What code section requires the neutral to case bond at a separately derived system?

The section you are looking for is 250.30(A)(1). It requires an unspliced system bonding jumper that complies with 250.28(A) through (D) to connect the equipment grounding conductors of the separately derived system to the grounded conductor. Except as permitted elsewhere in Article 250, that connection shall be made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device.

The single point rule

The words at any single point are what matters in the field. You may make the bond in the transformer enclosure or in the first disconnect or panelboard downstream, but not in both. Bonding in two places creates parallel paths for normal neutral current on the equipment grounding conductors and the metal raceway, which is exactly the objectionable current 250.6 tells you to avoid.

Section 250.30(A)(1) also spells out where the jumper cannot go. It shall not be installed at more than one point, and where the equipment grounding conductor and the grounded conductor are connected inside the transformer enclosure, the system bonding jumper shall not be installed at the first disconnecting means as well.

Sizing the system bonding jumper

Section 250.28(D)(1) sizes the system bonding jumper in accordance with 250.102(C)(1), which points to Table 250.102(C)(1). You size it based on the total circular mil area of the largest ungrounded derived phase conductor. For derived conductors up to 2 AWG copper, the jumper is 8 AWG copper. Where the derived phase conductors exceed 1,100 kcmil copper, the jumper must be at least 12.5 percent of the area of the largest phase conductor.

Section 250.30(A)(3) covers the grounded conductor brought to the first disconnect. Where the system bonding jumper is installed at the source, a supply side bonding jumper is run with the derived conductors to the first disconnecting means. Where the bond is made at the first disconnect instead, the grounded conductor itself must be routed with the derived conductors and sized per 250.30(A)(3), not smaller than the values in Table 250.102(C)(1) based on the largest ungrounded derived conductor.

The grounding electrode connection

Section 250.30(A)(4) requires a grounding electrode conductor connecting the grounded conductor of the derived system to a grounding electrode, and 250.30(A)(5) specifies the electrode: metal water pipe within 1.52 m, or 5 feet, of entry, or the structural metal frame of the building, or another electrode from 250.52 where neither is available. The grounding electrode conductor connects at the same single point where the system bonding jumper is installed.

Section 250.30(A)(6) permits a common grounding electrode conductor for multiple separately derived systems, with tap conductors sized per 250.30(A)(6)(a) and the common conductor not smaller than 3/0 copper or 250 kcmil aluminum.

What counts as separately derived

Article 100 defines a separately derived system as an electrical source, other than a service, having no direct connection to circuit conductors of any other electrical source other than through bonding connections of equipment grounding conductors. A transformer with a wye secondary is separately derived. A generator with a switched neutral in the transfer switch is separately derived. A generator with a solidly connected neutral through the transfer switch is not, and gets no bond at the generator.

Field takeaway

Cite 250.30(A)(1). One bond, one location, sized from Table 250.102(C)(1), with the grounding electrode conductor landing at the same point. Verify whether your generator transfer switch switches the neutral before you decide whether the generator is separately derived at all.