I agree with Don about a building having only one supply source.
BUT, what would happen if a 1 to 1 isolation transformer was installed in the feeder of a second source to a building. Would this be considered separately derived?
Article
100 Service
100 Feeder
100 Separately Derived System
100 Premise Wiring (System)
230 cover utility supplied
250 covers GROUNDING.
and probably other Articles.
