This section has been reorganized and includes many revisions and notes to clarify the grounding and bonding requirements of separately derived systems.

(A) Grounded Systems. Separately derived systems must be grounded and bonded in accordance with (A)(1) through (A)(8).

(3) System Neutral Conductor Size. If the system bonding jumper is installed at the disconnecting means instead of at the source, the following requirements apply:

(a) Sizing for Single Raceway. Because the neutral conductor of a derived system serves as the effective ground-fault current path for ground-fault current, it must be routed with the ungrounded conductors of the derived system and be sized not smaller than specified in Table 250.66, based on the area of the ungrounded conductor of the derived system. (click here to see Fig. 6)

(b) Parallel Conductors in Two or More Raceways. If the conductors from the derived system are installed in parallel in two or more raceways, the neutral conductor of the derived system in each raceway or cable must be sized not smaller than specified in Table 250.66, based on the area of the largest ungrounded conductor of the derived system in the raceway or cable. In no case is the neutral conductor of the derived system permitted to be smaller than 1/0 AWG [310.10(H)].

(6) Grounding Electrode Conductor, Multiple Separately Derived Systems.

(a) Common Grounding Electrode Conductor. The common grounding electrode conductor can be one of the following:

(1) A conductor not smaller than 3/0 AWG copper or 250kcmil aluminum.

(2) The metal frame of the building/structure that complies with 250.52(A)(2) or is connected to the grounding electrode system by a conductor not smaller than 3/0 AWG copper or 250kcmil aluminum.

(C) Outdoor Source. If the separately derived system is located outside the building/structure, a connection to the grounding electrode must be made at the separately derived system location.

Analysis: Considering the amount of changes that have occurred in this section, it wouldn’t be entirely inaccurate to say that the whole section has been rewritten. Here are a couple of items worth noting.

Section 250.30(A)(3) mainly borrows the text that was previously in 250.30(A)(8). It does, however, add new text to provide guidance on sizing the grounded conductor for a delta (corner grounded) system. In these applications, the grounded conductor must be the same size as the ungrounded conductors.

In 250.30(A)(6), the grounding electrode conductor(s) for multiple separately derived systems has been changed to clarify that structural metal can be used to ground multiple separately derived systems, provided that the structural metal complies with 250.52(A)(2) or is connected to the grounding electrode system by a conductor not smaller than 3/0 AWG CU or 250kcmil AL.

Section 250.30(C) is new to the NEC. This subsection addresses separately derived systems that are installed outside of a building or other structure. When this is the case, a grounding electrode connection to the transformer must be provided.