Figure 3 (Lights) Commentary New window.
1 [L-L]- To run, a light fixture needs a neutral and a switched wire. (Fixtures with pull or twist switches right on them would use an unswitched hot wire, of course.) Whatever else goes on electrically in a light box is to help the switching happen and/or for the sake of other parts of the circuit. Box-1 represents the classic "black-to-blacks and "white to whites" instruction. Fixture-1 itself only needs the black and white from whichever cable-L is bringing hotness and neutralness. The other cable-L is going to another light box whose fixture wants to be switched along with this one. A light box like this might have three or four or just one cable, and the black-to-black, white-to-white idea would apply, as long as we were sure that all these were two-wire cables and that (other than the one bringing neutral and switched) they were all going to other lights, to be switched together. If that were the case, in our minds we could label those cables L-L-L, L-L-L-L, and L... In any case, when there are more than one neutral wire or switched wire for the fixture to attach to, it can be a good idea to pigtail a single extra piece of wire from those wirenuts for the fixture's wires to attach to. That way, whenever the fixture is taken down or replaced, there is less chance of bothering the good connections that were first established. In these light diagrams I have not shown such pigtailing -- to simplify things... [What if the Fixture's own wires are not black and white?]
2 [p-h]- Here, however, we have two flat cables hooked up differently. Why? Well, there is no light but this one that is to be switched, and the neutral is arriving from a cable-p, whose black is constantly hot. So that cable is not coming from the switch -- not able to provide the light's switched wire. Rather cable-h does come from the switch, and its white is not neutral ("h" is for hot-white)! This is a way, more common before 1970, to switch a light when there are a neutral and a hot at the light box. It is commonly called a switch "loop". The hot makes cable-h's white be hot. This same cable-h at the switch's box may have both of its wires attached to the switch's terminals (see switch-2 or switch-12 above), so its black back at our light box is the switched wire we are looking for, to control this fixture. Could cable-p's black just as easily wirenut to cable-h's black, with cable-h's white serving as the switched wire (to hook to the fixture's black wire)? Functionally, yes, but that is against Code, because then you would have a confusion whenever the fixture was being replaced. Namely, which white wire is which for hooking up correctly to the fixture?... One rarer use of cable-h from a light box is shown in switch-18 above. There the end result for the light is the same, but instead of a single switch handling the wires, a 3-way switch system works on deciding the on- or off-ness of cable-h's black at our light.
3 [p-f]- Box-3 shows an additional non-white wire in cable-f, so we don't have to use its white to get hotness down to the switch. But why does the switch box need a neutral to be sent down to it? We don't know, but maybe that box has another switch in it, wired more like light-1's switch, that needs a neutral. Or maybe that switch box is going to pass the circuit's hot and neutral both along to enable a nearby receptacle... Although I have spoken of cable-p as the source of neutral and hot in this box, you would see the exact same connections if cable-f were the source. In that case, hot and neutral come up from the switch box in cable-f and are passed out by way of cable-p to somewhere further on the circuit.
4 [p-p-f]- Add another power cable. Which of these three cables is bringing hot and neutral to this box? It doesn't matter. The circuit's power is passing through the box in any case, and in the process delivering neutralness and a switched (red) wire to the fixture.
5 [p-L-f]- Box-5 lets us see a combination of the principles from box-3 and box-1. One two-wire cable (L) feeds neutral and switched black to the next light. The other (p) gives/gets neutral and hotness to/from the light's switch box, for the sake of other parts the circuit.