An adopted code is a law; the last I looked, the only folks who get to set aside laws are judges.
Sure, there is often room for judgment in the application of a law, and often that's where additional documentation can be handy.
The NEC has numerous places where a provision mentions "engineering supervision." In places other than where such is specifically allowed, I read the code to say "I don't care if you're Thomas Edison - this is the rule!"
As for being part of the "building code," we have to be careful of our language. While there is usually a general 'building code,' such as the IBC, all of the locally adopted codes, as a group, can be said to comprise the "building code."
Some have chided me in the past for having a poor attitude towards engineers. Any cynicism I may have has been influenced by encounters with engineers who felt they had carte blanche to ignore all convention, and do whatever they wished. This is America, however, where the same laws apply to all ... even those with PE tickets.
It is also why I am so strident in asserting that it is the electrician, not the engineer, who is the expert in code matters. An electrician has years of formal training in electrical matters; even an "electrical engineer" has absolutely none. (Check the curriculum at the engineering school of your choice- don't just take my word for it!) Engineers, even EE's, are valuable applied trades, but electrical work is not their trade.
I will concede that the NEC is conservatively written; you can very often 'get away' with pushing things. It also ... pay attention here ... is not a design manual, an instruction manual, or interested in efficiency.
As for de-rating, I admit that applying the tables does not constitute a complete 'engineering analysis' of every possible situation. The tables, for example, do not make any distinction between a lightly loaded convenience circuit, and a fully loaded dedicated circuit. As I said earlier, the NEC is a conservative document.
The irony is that these code rules, as well as many unwritten trade practices, are even today proving their worth in ways that could never have been anticipated when they were written. We are only beginning to understand power quality, harmonics, and transients. Sometimes, the 'little things' make all the difference!
