), but I will say that I do not understand the installation. It is not clear to me what is connected to what, and how, and whether any or all of the switch legs can be carrying current at the same time. I will say that if, by virtue of the manner of connecting wires to switches and to loads, it is not physically possible for (let us say) more than three of the ten wires to be carrying current at the same moment, then IMHO the total count of "current carrying conductors" is three, not ten. In order to make that declaration for any specific circuit, I would have to see the wiring diagram. If I can discern that (let us say) changing the position of one switch will allow current to flow in a new wire, while at the same time stopping current flow in another wire, then I won't count both wires as current-carrying.
I won't disagree with the others (not yet, anyway
So what I am now envisioning is nine switches, each controlling a separate light, such that if all nine switches are ON, then all lights are ON, and all eleven conductors are carrying current. In that case, I do agree that (1) derating is required, (2) derating isn't necessary from a physics perspective, and (3) item 2 is irrelevant.
I will add that this would be a difficult code revision to make, in that the wording of an exception would be tricky. You would need to make it possible to avoid derating, but make it clear when you can and cannot apply the exception. What makes derating for this particular example "unnecessary from a physics perspective" is that the total current on all conductors is less than the rated ampacity of any one of them. How can a new exception be worded such that it is both clear and enforceable?
