Outlets are wired from the breaker box to the first outlet, with a second cable running from the first outlet to the second, and so on until the final outlet is reached. The last outlet has one set of black, white and ground wires, where the other outlets have two sets. Connecting an outlet with two sets of wires (6 wires total) requires cutting a section of ground wire, joining one end to the existing ground wires, and the other end to the outlet's ground terminal, as electrical code dictates that each terminal can only connect to one wire.
Things You'll Need
6-inch ground wire
Strip back each cable's outer sheathing to within one inch of where it enters the outlet wall box. Run the tip of a utility knife blade along the center of the sheathing, making sure it does not cut into the plastic coating of the wires inside. When stripped, cut off the sheathing from the cable using the knife.
Rest each black and white cable in turn, in the appropriate slot on one jaw of a pair of wire strippers, 3/8-inch from the end of each wire. The stripper's jaws have various size slots that fit certain gauges of wire. Squeeze the stripper's handles together, and push the stripper towards the end of the wire to remove the small sections of plastic coating.
Measure and cut to length a 6-inch piece of ground wire, using a tape measure and pair of wire cutters. Attach one end of the 6-inch piece of ground wire to the ends of the other two ground wires. Screw on a wire nut (this process is called pig-tailing). Bend the other end of the ground wire, as well as the bare ends of the black and white wires, into clockwise hook shapes using long-nosed pliers.
Attach the ground wire to the outlet's ground terminal, one black wire to each of the outlet's brass terminals, and one white wire to each of the silver terminals. Tighten each screw head to its wire with a screwdriver.
Push the wires and wire nut carefully into the box. Attach the outlet to the wall box with the two screws, and attach the faceplate to the front of the outlet with the single screw -- in both cases, use the screwdriver.
If for some reason the outlet circuit has been connected to the breaker box prior to installing the outlet, TURN OFF the circuit breaker powering the outlet circuit before installing the outlet. When the outlet has been installed, turn the breaker back on (if all other outlets have been installed).