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.

(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. (click here to see Fig. 7)

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

(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

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.

(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.

(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 58 in., unless listed.

(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.

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(!).