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structures such as piping systems and underground tanks. (c) Rod and Pipe Electrodes. Rod and pipe electrodes shall not be less than 8 ft (2.44 m) in length, shall consist of the following materials, and shall be installed in the following manner. 1. Electrodes of pipe or conduit shall not be smaller than % in. trade size and, where of iron or steel, shall have the outer surface galvanized or otherwise metal-coated for corrosion protection. 2. Electrodes of rods of iron or steel shall be at least 5/8 in. (15.87 mm) in diameter. Stainless steel rods less than 5/8 in. (l5.87 mm) in diameter, nonferrous rods, or their equivalent shall be listed and shall not be less than ~ in. (l2.7 mm) in diameter. 3. The electrode shall be installed such that at least 8 ft (2.44 m) oflength is in contact with the soil. It shall be driven to a depth of not less than 8 ft (2.44 m) except that, where rock bottom is encountered, the electrode shall be driven at an oblique angle not to exceed 45 degrees from the vertical or shall be buried in a trench that is at least 2~ ft (762 mm) deep. The upper end of the electrode shall be flush with or below ground level unless the aboveground end and the grounding electrode conductor attachment are protected against physical damage as specified in Section 250-10. i;comm~t;riJ (d) Plate Electrodes. Each plate electrode shall expose not less than 2 ft2 (0.186 m2) of surface to exterior soil. Electrodes of iron or steel plates shall be at least ~ in. (6.35 mm) in thickness. Electrodes of nonferrous metal shall be at least 0.06 in. (1.52 mm) in thickness. Plate electrodes shall be installed not less than 2~ ft (762 mm) below the surface of the earth. (e) Aluminum Electrodes. Aluminum electrodes shall not be permitted. . . 250-54. Supplementary Grounding Electrodes Supplementary grounding electrodes shall be permitted to be connected to the equipment grounding conductors specified in Section 250-118, but the earth shall not be used as the sole equipment grounding conductor. I;'E~~wJj . . 250-56. Resistance of Made Electrodes A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall 'be augmented by one additional electrode of any of the types specified in Sections 250-50 or 250-52. Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 6 ft (l.83 m) apart. FPN: The paralleling efficiency of rods longer than 8 ft (2.44 m) is improved by spacing greater than 6 ft (1.83 m). Tc~.~..'.=ia;r..l..... :.'1IiIrrl.,'n" " . . 250-58. Common Grounding Electrode Where an ac system is connected to a grounding electrode in or at a building as specified in Sections 250-24 and 250-32, the same electrode shall be used to ground conductor enclosures and equipment in or on that building. Where separate services supply a building and are required to be connected to a grounding electrode, the same grounding electrode shall be used. Two or more grounding electrodes that are effectively bonded together shall be considered as a single grounding electrode system in this sense.