250.52(A)(2) Metal In-ground Support Structure(s).
Code Change Summary: The code language describing the metal frame of a building or structure as a grounding electrode has been removed and replaced.
Section 250.52(A) 1-8 describes all of the permitted grounding electrodes. There is one thing that all grounding electrodes have in common; they are all installed underground.
The previous code language in 250.52(A)(2) did not make sense. It described the metal frame of a building or structure that is “connected to earth”. As we all know, the metal frame of a building does not normally have direct earth contact but it is usually connected to the earth through hold down bolts connected to a concrete footing filled with rebar. Technically, the previous code language mainly described the metal frame of the building as more of a grounding electrode conductor rather than an actual grounding electrode.
The new code language is now all about “metal in-ground support structure(s)”. In order for a metal in-ground support structure to qualify as a grounding electrode, it must be in direct contact with the earth vertically for 10 feet or more, with or without concrete encasement.
The new informational note makes it clear that metal in-ground support structures include, but are not limited to, pilings, casings, and other “structural metal”.
The previous code language in this section mentioning hold down bolts has been relocated to 250.68(C)(2) which pertains to the grounding electrode conductor “connections”.
Below is a preview of Article 250. See the actual NEC® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC® edition of NFPA 70.
2014 Code Language:
250.52(A)
(2) Metal Frame of the Building or Structure. The metal frame of the building or structure that is connected to the earth by one or more of the following methods:
(1) At least one structural metal member that is in direct contact with the earth for 3.0 m (10 ft) or more, with or without concrete encasement.
(2) Hold-down bolts securing the structural steel column that are connected to a concrete-encased electrode that complies with 250.52(A)(3) and is located in the support footing or foundation. The hold-down bolts shall be connected to the concrete-encased electrode by welding, exothermic welding, the usual steel tie wires, or other approved means.
2017 Code Language:
250.52(A)
N (2) Metal In-ground Support Structure(s).
One or more metal in-ground support structure(s) in direct contact with the earth vertically for 3.0 m (10 ft) or more, with or without concrete encasement. If multiple metal in-ground support structures are present at a building or a structure, it shall be permissible to bond only one into the grounding electrode system.
Informational Note: Metal in-ground support structures include, but are not limited to, pilings, casings, and other structural metal.