250.20 Systems Required to be Grounded and Bonded Continued
(D) Separately Derived Systems. Separately derived systems that are required to be grounded (bonded) by 250.20(A) or (B), must be grounded and bonded in accordance with 250.30(A).
A separately derived system is a premises wiring system with no direct electrical connection to conductors originating from another system [Article 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.(E) Impedance Grounded Neutral Systems. Impedance grounded (bonded) neutral systems must be grounded and bonded in accordance with 250.36.
Generators that supply a transfer switch that opens the grounded neutral conductor are also considered separately derived. Figure 250.47
FPN 1: A generator isn't a separately derived system if the grounded neutral conductor from the generator is solidly connected to the electrical supply source.
In other words, if the transfer switch doesn't open the grounded neutral conductor, then the generator isn't a separately derived system. Therefore, a neutral-to-case bond (system bonding jumper) cannot be made at the generator because it will cause objectionable current to flow on metal parts in violation of 250.6(A). Figure 250.48
FPN 2: If a grounded neutral conductor is supplied at a transfer switch, and the transfer switch doesn't open the grounded neutral conductor, then the grounded neutral conductor must be sized:
- To carry fault current back to the generator in accordance with 445.13. Figure 250.49
- Not smaller than required to carry the unbalanced load in accordance with 220.61.
NEC refers to the practice of deliberately placing resistance between the system winding and the metal case as an impedance grounded neutral system. Figure 250.50
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