Grounding and Bonding
250.52 Grounding Electrodes
The language in the 2002 NEC was vague and subject to wide interpretation as it described when the metal frame of a building or structure could serve as a grounding electrode. The new text establishes the requirement for the structural metal electrode. In addition, listed galvanized rods of the 1/2 in. diameter are now permitted.
(A) Electrodes Permitted for Grounding.
(2) Metal Frame of the Building or Structure Electrode. The metal frame of the building or structure can serve as a grounding electrode, where any of the following methods exist:
(a) 10 ft or more of a single structural metal member is in direct contact with the earth or encased in concrete that is in direct contact with the earth
(b) The structural metal is bonded to an electrode as defined in 250.52(A)(1), (3), or (4)
(c) The structural metal is bonded to two ground rods if the ground resistance of a single ground rod exceeds 25 ohms [250.52(A)(5) and 250.56]
(d) Other means approved by the authority having jurisdiction
The intent is that the metal used in the building structure be of substantial cross-sectional area (a€˜I' beams, columns, channel and angle iron). It doesn't include items such as sheet metal studs that are made of much thinner metal.
The grounding electrode conductor to the metal frame of a building or structure must be sized in accordance with Table 250.66. Figure 250-16
(5) Ground Rod Electrodes. Ground rod electrodes must not be less than 8 ft long and must have not less than 8 ft of length in contact with the soil [250.53(G)].
(a) Electrodes of pipe or conduit must not be smaller than 3/4 in. and, where of iron or steel, must have the outer surface galvanized or otherwise metal-coated for corrosion protection.
(b) Rod. Unlisted ground rod must have a diameter of at least 5/8 in., where as listed ground rods only require a diameter of 1/2 in.
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