110.2 Approval.
The conductors and equipment required or permitted by this Code shall be acceptable only if approved.
110.3 Examination, Identification, Installation, and Use of Equipment.
(A) Examination. In judging equipment, considerations such as the following shall be evaluated:
(1) Suitability for installation and use in conformity with the provisions of this Code
FPN:Suitability of equipment use may be identified by a description marked on or provided with a product to identify the suitability of the product for a specific purpose, environment, or application. Suitability of equipment may be evidenced by listing or labeling.
(2) Mechanical strength and durability, including, for parts designed to enclose and protect other equipment, the adequacy of the protection thus provided
If the AHJ "approved" it you could use cardboard boxes and paper towel tubes for raceways. The reality is most AHJs cover their ass by requiring listings from a NRTL.
In some cases they will allow site built equipment if it is not readily available from a regular source. They may still want a NRTL test or them may be happy letting their inspectors approve it.
In my state job I did get the power of the pencil to approve a few things. I was pretty dilligent to see that it was equal to or better than a similar listed product.
