In section 250.52(A)(2) of the 2005 and the 2008 editions of the National Electrical Code (NEC), the metal frame of a building or structure is considered to be a grounding electrode if the metal frame is connected to earth by one of four different methods. Upon analyzing the four different methods, many electricians’ question whether the four methods provide an appropriate connection to earth. Careful study of the NEC text is necessary to answer that question.
Establishing the reason for the earth connection is part of determining if the metal frame of the building or structure is appropriately connected to earth is in the first place. Section 250.4(A)(1) states a grounded electrical system must be connected to earth to limit the electrical system voltage imposed by lightning and provide a path to earth to dissipate the lightning. The voltage at the premises can be affected by line surges caused by large electrical equipment (located either in the building’s internal or external spaces) being turned on or off or by other causes of line voltage disruption. Providing proper reference to ground can help stabilize the voltage. Unintentional contact of the premise source of power with higher voltage lines may cause line voltage disruption or surges that can have detrimental effects on the electrical system.
The electrical system must also be connected to earth to stabilize the voltage during normal operation of the electrical system. With the proper connection to ground and the electrical system stabilized, any abnormally high line voltage on the electrical system will permit surge protection and overcurrent protective devices to guard the system.
Part III of Article 250 provides the grounding-electrode requirements with Section 250.50 stating that all grounding electrodes that are available as stated in the 2005 NEC or that are present as stated in the 2008 NEC must be bonded together to form the grounding-electrode system. Based on 250.52, metal water pipe that is in direct contact with the earth for 10 feet or more, a concrete-encased electrode (“Ufer” ground), ground ring, rod or pipe electrodes, plate electrodes, other listed electrodes, other metal underground systems or structures, and the metal frame of the building or structure are all considered to be electrodes used to provide the reference to ground for the grounded electrical system and must be connected together to form the grounding-electrode conductor. Each electrode, by itself, qualifies as a path to ground for the premises electrical system to establish as close to a zero reference to ground as possible with the exception of certain aspects of the metal frame of the building or structure.
Section 250.52(A)(2) states that the metal frame of a building or structure can be connected to earth by one of four methods. The first method is by ensuring that 10 feet or more of a single structural metal member of the building or structure is in direct contact with the earth or the metal member is encased in concrete in direct contact with the earth. The second method of connecting the metal frame of the building or structure to earth is to connect the structural metal frame of the building or structure to the reinforcing bars of a concrete-encased electrode as provided in 250.52(A)(3) or to a ground ring in accordance with 250.52(A)(4). The third method is to bond the structural metal frame of the building or structure to one or more of the rod, pipe or plate electrodes that comply with the requirements in 250.56. The fourth method permitted is to connect the metal frame to other approved methods of establishing a connection to earth.
The second and third methods, using the ground ring or rod, pipe or plate electrodes, become appropriately questionable connections to earth. For example, the grounding-electrode conductor size for a 400-ampere service with 500 kcmil phase conductors would normally be a 1/0 copper conductor based on Table 250.66 for building steel that is not otherwise connected to earth. The minimum size for a rod, pipe or plate electrode is a 6 AWG copper conductor or the ground ring minimum size is a 2 AWG bare copper conductor based on Section 250.66(A) and (C) respectively. Basically, this installation would need to have a 1/0 copper connected to the metal (steel) of the building, and then the building steel, based on 250.52(A)(2)(2) or 250.52(A)(2)(3), would be connected to earth to a 2 AWG to the ground ring or a 6 AWG to the ground rod, pipe or plate. In effect, either a 2 AWG conductor or a 6 AWG would connect the building steel to earth. The requirements in 250.52(A)(2) must be changed for the 2011 NEC.
ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 and email@example.com.