The wiring method in question is open wiring on insulators. That doesn't mean bare conductors on insulators; the conductors are insulated [398.2]. The "open" part means the wiring is exposed. It's cheap and it's ugly, but it works. This wiring method uses cleats, knobs, tubes, and flexible tubing to protect and support single insulated conductors run in or on buildings.
The NEC lists four situations in which this wiring method may be used [398.10], but the first two don't make any sense. Those are "indoors or outdoors" and "in wet or dry locations." Maybe they should also state you can install it right side up or upside down? The two other conditions are:
A. Where subject to corrosive vapors.
B. For services.
The four conditions are preceded by saying this wiring method is permitted only for industrial or agricultural establishments on systems of 600V or less.
You can't use it where concealed by the building structure [398.12].
- Article 398 does not specify the hardware you can use, but it does specify support requirements [398.30]. Note that you can use nails to mount knobs [398.30(D)] but that's really a bad practice. It's faster to drive screws (if you have an electric screwdriver) and this will give you a far more reliable installation. So wherever possible, drive screws rather than nails.
- Make sure any conductor splices and taps are made and insulated by approved methods. Just because you've gone with "fast and cheap but ugly" doesn't mean you can also go with dangerous.