Figure 5 Commentary New window.
11 [L-m]- Don't worry, I am expecting no double circuits at light boxes. I don't remember ever seeing that, but I suppose it is possible. No, box-11 shows cable-m switching multiple items. Well, two really -- our light and whatever cable-L is going to. It could be another light. It might be a separate fan. Cable-m's red is a switched wire from one switch and its black is switched from a second switch in the same switch box. See switches 4 and 5 in FIG.1 above. If this box-11 were for a ceiling "paddle" fan assembly that included a light, and if those two were to be switched separately, then this diagram would generally apply. But instead of there being any cable-L at all, the black of cable-m would be connecting to a black of the assembly. There might be two whites from the assembly or just one; either way, they would wirenut to cable-m's white. Similar attachments would be made at a combination exhaust-fan and light unit that was to have those functions switched separately; this would be common in bathrooms.
12 [L-p-m]- Box-12 is somewhat similar to box-11, but instead of a switch box sending the neutral to this light box, we have it provided here by cable-p. The white of cable-m is now made to be a hot, taken from cable-p and attached in some fashion in the switch box to at least the two switches that control fixture-12 and cable-L. See the switch end of things at switches 14 and 15 in FIG.2 above.
13 [t-n]- Not even an electrician recognizes the connections in this box immediately. Knowing that the light is part of a 3-way switching system helps. Two of the wires of cable-t are travelers "tying through" our box to the two travelers in cable-n. Cable-t is coming from a 3-way (and any 4-way) switch that is on the "hot end" of the system; it is also supplying the neutral path for our light. Cable-n is returning the on-off verdict from a 3-way (and any 4-way) switch on the "leg end" of the system. See switches 16 and 8 in Figures 2 and 1 above and this 3-way diagram. Cable-t must use its red and black wires as its travelers because the white wire is set as neutral. Cable-n could not have used its white as the switched leg, but theoretically it could have used its black as one of the travelers, with the red switching the fixture... How would an additional light be fed out from this box and be controlled along with this fixture-13? In other words, how would you hook up wires in a light box with t-n-L cables?
14 [p-n-n]- Once again we are dealing with a light controlled by a 3-way switch system. But here the neutral, and even a hot for the 3-way system, are provided by the new cable-p. So the white wires of both n-cables are not going to be neutrals. The only color restriction on these n-wires is that the returning leg that will attach from the right-hand cable-n must not be white. So cable-p's black could have been wirenutted to any one of the wires of left-hand cable-n, with the other two being travelers. Actually, we could have hooked cable-p's black to a wire of the right-hand cable-n, making it the hot-end cable, as long as we then treated the left-hand cable-n as the leg-end, hooking its non-white non-traveler to the fixture's black. The 3-way system would work either way. How would you deal with connections at this box if an additional light to be switched with this one were now fed by a 2-wire cable? That would make the box's cables p-L-n-n.
15 [p-p-n-n]- With box-15 we do add a 2-wire cable to the previous example, but it is not for adding another light. It is to pass the neutral and hot of the circuit on to some other needy box.