Seperately-derived Systems

 
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Old 11-11-2007, 09:53 AM   #1
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Seperately-derived Systems


Second question:
The term "seperately derived system" is defined in Art. 100 of the 2008 NEC on page 70-29. I thought about it awhile and decide that it refers to something like wind power, photovoltaic power, on-site steam co-generation--i.e., power derived from other than a service.

Systems comes up again in 200.6 (D), Grounded Conductors of Different Systems. Thought about that one for awhile, too, and decided that the "different systems" refer to feeders and/or branches coming from distinct services or distinct main panels fed by the same service drop or lateral.

Now, I'm at 250.20 (D), specifically the first sentence: "Separately derived systems, as covered in 250.20 (A) or (B)...". This kinda' threw me for a loop, as (A) and (B) are all about alternating-current systems powered by transformers, such as low-voltage AC supplied from a transformer whose primary is powered from line voltage [250.20 (A)], or a household-type system where the hot-to-ground doesn't exceed 150 volts [250.20 (B)(1)], or 3-phase, 4-wire, wye- and delta-connected systems [250.20 (B)(2) and (3)].

So, what gives??? What makes a 3-phase, 4-wire, wye-connected system a separately-derived system? To me, that is referring to an industrial application, where a transformer powers a motor, but it doesn't seem like a separately-derived system to me, because the transformer is still powered off a main panel with a service on its line side.

Can anyone offer some clarification?

P.S. I've got some Delmar books on my shopping list--just waiting for the next paycheck. :-)
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Old 11-11-2007, 09:57 AM   #2
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Re: Seperately-derived Systems


A transformer is absolutely a separately derived system. There is no direct electrical connection between primary and secondary. (Auto transformers excepted)

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Old 11-11-2007, 10:01 AM   #3
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Re: Seperately-derived Systems


Thanks. That totally makes sense. It also explains why the question of where the secondary gets its neutral/ground is covered in the code. I have a lot to learn.
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Old 11-11-2007, 10:18 AM   #4
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Re: Seperately-derived Systems


The secondary of a transformer is just like a service, for the most part - GEC, bond EGC to X0 (neutral), etc.
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