What’s New for NEC-2008?
Concrete Encased Electrodes.
By Keith Lofland

A proposal was submitted for NEC-2008 to recognize a vertically run concrete-encased electrode as well as the currently accepted horizontal concrete-encased electrode. This proposal, ROP 5-152, was originally rejected but was ultimately accepted in the comment stage by comment 5-86. There seems to be no technical justification to require that concrete-encased electrodes (rebars) be located horizontally and not to accept the vertical position within that portion of a concrete foundation footing or pier.

One of the reasons that a concrete-encased electrode is so effective is due to the fact that the concrete has a much larger surface area in contact with the earth than other electrodes. In addition, the steel rods or copper conductors in the concrete are in direct contact with the concrete. These two facts exist whether the rebars are at the bottom of the foundations or vertical in a column or pier. Concrete gives out moisture slowly wherever it is in contact with the earth, not just at the bottom of the foundation. Concrete absorbs moisture quickly and loses moisture very slowly. The mineral property of concrete and its inherent pH means concrete has a supply of ions to conduct current. The moisture present, in combination with the surrounding soil, makes for a good conductor for electrical energy or lightning currents.

Another element involving concrete-encased electrodes that received some clarification for NEC-2008 was multiple concrete-encased electrodes present at a building or structure. At several large commercial (and some residential) buildings, it is very possible to have several sections and perhaps multi levels of a foundation with isolated rebar sections that individually would meet the definition of a concrete-encased electrode, but with none of these separate sections being tied together by any intentional conductive path. The question arose in the past concerning these multiple concrete-encased electrodes pertaining to whether or not one or all of these multiple qualifying electrodes are required to be connected and used in the grounding electrode system. It was made clear by the addition of the last sentence of Section 250.52(A)(3) in NEC-2008 that only one of these qualifying sections of a concrete-encased electrode be required to be connected and used in the grounding electrode system. The conclusion can be reached that subsidiary conductivity is provided between these separate sections of steel reinforcing bars by the eventual common concrete encasement.