Grounding Electrode System, NEC 250-81

If available on the premises at each building or structure served, each item (a) through (d) below, and any made electrodes in accordance with Sections 250-83(c) and (d), shall be bonded together to form the grounding electrode system. 

The bonding jumper(s) shall be installed in accordance with Sections 250-92(a) and (b), shall be sized in accordance with Section 250-94, and shall be connected in the manner specified in Section 250-115. 

The unspliced grounding electrode conductor shall be permitted to run to any convenient grounding electrode available in the grounding electrode system. It shall be sized for the largest grounding electrode conductor required among all the available electrodes. 

Exception No. 1: It shall be permitted to splice the grounding electrode conductor by means of irreversible compression-type connectors listed for the purpose or by the exothermic welding process. 

Interior metal water piping located more than 5 ft (1.52 m) from the point of entrance to the building shall not be used as a part of the grounding electrode system or as a conductor to interconnect electrodes that are part of the grounding electrode system. 

Exception No. 2: In industrial and commercial buildings where conditions of maintenance and supervision ensure that only qualified persons will service the installation and the entire length of the interior metal water pipe that is being used for the conductor is exposed. 

(a) Metal Underground Water Pipe. 

A metal underground water pipe in direct contact with the earth for 10 ft (3.05 m) or more (including any metal well casing effectively bonded to the pipe) and electrically continuous (or made electrically continuous by bonding around insulating joints or sections or insulating pipe) to the points of connection of the grounding electrode conductor and the bonding conductors. Continuity of the grounding path or the bonding connection to interior piping shall not rely on water meters or filtering devices and similar equipment. A metal underground water pipe shall be supplemented by an additional electrode of a type specified in Section 250-81 or in Section 250-83. The supplemental electrode shall be permitted to be bonded to the grounding electrode conductor, the grounded service-entrance conductor, the grounded service raceway, or any grounded service enclosure. 

Where the supplemental electrode is a made electrode as in Section 250-83(c) or (d), that portion of the bonding jumper that is the sole connection to the supplemental grounding electrode shall not be required to be larger than No. 6 copper wire or No. 4 aluminum wire. 

Exception: The supplemental electrode shall be permitted to be bonded to the interior metal water piping at any convenient point as covered in Section 250-81, Exception No. 2. 

(b) Metal Frame of the Building. The metal frame of the building, where effectively grounded.

(c) Concrete-Encased Electrode (Known in the trades as UFER)

An electrode encased by at least 2 in. (50.8 mm) of concrete, located within and near the bottom of a concrete foundation or footing that is in direct contact with the earth. 

Consists of at least 20 ft (6.1 m) of one or more bare or zinc galvanized or other electrically conductive coated steel reinforcing bars or rods of not less than 1/2 in. (12.7 mm) diameter, or

Consists of at least 20 ft (6.1 m) of bare copper conductor not smaller than No. 4.  ,

(d) Ground Ring. A ground ring encircling the building or structure, in direct contact with the earth at a depth below earth surface not less than 2 1/2 ft (762 mm), consisting of at least 20 ft (6.1 m) of bare copper conductor not smaller than No. 2.