The NEC doesn’t allow you to use equipment or conductors unless they are “approved” [110.2]. Article 100 defines “approved” as “Acceptable to the Authority Having Jurisdiction.”
What does this really mean? How does it affect your work in a plant?
First of all, it doesn’t mean you need to send the AHJ a bill of materials and/or samples for inspection to get approval to use those materials. The AHJ generally doesn’t do the actual approving.
The AHJ for an industrial plant may be the city engineer or even the fire chief. Their concern is that you are following industry standards, including the NEC. If you buy a spool of THHN, there’s no question of approval because that conductor is listed in the NEC ampacity tables. The approval, however, is contingent upon using THHN per its intended use; For example, THHN isn’t approved for use in wet locations (but THWN is).
If you want to use EMT, Art. 360 is a big clue that EMT is approved. But suppose your boss tells you to use pipe elbows in that run? How can you prove they are unacceptable to the AHJ? Water pipe isn’t listed or labeled for electrical work. Section 110.3 provides more criteria.
