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Friday, November 5, 2021
Question:
Thank you for CQD. I have been following it for years. Here is a situation I have…
Existing conditions; All concrete building (i.e., slab, walls, columns, beams, and roof deck are all concrete). It appears that the grounding electrodes used are the underground metal water pipe, a concrete encased electrode, and a ground rod triad and that all bonding is at the main switchboard.
Task; Add a step-down transformer and 208Y120-volt panelboard in the middle of the plant. Quandary: Grounding the separately derived system. A building grounding electrode system is not present (everything is concrete and we're 100's of feet from an exterior wall).
Question: If sized per table 250.102(C)(1), can the equipment grounding conductor pulled with the feeder to the transformer primary simultaneously serve as the grounding electrode conductor for the transformer secondary? Use Exhibit 250.15 in the NFPA 70-2020 Handbook as a pictorial reference. The building grounding electrode system is not present. The EGC serves as the EGC and GEC. SSBJ's as shown, sized per table 250.102(C)(1).
Thanks in advance for your time, Paul Briggs
A
Answer:
Hi Paul,
Thanks for the question. The exception to Section 250.121 provides for this alternative mentioned in your question, but specific conditions must be met. Section 250.121 and the exception indicate the following: 250.121 Restricted Use of Equipment Grounding Conductors. An equipment grounding conductor shall not be used as a grounding electrode conductor.
Exception: A wire-type equipment grounding conductor installed in compliance with 250.6(A) and the applicable requirements for both the equipment grounding conductor and the grounding electrode conductor in Parts II, III, and VI of this article shall be permitted to serve as both an equipment grounding conductor and a grounding electrode conductor.