A junction box is used when multiple runs of electrical wires need to be connected. They are available in a number of sizes and shapes, including square and octagon. Installing one is a straightforward process requiring only a few tools and easily obtained materials. For this example, we'll assume you wish to connect two floodlight fixtures installed on the roof trusses in your barn, so that both fixtures can be controlled by an existing operational wall switch.
Shut off the circuit breaker controlling the switch. Locate a spot roughly equidistant between the fixtures. Attach the junction box to another of the roof trusses with the wood screws. There are small holes in the bottom of the junction box for this purpose. If needed, use the drill to make pilot holes in the truss before driving the screws.
Run the wires from the fixtures and the switch to the junction box. Then, using the screwdriver, force open three conduit knockouts and insert the screw-in wire clamps.
Loosen the screws on the face of the clamps and insert the wires from the fixtures through the clamps and into the junction box. Let 5 or 6 inches of wire extend inside. Use the wire cutters to trim off any excess.
Tighten the screws on the clamp faces to secure the wires you ran from the fixtures and the switch.
Remove 3 or 4 inches of the outer covering on the wires with the utility knife. There will be three wires inside; one black, one white and a bare copper ground wire.
Strip 1 inch of insulation from the end of each black and white wire with the utility knife.
Twist the ends of the tree black wires together. If needed, use the lineman's pliers to ensure all three are tightly twisted and make good contact with one another. Twist a wire nut down on top of the three wires. Tighten it securely but do not overtighten or you might break one of the wires.
Perform the same procedure with the white wires and the bare copper ground wires, respectively.
Install the junction box cover using the screws provided on the face of the box. Octagon boxes usually come equipped with two screws just for this purpose.
Restore the power and flip on the switch. Both fixtures should light up.
If you will be connecting a larger group of wires like four or five, you should use what is called a "deep" box, and correspondingly larger wire nuts. Keep in mind that a junction box is not a special type of electrical box. It derives its name because it is installed at the junction point of several wires.