Most of us seem to be confused here.

Electrical design is based on physical laws.

The NEC is a set of perscriptive rules that approximate the physical laws. These rules will be sufficient under most circumstances. The NEC points out some circumstances where the rules may not be sufficient.

An engineered design will be based on the physical laws and the specific circumstances. An engineered design will always be sufficient.

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With regard to the neutral problem. An engineer can make a decision that the NEC fill/derating rules are too conservative and another neutral can be pulled without derating the condutors.

For course this incurs the rath of the AHJ who thinks the NEC is to be followed. A lot of yelling follows. A few people land in jail and the hospital and then the lawyers go to work. [Linked Image]

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I should point out the most engineers prefer to use perscriptive rules that cover their area of practice better than the NEC covers their area of practice.

Engineers do a good job at their work.