The year was 2005. It was a typical spring afternoon in this typical Midwest town of Anywhere, USA. The skies were a cloudy gun-barrel gray as the light rain fell and the thunder rolled in the distant background. Suddenly, without warning, the rain intensifies and the skies turn darker and darker. That distant thunder is suddenly not so distant and is now all around with almost deafening crackling from the skies to the ground (or is it ground to sky)?
The lights inside this comfort-giving dwelling unit start to flicker. Will they stay on or go out? On again, off again. Now the lights are out, along with the rest of the electricity that we too often take for granted until times like this. BANG!
The overhead service mast, sticking straight up in the air as if to defy the surrounding atmosphere, abruptly takes a direct hit by lighting. Our grounding electrode system immediately jumps into action. This overcurrent, perhaps as much as one million volts, is looking to go to ground and will take all paths available, with the paths of least resistance sustaining the most current. Some of this unwanted current takes the path of the metal water piping system to reach its ultimate destination, while other amounts of current find a hardy route to ground in the form of that ground rod driven below the service that the homeowner silently cursed every time he ran over it with his lawnmower.
With this entire scenario taking place quicker than the blink of an eye, the largest amount of this current finds little resistance traveling along the path of the reliable concrete-encased electrode. Traveling past the service equipment, finding solace with the grounded conductor and the main bonding jumper, this current finds its way onto the grounding electrode conductor leading directly to the concrete-encased electrode.
Traveling horizontally along the footings that are now performing double-duty as a structural component and concrete-encased electrode, this menacing surge of current suddenly encounters a vertical path in the form of a pier doing the same job it’s done since the day it was poured, helping to support the foundation. Something strange and unusually unexpected happens at this point.
This lightning-generated current, having full knowledge of the rules of the National Electrical Code, knowing that it can only take horizontal paths of concrete-encased electrodes and that vertical paths are not recognized, abruptly reverses its path to avoid this vertical pier. The rules of the 2005 NEC are satisfied, and everyone lives happily ever after…yeah, right!
With this story as our backdrop, let’s take a look at some of the changes to the 2008 NEC pertaining to concrete-encased electrodes and, in particular, Section 250.52(A)(3).
Some History about Concrete-Encased Electrodes
In the past, this section of the NEC addressed only that section of a concrete-encased electrode that was located within and near the bottom of a concrete foundation or footing that was in direct contact with the earth. Since most foundations and footing are constructed in the horizontal position, this would imply that only a horizontally installed concrete-encased electrode would comply with the previous requirements of Section 250.52(A)(3).
Concrete-encased electrodes were first introduced into the NEC in the 1968 edition with the 4 AWG copper conductor concept. The steel reinforcing bar concrete-encased electrode was first introduced in the 1975 NEC. These concepts came about primarily as a result of the work of Mr. Herbert G. Ufer on twenty-four buildings located in Arizona, beginning in 1942. These buildings had no metal water piping system available, and Mr. Ufer used installations of 13 mm (½ in.) steel reinforcing rods installed in concrete footings. The resistance values were monitored bimonthly over the next eighteen years, with the results proving the value of this very effective grounding electrode.
One of the reasons that this grounding electrode, commonly referred to as a Ufer ground, is so effective is the fact that the concrete has such a large surface area in contact with the earth. In addition, the steel rods (or minimum 4 AWG copper conductor) in the concrete are in intimate contact with the concrete as well. One can trace the origins of the current language in Section 250.52(A)(3) to the findings of Mr. Ufer, based on concrete-encased grounding electrodes that were installed primarily in the horizontal position.
Why not vertical? Is there something magical about the horizontal position versus the vertical position?
What’s New for NEC-2008?
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.
Questions about NEC-2008 Requirements
We are very early in the 2008 cycle, but a few questions have arisen concerning the new requirements for concrete-encased electrodes for NEC-2008. Let’s take a look at a few of those questions.
Question: Can I use a combination of both horizontal and vertical positioned concrete-encased electrodes?
Answer: To be clear here, could someone use a combination of a continuous “L” shaped 6.0 m (20 ft) section of rebar with 1.2 m (4 ft) in the vertical position and 4.9 m (16 ft) in the horizontal position? If the Code is going to accept a horizontal position as well as a vertical position, then surely a combination of the two would satisfy this Code requirement also. Even though this would seem to make sense, the language in the Code does not seem to support such a stance. The actual Code language at Section 250.52(A)(3), in describing the first two of five mandatory conditions that must be met in order to qualify as a concrete-encased electrode, states that a concrete-encased electrode would consist of “an electrode encased by at least 50 mm (2 in.) of concrete, located horizontally near the bottom or vertically, and within that portion of a concrete foundation or footing that is in direct contact with the earth….” The actual Code language seems to indicate that the electrode must be horizontal or vertical, but not a combination of the two. Perhaps a well-worded Code proposal for NEC-2011 would be justified for this section.
Question: Would a 1.5 m (5 ft) deep vertical pier with four 1.5 m (5 ft), 13 mm (½ in.) rebars bonded together by the usual steel tie wires and a continuous spiral-shaped rebar consisting of 6.0 m (20 ft) or more in length located within that portion of the concrete pier that is in direct contact with the earth meet the new vertical concrete-encased electrode requirements?
Answer: In reviewing the original work preformed and documented by Mr. Ufer, it seemed to rely heavily on the fact that the 6.0 m (20 ft) continuous length of concrete in contact with the earth was the key to the effectiveness of a concrete-encased electrode. In answering the question concerning the 1.5 m (5 ft) deep vertical pier, the question could be asked, “Well, what were you accepting under NEC-2005 requirements?” Nothing has changed from the 2005 to the 2008 NEC concerning the 6.0 m (20 ft) of one or more of steel reinforcing bars or rods. In other words, if an AHJ accepted a 1.5 m (5 ft) horizontal footing with the same characteristics as the vertical pier described in the question, then the 1.5 m (5 ft) deep vertical pier would be acceptable. Most AHJs would not accept the 1.5 m (5 ft) horizontal footing and, therefore, should not accept the 1.5 m (5 ft) deep vertical pier. It would seem that the new vertical requirements would call for a minimum 6.0 m (20 ft) deep vertical pier in order to qualify as a concrete-encased electrode. This seemed to be the overwhelming opinion among Code-Making Panel 5 members for NEC-2008 in an informal poll of same.
Summary
The minimum accepted criteria for concrete-encased electrode have been expanded for NEC-2008. Concrete-encased electrodes can now be installed horizontally near the bottom or vertically and within that portion of a concrete foundation or footing that is in direct contact with the earth, as long as it meets all the requirements of Section 250.52(A)(3).
In addition to this requirement, if more than one qualifying concrete-encased electrode is present on the jobsite, only one of these qualifying electrodes is required to be used in the grounding electrode system for the building of structure involved. A better understanding of this rule for concrete-encased electrodes by both the installer and the enforcement community will only enhance the safety of the environment of all who come in contact with the structures and buildings involved.
Now when the storms come rolling in, we can worry about the leaky roof and less about the path, whether horizontal or vertical, that the overcurrents will take in getting to the earth.
L. Keith Lofland is education, codes and standards coordinator and serves as seminar specialist for IAEI. Prior to his position with IAEI International, Keith spent sixteen years with the city of Garland, Texas, serving as their chief electrical inspector. He served as chair of the Texas Chapter in 1989. He served as secretary/treasurer for the Texas Chapter for ten years. Keith has taught seminars for IAEI since 2000. |
