[280 and 285]. The NEC differentiates them by
voltage level. In actual practice and application, they differ in
several ways. For example, the kinds of devices used, the location
of the devices, and the actual power ratings of the devices. You can
generally think of Article 280 as "outside" and 285 as "inside,"
because of the typical voltage levels in those locations.
[285.3]. In three places: On circuits exceeding
1KV, on systems with certain grounding arrangements unless
specifically identified for those systems, and where the SPD rating
is less than the maximum continuous voltage.
[285.26]. Line and grounding conductors must be
at least 14AWG copper or 12 AWG aluminum.
[285.27]. Yes, but doing so doesn't make sense.
You can connect the SPD across any two conductors. The idea is to
conduct the overvoltage from the current-carrying conductor to the
neutral or ground--not to another current-carrying conductor.
[285.28]. Part III.