Requirements for electrical installations are noted in Art. 110. Unfortunately, many of these are overlooked or misunderstood in the typical facility.
While your electrical distributor is no doubt a valuable asset, it's not true that everything they sell is "Code-compliant" under all conditions of use. It's up to you to understand the application and judge whether the equipment is suitable. Section 110.3 lists eight factors to consider.
Sometimes, it's tempting to save money and get "creative" with the components you have on hand. However, custom modifications of an electrical component will typically void its listing and labeling for any use. A certified testing lab hasn't tested it with that particular modification. This kind of "cost savings" is prohibited by 110.3(B). If you count the labor costs, it's also likely that simply purchasing the correct component costs less than doing the modification work.
You can use only the wiring methods that are included in the NEC [110.8]. The term "wiring method" includes raceways, each type of which has its own Article in Chapter 3. An example of a violation is using a pipe nipple in a run of rigid metal conduit.
Breakers must do more than just protect the wiring. The NEC requires they clear a fault without extensive damage to the equipment [110.10].