Mike,

I find it truly amazing that I cannot find a definitive basis for selecting a separately derived system vs. a nonseparately derived system when using a generator. Certainly, IEEE, reference books, vendors, etc. parrot NEC Article 250, but they have no discussion concerning the reason for selecting either.

Mike Holt's Comment: For the generator to be considered a separately derived system, the supply to the transfer switch must include a grounded (neutral) conductor and the transfer switch must switch the grounded (neutral) conductor (example a 4-pole transfer switch for a 3-phase, 4-wire system). If there is no grounded (neutral) conductor run to the transfer switch or if the transfer switch does not switch the grounded (neutral) conductor, then the generator is not a separately derived system.

Separately Derived System. If the generator is a separately derived system, then the grounded (neutral) conductor/terminal of the generator MUST be bonded to the metal parts of the electrical installation in accordance with 250.30(A)(1) and the generator is REQUIRED to be grounded in accordance with 250.30(A)(2).

Nonseparately Derived System. If the generator is not a separately derived system, then the grounded (neutral) conductor/terminal of the generator MUST NOT be bonded to any metal parts of the electrical installation and the generator is NOT REQUIRED to be grounded to any electrode.

Mike, I have done a search on your site for "separately derived systems" and yes the references do an outstanding job in the discussion the grounding and bonding requirements for separately derived systems and it seems to concentrate on the dangers of an improperly grounded and/or bonded system.

By inference, the solidly grounded nonseparately derived system (no switched neutral) seems to be the logical choice and a separately derived system (switched neutral) is only required for special applications. I am curious about which applications would demand the use of a separately derived system.

Did I miss some pearl of wisdom concerning this subject in your older columns?

Thanks, Dick

Mike Holt's Comment: Dick you are correct, all of my article focus on when a system is considered separately derived by the NEC (Article 100) and how the NEC requires them to be grounded and bonded. I also focus on the dangers of electric shock to fire safety from an improperly grounding or bonding system. But I have not written any articles on when a separately derived system (generator supplying a switched neutral transfer switch) is required or preferred. But your question is going to over 20,000 electrical professionals and I'm sure we'll get lots of positive feedback.

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