What is/are the appropriate electrode(s) required to ground a 112.5kVA transformer installed as a separately derived system? The transformer is located 500 feet from the service equipment in a separate electrical room. Metallic water piping runs throughout the structure. The framework of the building is structural steel, and there's neither a ground ring nor access to rebar installed in the footings. The building electrical system is grounded in accordance with all the applicable rules of Art. 250 of the 2005 NEC.
A) A grounding electrode conductor connection to the metal water piping system in the area of the separately derived system only.
B) A grounding electrode conductor connection to the structural steel in the area of the separately derived system only.
C) Either one of the electrodes in A or B by itself is acceptable.
D) Either one of the electrodes in A or B is acceptable as the grounding electrode only if the other electrode is also connected (bonded) to the same point of the separately derived system where the grounding electrode conductor connection of the system is also made.
Answer: D)
Explanation: Sec. 250.30(A)(7) Grounding Electrode intends that the grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system. It’s clear that the nearest electrode to the separately derived system shall be used. There’s no rule that doesn’t permit the use of both electrodes (metal water piping system or structural metal), but the use of only one of the electrodes is acceptable.
Per a 2005 NEC change, 250.104(D)(1) requires that both the structural steel and the metallic water piping in the area of the separately derived system (if both are in the area of the separately derived system) be bonded to the same point on the separately derived system where the grounding electrode conductor connection to the system is made.
In-other-words, use the electrode of your choice (closest to the separately derived system), but don't forget to bond the metal grounding electrode, and the metal water pipe electrode in the area of the separately system to your separately derived system at the same point as the connection of the grounding electrode conductor.
Owen is the owner and president of National Code Seminars and the holder of master electrician certifications in 46 states. He can be reached at [email protected].
