2011 Changes to the NEC – 250.52

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• Each Code section which contains a change includes a summary of the change, followed by a paraphrase of the NEC text affected by the change. Any specific change is denoted by underlined text and in the corresponding chapter color
• Graphics with green borders and 2011 CC icons next to the heading are graphics that contain a 2011 change; graphics without a green border or icon are graphics that support the concept being discussed, but nothing in the graphic was affected by a 2011 Code change.

250.52(A) Electrodes Permitted for Grounding

The rule explaining when a structural metal frame can serve as a grounding electrode has been changed…again, and the requirements for concrete encased electrodes, ground rods, and ground plates have been clarified.

250.52 Grounding Electrode Types.

(A) Electrodes Permitted for Grounding.
(1) Underground Metal Water Pipe Electrode. Underground metal water pipe in direct contact with the earth for 10 ft or more can serve as a grounding electrode. Figure 250–44

Figure 250-44 (Click on image to enlarge)

Author’s Comment: Controversy about using metal underground water supply piping as a grounding electrode has existed since the early 1900s. The water industry believes that neutral current flowing on water piping corrodes the metal. For more information, contact the American Water Works Association about their report Effects of Electrical Grounding on Pipe Integrity and Shock Hazard, Catalog No. 90702, 1.800.926.7337. Figure 250–45

Figure 250-45 (Click on image to enlarge)

(2) Metal Frame Electrode. The metal frame of a building/structure can serve as a grounding electrode when it meets at least one of the following conditions:
(1) At least one structural metal member is in direct contact with the earth for 10 ft or more, with or without concrete encasement.

(2) The bolts securing the structural steel column are connected to a concrete encased electrode [250.52(A)(3)] by welding, exothermic welding, steel tie wires, or other approved means. Figure 250–46

Figure 250-46 (Click on image to enlarge)

(3) Concrete-Encased Electrode. At least 20 ft of either (1) or (2): Figure 250–47

Figure 250-47 (Click on image to enlarge)

(1) One or more of bare, zinc-galvanized, or otherwise electrically conductive steel reinforcing bars of not less than ½ in. diameter, mechanically connected together by steel tie wires, welding, or other effective means, to create a 20 ft or greater length.

(2) Bare copper conductor not smaller than 4 AWG.

The reinforcing bars or bare copper conductor must be encased by at least 2 in. of concrete located horizontally near the bottom of a concrete footing or vertically within a concrete foundation that’s in direct contact with the earth.

If multiple concrete-encased electrodes are present at a building/structure, only one is required to serve as a grounding electrode. Figure 250–48

Figure 250-48 (Click on image to enlarge)

Note: Concrete containing insulation, vapor barriers, films or similar items separating it from the earth isn’t considered to be in “direct contact” with the earth.

Author’s Comments:
•  The grounding electrode conductor to a concrete-encased grounding electrode isn’t required to be larger than 4 AWG copper [250.66(B)].
•  The concrete-encased grounding electrode is also called a “Ufer Ground,” named after a consultant working for the U.S. Army during World War II. The technique Mr. Ufer came up with was necessary because the site needing grounding had no underground water table and little rainfall. The desert site was a series of bomb storage vaults in the area of Flagstaff, Arizona. This type of grounding electrode generally offers the lowest ground resistance for the cost.

(4) Ground Ring Electrode. A ground ring consisting of at least 20 ft of bare copper conductor not smaller than 2 AWG buried in the earth encircling a building/structure, can serve as a grounding electrode. Figure 250–49

Figure 250-49 (Click on image to enlarge)

Author’s Comment: The ground ring must be buried not less than 30 in. [250.53(F)], and the grounding electrode conductor to a ground ring isn’t required to be larger than the ground ring conductor size [250.66(C)].

(5) Ground Rod and Pipe Electrode. Ground rod electrodes must not be less than 8 ft in length in contact with the earth [250.53(G)].

(b) Rod-type electrodes must have a diameter of at least 5⁄8 in., unless listed. Figure 250–50

Figure 250-50 (Click on image to enlarge)

Author’s Comments:
•  The grounding electrode conductor, if it’s the sole connection to the ground rod, isn’t required to be larger than 6 AWG copper [250.66(A)].
•  The diameter of a ground rod has an insignificant effect on the contact resistance of a ground rod to the earth. However, larger diameter ground rods (¾ in. and 1 in.) are sometimes installed where mechanical strength is desired, or to compensate for the loss of the electrode’s metal due to corrosion.

(6) Listed Electrode. Other listed grounding electrodes.

(7) Ground Plate Electrode. A bare or conductively coated iron or steel plate with not less than ¼ in. of thickness, or a solid uncoated copper metal plate not less than 0.06 in. of thickness, with an exposed surface area of not less than 2 sq ft.

(8) Metal Underground Systems Electrode. Metal underground piping systems, underground tanks, and underground metal well casings can serve as a grounding electrode.

Author’s Comment: The grounding electrode conductor to the metal underground system must be sized according to Table 250.66.

ANALYSIS: Over the last few Code cycles, the NEC has tried to make clear when the structural metal of a building or structure can be used as a grounding electrode. The first prescribed method will find the structural metal with direct earth contact for 10 ft or more. As an alternative, the hold-down bolts securing the structural metal column can be connected to a concrete-encased electrode. Previously, the Code allowed the structural metal to serve as an electrode if it was connected to a ground rod meeting the 25 ohm requirement of (formerly) 250.56. This option has now been removed, and is no longer a suitable method of bonding the structural metal to qualify it as a grounding electrode. The ways of creating a concrete-encased electrode have been changed into an easy-to-use list format, and a clarification has been made regarding the use of vapor barriers. When a vapor barrier (typically a plastic sheet) is installed beneath the footing, NEC users have debated whether or not the concrete is still considered to be in direct contact with earth. A new Informational Note was added to clarify that such a footing isn’t considered to be in direct contact with the earth, therefore the rebar or bare copper conductor can’t be used as a grounding electrode. Section 250.52(A)(5) has been changed to eliminate the minimum size for listed electrodes. Previous editions of the Code have stated that listed ground rods must be at least ½ in. in diameter. Because the NEC is typically not the place to find listing requirements, this text has been removed, which might open the door to smaller ground rods being listed. Lastly, a change was made to 250.52(A)(7) which clarifies that plate electrodes must be conductive(!).

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