To find the right size junction box in cubic inches, count the number of wires entering the junction box. Multiply the total by 2 when working with 14-gauge wire or by 2. 25 when working with 12-gauge wire. Then, select the largest ground wire and add 2 if it is a 14-gauge wire to 2. 25 if it is a 12-gauge wire. Choose a junction box with conductor fill volume (which is the number of conductors allowed in a junction box) higher or equal to the number of wires (after multiplying them by 2 and adding in the ground wire) entering the junction box. It’s best to choose a fill volume higher than the number of wires you have to ensure there will be enough room. [1] X Research source Stuffing a junction box with wires can lead to fires. All of the wires should have plenty of space to fit cleanly inside the box. When in doubt, always go with a bigger size.
You may also turn off power only to the room you’ll be working in. If you do turn off power to your entire home, be sure that your electronics and appliances won’t be negatively affected. You don’t need to contact the electrical company to install the junction box, but they can help you shut off the power supply.
Make sure you know how to use a voltmeter properly to prevent hazards. If the meter isn’t set correctly, it could damage the junction box or cause serious injury. Don’t use a current meter to check the circuit. It will give a reading of 0 amps when there aren’t any devices connected to it, but the circuit will still have at least 120 volts running through it.
For drywall, you can cut out a spot for the box and hold it in place with built-in clamps or Madison clips. The box should be flush with the wall For brick or concrete, attach it with masonry anchors.
If you use a metal connector to attach armored cables, you must also use a plastic bushing to protect the conductors utilized by the connector. Armored cable tends to be used in commercial wiring rather than residential homes. If you have a metal junction box, look for circles outlined on the box’s sides. Hammer these out to create openings for the cables. If you accidentally create too many openings, seal the ones you won’t use with a knock out seal, as there must not be unused openings in an electrical box. If the box has breakaway plastic tabs (most often found on boxes that will be nailed to the surface), push the tabs out by hand or with a screwdriver. Fasten the cable using a wire staple within 12 inches (30 cm) of the box to secure it. For junction boxes installed on the ceiling, run the cable from the ceiling down into the box.
When stripping a Romex cable, for example, you’ll notice 3 differently-colored wires. The wires must be the same size. The AWG (wire gauge) size will be printed on the outer sheathing. If the wires are new, find the gauge number on the packaging. Mismatched wires lead to fires.
If you are using a metal junction box, you need a scrap ground wire. The casing of these wires is green or copper-colored. When connecting three or more same-colored wires, get scrap wire for each color.
Some junction boxes have terminals. All you have to do is plug the wire ends into the terminals, then screw wire nuts over them. Don’t twist the wires too much or else they could break.
The color of the wire nut determines its size and how many wires can fit inside of it. For example, a red wire nut holds a minimum of two 14-gauge wires up to a maximum of four 12-gauge wires.
This is only necessary when using a metal junction box.
