# What is the intent of NEC 300.3



## bwenholz (Dec 28, 2009)

What is the difference between "different systems" as in 300.3 and "separately derived systems" elsewhere in the code?


----------



## wildleg (Apr 12, 2009)

the different systems applies to any different system - different voltage, ac, dc, class 2 or class 3, power limited, non power limited, etc.

separately derived system might be an example of a different system, but if you investigate exactly what its intent is relative to the code, it specifically addresses power sources and how they are to be connected (or not) to other power sources, especially as regards the grounding and bonding of the sources. (look up the definition of separately derived system)


----------



## Dennis Alwon (May 9, 2009)

Here is the NEC def. for an SDS. If you use a transfer switch that disconnects the service neutral from the generator then that generator is an SDS



> 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.


----------



## voltz (Jun 2, 2010)

Dennis Alwon said:


> Here is the NEC def. for an SDS. If you use a transfer switch that disconnects the service neutral from the generator then that generator is an SDS



would the secondary of a transformer be considered a sds (480/277 to 208/110)


----------



## alfein (Jun 1, 2010)

That would definitely be considered a separately derived system according to the code.


----------



## Dennis Alwon (May 9, 2009)

alfein said:


> That would definitely be considered a separately derived system according to the code.


I agree. Here is an excerpt from an article found *here*



> *250.30 Grounding and Bonding of Separately Derived AC Systems*
> 
> *A separately derived system* *is a premises wiring system with no direct electrical connection to conductors originating from another system* *[Art. 100 definition and 250.20(D)].* *All transformers, except autotransformers, are separately derived because the primary circuit conductors do not have any direct electrical connection to the secondary circuit conductors. Generators that supply a transfer switch that opens the grounded neutral conductor would be considered separately derived [250.20(D) FPN 1].*


----------

