Definition of Junction Box

junction box
••• © Big Stock Photo

Definition: Junction boxes are made from metal or plastic, measure 2 1/2 inches or 3 1/2 inches deep, and it's where the electrical wiring in your home goes to live when the wires are joined together. Boxes that measure 2 x 3 and 2 1/2 inches deep generally contain 3 wires. Boxes that measure 2 x 3 and 3 1/2 inches deep are made for five or more wires.

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). There are other colors of wires used for secondary functions and for lighting. A ROMEX 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 and distributed to other fixture boxes. All wire gauges should be the same.

It's like the communal meeting spot for electrical wires, where they connect before moving on. All junction boxes must be covered to be installed correctly and comply with building code. A cover also prevents moisture from getting inside.

The 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 will avoid circuit overload and helps to thwart 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 typically your best option because you do not want to plug your expensive big screen TV into an older two-wire receptacle. Nor would you want to hook up your computer to such a receptacle. Too much can go wrong.

I always recommend to my clients that they 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. While you are there, pick up a couple of extra covers for those "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 member 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 they will not support a light fixture. People sometimes call a receptacle box a junction box and use the words interchangeably, but a receptacle box is not a junction box.

For example, there are a variety of electrical boxes, and names for those boxes, which range from:

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


Securing Wires Inside a Junction Box

Turn off the power. Always turn off the power prior to working on a junction box. I know a handful of professional electricians who sometimes do not 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 that one guy. Don't be that one guy. Turn off the power first.

Also, don't guess at which breaker switch controls the room you are working in, just hit the main breaker and 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 swap life for death.

Punch a hole out to provide a path for the wires to enter the box. Use a cable clamp to secure 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 a needle-nose pliers to twist black to black and white to white, before attaching the wire nut. Ground the box.

At the time of writing, Elizabeth Weintraub, CalBRE #00697006, is a Broker-Associate at Lyon Real Estate in Sacramento, California.