Connections Tutorial: Figure 6
Eight Receptacle Hookups
1 [p-p] - Basic. Receptacle 1 presents perhaps the most common hookup to be found in outlet boxes. One cable-p (which one doesn't matter) is incoming power that is, a black hot wire and a white neutral. The other will take power out of the box and on to the next item of the circuit, once it receives that power by some connection to the wires of the incoming p-cable. In this case the receptacle's terminals -- side-screws and/or push-in-the-back holes -- provide a way for that to happen. Blacks attach to the brassy side and whites to the silvery side. In the process, the receptacle itself can now also pass power to what we plug into it.
2 [p-p-p] - Pigtail. Here a third power cable is present. Of any number of p-cables and/or f-cables, one will always be the source of hot and neutral, and the others, if any, will depend for their "powerness" on their connection to that one... Box-2 is mostly to show that, given the functions the wires are meant to play, there are often still many options for how to make the connections. The two blue wirenuts in box-2 are "pigtailing" wires to the receptacle. As you can see, wires not involved in a pigtail can still make their connection at a receptacle screw or hole.
4 [f] - Switched. At receptacle 4 we get our first look at a 3-wire cable in an outlet box. This is a case of a switched wire being fed with hot and neutral in the same cable. There is something else new to notice here -- right on the receptacle. If it looks like this receptacle is missing a gold tooth compared to the ones to its left, it is. What is missing is not a terminal screw, but a metal tab that is responsible for passing hotness between the top and bottom halves of these "duplex" receptacles. It is also there so that we can keep the two halves from being connected if we want -- by breaking the tab off (back-and-forth motion with needlenose pliers). Why would we do that? Receptacle-4 is set up to be a switched receptacle. Or I should say, half-switched. By convention, one use of the red wire from 3-wire cable is as the switched "leg" -- for a light or for a receptacle. The black is constantly hot. This red will only be hot (live) when the switch it comes from says so. What would happen if we had these wires hooked up here at receptacle-4, but we didn't break the tab off on the hot side? (People do this, when replacing receptacles blindly.) The constant hotness of the black would carry the day -- the red would have no effect. In fact, the red would be made to be constantly hot. So the switch across the room would do nothing, and no matter which half you plugged a lamp into, the lamp would stay on all the time (except for the twist-switch right on it). By the way, you are not at all likely ever to need to break the tab off from the neutral side of a receptacle. So don't do it. It could disrupt the circuit from that point on.
5 [p-f] - Top. This receptacle is also being switched. This time it is the top half that is switched. There is no rule saying to switch one half versus the other, but if other outlets in the room or the house are switched a certain way, following their lead makes sense. This receptacle-5 also has an additional 2-wire cable. Cable-f at box-4 brought a neutral and a hot to that receptacle. Here at box-5 it could be the same way, or the power might be coming in on cable-p and passing to the receptacle and (on cable-f) toward the switch box, with the red still coming "back" here. Although I show the hot-wire connections as a pigtail, the black wires of cable-p and cable-f could have both attached to terminals at the bottom of the receptacle's hot side.
6 [p-p-f] - Bottom. Receptacle 6 is back to having the bottom half switched. At this box, another 2-wire cable has been added, sending power to more of the circuit, probably to another outlet, one that is not to be switched. I show all three blacks pigtailed to the terminal. If only two hot-side terminals were available at the top half, we would be forced to pigtail from at least two of the three blacks. On the neutral side, we could have pigtailed, but enough terminals were available to attach directly to them; both top and bottom of a switched receptacle are available for neutrals to attach to.
7 [d] - Double. Notice that receptacle 7 is hooked up just like #4. (The position of the white wire connection doesn't matter.) This #7 is not a switched receptacle, however. It is a receptacle whose top half is from one circuit and bottom from another. The circuits share one neutral and are from two circuit breakers, which are 240 volts apart! Since 1981 Code has wanted those two breakers physically tied together so that no one working on the receptacle could think it was dead from turning off just one breaker. Where would such a double-circuit receptacle ever be encountered? Dining/Kitchen receptacles in the 1960s were often wired this way (see 2-circuit); receptacle-7 would occur at the end of such a run. The only other case of a double-circuit duplex receptacle would probably be for two special appliances located together and each getting a circuit dedicated to it alone. Two freezers perhaps. Or the pump and the heater for a whirlpool tub.
8 [d-p] - Shared. Here is another case of the 3-wire cable bringing two circuits to the box. In this case, the receptacle is only being run by the black circuit. The red circuit is tied through to feed cable-p, which may go to many more items. The black circuit here seems to be serving only receptacle-8, however; this is not as common as what will be shown in box-18. Notice one other thing. The neutrals of box-8 are pigtailed to the receptacle. Up to now, we have had the option of attaching neutrals directly to the receptacle, but where a double circuit is involved, it is important (and Code) for the neutral connection for the two circuits to be independent of the receptacle. This more reliable connection makes a loose shared neutral -- a condition that allows sustained high voltage to damage electronics and appliances -- less likely. See Double circuits.