How to Design Grounding Electrodesby: Matt Buecker The process of electrically connecting an electrical system to the earth is referred to as "earthing" or "grounding". NEC Arcticle 250, Section III addresses the Grounding Electrode System and Conductor. The language used in the Code leaves a lot to the imagination so the purpose of this article is to clearly explain how the electrical engineer should design grounding electrode conductors and systems. The important code sections are 250.50 and 250.52(A), which state the following: NEC 250.50 All grounding electrodes as described in 250.52(A)(1) through (A)(7) that are present at each building or structure served shall be bonded together to form the grounding electrode system. Where none of these grounding electrodes exist, one or more of the grounding electrodes specified in 250.52(A)(4) through (A)(8) shall be installed and used. Exception: Concrete-encased electrodes of existing buildings or structures shall not be required to be part of the grounding electrode system where the steel reinforcing bars or rods are not accessible for use without disturbing the concrete. NEC 250.52(A), Electrodes Permitted for Grounding: (1) Metal Underground Water Pipe  What does all this mean? Well, let's start with 250.52(A)(1)-(3). These three items are required to be installed/designed into the electrical systems of all new buildings as they are items that will likely be present during construction as the building will most likely have domestic water piping, a metal structure and a concrete footing system. The main ground rod for the electrical service is then connected to these three grounding electrodes whereby they are all bonded together as a system. The second part of 250.50 refers to situations where one or more of items 250.52(A)(1) - (3) is not present. Then, and only then, is it necessary to install one or more of items (4) - (8). Quite often, these are commonly used in renovation situations where it is not possible to access the concrete encased steel electrode, for example. Another example would be for a building that is constructed of wood frame and does not have a steel frame. And the ground ring (also called a counterpoise loop) is a popular means of grounding an electrical system for a data center.Items (4) - (8) should be viewed as grounding electrode alternatives when items (1) - (3) are not possible. The biggest takeaway is this: The electrician cannot install only one form of grounding electrode for the grounding system. There needs to be multiple (three is recommended) and they all need to be bonded together. The Code continues to improve with each new version but the grounding system seems to be one of the first electrical VE items.
To learn more about the different types of grounding electrodes, click here. For more information about changes/additions to NEC 2011, click here. |
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