electrical panel. You do not want to plug an expensive big screen TV or a computer with essential data into an older, two-wire receptacle.

You can buy junction boxes at most hardware or home improvement stores. 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, 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, which is essential if the box supports a light fixture. Some junction boxes have wings that fit within a cut-out drywall hole, but these boxes are not sufficiently sturdy or stable for supporting 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 is not always referring specifically to a junction box. Other types of electrical boxes that serve different roles include:

  • Mounting box
  • Fixture box
  • Handy box
  • Remodeling box
  • Light switch box
  • Receptacle box
  • Outlet box
  • Electrical box
  • Ceiling fan box