How about also including a electrical theory category? Or a category that in forces the reason we do what we do. Its easy to remember 210.52(C)(5) exception. When I rough in a outlet on a island countertop without wall space, its easy to think, this outlet needs to be less than 12" from the top of the countertop. All I need to think about is the horrible example of the little toddler that reached up and pulled the toaster down on top of her head by grabing the cord end. For the electrical Theory aspect, just a few things that come to my head. Why do we size a fuse different for different cat. of motors? Why does a conductor heats up when overloaded? I guess this same principle could be used for continues loading. Conductors in ambient temps over 30C. Why the grounding conductor needs to be oversized if you oversize your hot conductors. Harmonic loads, over sizing your grounded conductor. These are just a few off the top of my head. Maybe this is the wrong forum as the people on this site should be professionals, and all this should be known. I do know however that with some practical applications, or examples, some things become extremely clear!