Article 100 Definitions. Separately Derived System.
Code Change Summary: The existing definition of a separately derived system has been simplified.
The previous definition mentioned the “premises wiring system” which really pulls the focus away from the actual separately derived system itself. The definition and associated requirements were really about what the separately derived system supplies rather than the actual source itself.
Most transformers and some generators are separately derived systems. A transformer that is a separately derived system is supplied only by ungrounded conductors and creates a grounded neutral conductor on the secondary side. The ungrounded phase conductors on the primary and secondary sides of the transformer have no direct physical connection. Ungrounded secondary conductors receive voltage through induction.
For generators, the type of transfer switch used, and if a system bonding jumper is present at the generator, dictates whether or not a generator is wired as a separately derived system. A generator is a separately derived system if using a transfer switch that also switches the grounded neutral conductor. If the grounded neutral conductor is unswitched and solidly connected to the main service grounded neutral conductor inside the transfer switch, then the generator is NOT considered a separately derived system.
Below is a preview of the NEC®. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).
2011 Code Language:
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 connection from circuit conductors of one system to circuit conductors of another system, other than connections through the earth, metal enclosures, metallic raceways, or equipment grounding conductors.
2014 Code Language:
Separately Derived System. An electrical source, other than a service, having no direct connection(s) to circuit conductors of any other electrical source other than those established by grounding and bonding connections.