How To Install A Junction Box

Each type is available in metal or plastic. Metal boxes are used primarily to house metal conduit or armored (BX) cable. Plastic boxes are used to house the more common non-metallic (plastic) sheathed cable. Junction boxes are attached to the wall framing at the surface of the drywall with their solid (hole-free) covers exposed. A junction box can also be mounted on an exterior wall, with a waterproof cover to keep the connections dry. Wherever a junction box is located, its cover must remain accessible.

Junction Box Size

Junction boxes are sized according to how many conductors they can hold—because, among other things, an overstuffed junction box is a fire hazard. The National Electrical Code has published a chart that determines a junction box’s correct size, based on the number and size of the conductors it must accommodate. The size of a conductor is expressed as AWG (American Wire Gauge); the smaller the number, the larger the wire. Common household wiring is usually 16, 14, or 12 AWG. As an example, the code allows 2 cubic inches of space for each 14-AWG conductor, so if you have two cables with three conductors each (a total of six), you’ll need a box that’s 4” x 2-⅛” x 1-⅞”, a minimum volume of 13 cubic inches.

Steps for Wiring and Installing a Junction Box

First, turn off the power for the circuit you’ll be using. (If there’s a light on that circuit, turn it on before you flip the circuit breaker, and if it turns off, you can be reasonably sure that the circuit is disconnected. But don’t do anything yet!)

What You Need

Materials

Code-approved electrical box Cable clamps (for a metal box)

Tools