When an outlet falls in the middle of a circuit run rather than at the end, the power source cable will enter the outlet box from one side and exit the other side on its way to serve additional outlets or lights on the circuit. (There may also be a third cable if the power source is splitting at this point). There are two basic options for wiring the receptacle to the two cables running through the box.
First, the circuit can be direct-wired through the receptacle—that is, the entry cable can be attached to one pair of hot and neutral screw terminals on the receptacle, while the exit cable can be attached to the other set of screws. In this configuration, the circuit flows through the receptacle at all times, using the connecting tabs on the receptacle to establish the continuous path.
The second method of wiring a mid-run receptacle is to connect the receptacle to the circuit wires with "pigtails" that splice into the circuit wires passing through the box. In this case, the circuit load flows both to the receptacle and to any "downstream" receptacles without being dependent on flowing through the receptacle's connecting tab. Both methods are acceptable by Code, but pigtailing is preferred for several reasons.
Direct-Wiring Through the Receptacle
With two cables in an electrical box, one is the incoming power feed or "line" cable and one is the outgoing power or "load" cable. The load cable feeds any receptacles or other devices falling downstream on the circuit.
On a standard 120-volt receptacle, there are three types of screw terminals: brass-colored screws that accept black hot circuit wires, silver-colored screw terminals that accept white neutral wires, and a green screw terminal that accepts the bare copper grounding wires.
Be aware that in old wiring, you may not see the familiar black and white jackets on the circuit wires—the important thing to remember is that brass screws accept hot wires, and white wires accept neutrals. It's also possible that in some configurations, a hot wire may be indicated by red insulation on the wire jacket.
To direct-wire through the receptacle, connect one of the black hot circuit wires to one of the brass-colored terminals, and connect the other black wire the other brass terminal. Similarly, each white neutral wire is connected to a silver neutral terminal. That leaves the two ground wires—usually bare copper wires, or sometimes green insulated wires. These need to be twisted together with one or two pigtail wires joined with a wire connector. One ground pigtail connects to the ground screw terminal on the receptacle. If the electrical box is metal, you also need a second pigtail to connect to the ground terminal on the box itself. Usually, this ground terminal is a green screw threaded into the back of the metal box, but it is also acceptable by Code to make this connection with a green clip that attaches to the side of the box.
Some receptacles also have holes in the back of the receptacle body, used for "back-wiring." Use these only if they are the type that can be clamped down with a screw. The push-in type of connector is unreliable and can lead to loose wires and other hazards. Professionals almost never use the push-in connectors.
Pigtailing the Receptacle
To connect the receptacle with pigtails, each of the black, white, and ground wires in the two cables in the box are joined with a short length of wire called a pigtail. Normally the pigtail will be a wire with the same color coding as the circuit wires: a black pigtail is hot, a white pigtail is neutral, and a green or bare copper pigtail is the ground.
The hot pigtail connects to one of the hot brass terminals on the receptacle. The neutral pigtail connects to the neutral terminal, and the ground pigtail connects to the green ground terminal on the receptacle. Again, if the box is metal, you need an additional ground pigtail connecting to the box terminal. With this method, only three wires are connected to the receptacle, as opposed to five wires with the direct wiring method.
Why Pigtailing Is Preferred
One drawback to direct wiring through a receptacle is that the receptacle is in the middle of the circuit and any trouble in the wiring or receptacle spells trouble for any receptacles connected downstream from it. Any problem with the receptacle, or even loose wire under one of the screw terminals, could cause you to lose power to the downstream circuit receptacles as well. Even diagnosing the problem can be difficult, because if all receptacles lose power, it's hard to determine which one is causing the issue.
Direct wiring also complicates repair or replacement, because if you have to take one receptacle out of the circuit, you interrupt the remaining downstream receptacles. The wires must be reconnected to another receptacle before the circuit downstream can function again, leaving the circuit out of commission in the meantime.
With pigtail wiring, all of the above drawbacks are eliminated. A problem or loose connection with one receptacle will not affect the receptacles downstream. You can also remove a receptacle in the middle of the circuit without affecting the others.
One situation in which pigtailing may not be advised is if the wall box is very shallow or is otherwise not big enough to handle the volume of extra wire nuts and pigtail wires. If the box is too small to comfortably handle all the pigtails and connectors, then the solution may be to install a larger wall box—or to make do with direct wiring through the receptacle.
More Than Two Cables in the Box
Sometimes you may open a receptacle box and discover that there are three cables, not just one or two. In this case, one cable is the line (incoming power) cable and the other two are the load (feeding downstream devices). The only appropriate way to wire a receptacle in a box with three cables is to use pigtails to connect the receptacles. Never connect more than one wire under a single screw terminal. It's also not a good idea to direct-wire all three cables to the receptacle by utilizing both the screw terminals and the back push-in terminals, as this allows two load currents to pass through the receptacle, exacerbating the problems described above.