What Is a Junction Box?

A junction box is an essential part of the electrical wiring systems for homes and commercial buildings. These metal or plastic boxes house and safely protect a structure's electrical connections. The electrical casings come in many sizes and types for various applications. National and local building codes necessitate the type of junction boxes needed for certain applications.

Purpose and Function of a Junction Box

The junction box serves as the communal meeting spot for electrical wires, where they connect before moving on throughout the house. 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.

How to Choose a Junction Box

Sizes

The typical junction box size for residential use is a 4-inch square box, which is roomy enough for multiple wires and cables. Residential junction boxes come in other sizes, as well. These boxes are usually much smaller than a junction box used in a commercial setting which will have more connectors in one spot.

Materials

Junction boxes are made of either metal or plastic and each material has its pros and cons.

Metal boxes are strong, and fireproof, and must be used with metallic-sheathed wire to provide proper grounding. They are a little more complex to work with because they have knock-outs for clamps and tricky sharp edges. Metal boxes can be recessed in walls and they are designed to mount heavy items.

Plastic or PVC boxes can be used indoors with non-metallic wires but not with metal conduit since there is no grounding provided. They are not as strong and can melt in a fire, but they are less expensive than metal boxes, are lightweight, and have built-in clamps. Plastic cannot be recessed in walls.

Covers

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. The cover also prevents any sparks from igniting combustible materials should arcing occur due to a loose connection or short circuit.

Safety First

Always turn off the main power before working with a junction box. It can sometimes be a guessing game to figure out which breaker switch controls the room you're working on and often several different branch circuits feed through a single junction box. Turning off the home's power eliminates any confusion and removes the risk of a potentially fatal situation.

Tips for Installing the Junction 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.
• 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 most of these boxes are not sufficiently sturdy or stable for supporting heavier light fixtures.
• Be sure to check the listed weight capacity of the box used and the weight of the fixture you are installing, and never use a winged box for a ceiling fan installation.

Other Types of Boxes

The terms receptacle box, electrical box, and junction box are often interchangeable. However, other types of electrical boxes serve various roles, including the following:

• Fixture box: Holds smoke and carbon monoxide alarm fixtures and some lights.
• Handy box: Wall mount for light switches or receptacles located in difficult-to-reach spaces.
• Remodeling box: Known as "old work box" and installed after drywall is hung for new outlets.
• Light switch box: Holds wiring for a light switch.
• Receptacle box: Holds wiring for a receptacle.
• Outlet box: Holds an outlet.
• Ceiling fan box: Houses wiring for fans weighing up to 70 pounds or light fixtures up to 90 pounds, typically mounted to ceiling joists.