Figure 4 Commentary (Compared to Fig.3 Above) New window.
6 [p-h-p]- These connections are functioning as they do in box-2, but with the addition of a another cable-p, which can be a recipient or the source (this time) of circuit power. The red tape or ink on cable-h's white wire is to remind anyone installing this wiring that Code now requires this sort of re-identification of all white wires that are not neutrals. I show most of them as still white because that is what most people visiting my website will be finding in their homes. In fact, if you have a newer home (this century), realize that the underlying color of some of your red- or black-looking wires may be white.
7 [p-h-L]- Like box-2, this one has a switch loop going down to the switch, but the added cable now is for a second light to be switched with light-7.
8 [p-h-p-L]- Here we combine the ideas in boxes 6 and 7 by adding a fourth cable. So what is going on here is the circuit's hot shared among the p-cables and the white of cable-h; the circuit's neutral being shared among p-cables, the fixture, and outgoing cable-L; and the switched black from cable-h being shared to the fixture and cable-L. In other words switch loop "h" returns switchedness to control not only this light-8 but another light fed out on cable-L. You might test your understanding of the connections we have been dealing with by diagramming for yourself two more similar sets of light box connections: p-p-p-h and p-L-L-h.
9 [p-p-L-f]- Three 2-wire cables with one 3-wire cable in a light box may be functioning as I am showing them here. This is actually the same as box-8, except that cable-f's added wire lets a neutral be brought from or given to our light's switch box, for other purposes. But other functions for three flat and one round cable are possible. If you have grasped the principles shown in boxes 3, 4, 5, and 9, you ought to be able to diagram these others: p-p-p-f and p-L-L-f.
10 [f-f]- Box 13 will be giving the only scenario other than this one, for the connection of only two 3-wire cables at a light box. Here at box-10, the circuit's hot and neutral are being passed through from/to a second light box whose fixture will be switched along with ours via the red wire. At that box there will probably be one or two 2-wire p-cables passing power between that box and more things of the circuit. The arrangement of box-10 is not common... What would happen if someone goofed and connected the fixture's black to the other blacks?