It was my understanding that generators and UPSs are are always
a seperately derived source, regardless of how the neutral is handled.
Quoth NEC 2005:
Separately Derived System. A premises wiring system whose power is derived from a source of electric energy or equipment other than a service. Such systems have no direct electrical connection, including a solidly connected grounded circuit conductor, to supply conductors originating in another system.
250.30(A)(1) Exception No.1: For separately derived systems that are dual fed (double ended) in a common enclosure or grouped together in separate enclosures and employing a secondary tie, a single system bonding jumper connection to the tie point of the grounded circuit conductors from each power source shall be permitted
[This message has been edited by SteveFehr (edited 01-30-2007).]