Code Change Summary: Proper methods for grounding a separately derived AC system have been revised.

For many code cycles, a separately derived system such as most transformers, was required to be connected to a grounding electrode that was as near as practicable to the grounding electrode conductor connection to the system. As well, the actual grounding electrode itself was required to be either a metal water pipe electrode or a structural metal grounding electrode unless neither of the two were available.

In the 2017 NEC^®, much of this code language in 250.30(A)(4) was deleted and simplified. Now, the building or structure grounding electrode system shall be used as the grounding electrode for the separately derived system even if the building or structure grounding electrode system is nowhere near the separately derived system.

As well, in the 2017 NEC^®, section 250.30(A)(5) now specifically mentions the methods in 250.68(C)(1) and (2) as acceptable ways to interconnect the grounded conductor from the separately derived system to the building or structure grounding electrode.

Section 250.68(C)(1) and (2) allows the first 5 feet of an emerging metal underground water pipe that qualifies as a grounding electrode, or structural building steel to be used to extend or interconnect the connection to an electrode (see image).

Below is a preview of Article 250. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.