9.3 Grounding Electrode Conductor Sizing and Installation

Table 250.66

The grounding electrode conductor is sized on the largest ungrounded service-entrance conductor or its equivalent area for parallel conductors:

Notice the table tops out at 3/0 copper. However large the service, the grounding electrode conductor never exceeds 3/0 copper or 250 kcmil aluminum. The GEC is not sized for fault current — it is sized for lightning and surge energy, which does not scale with service size.

Where the service-entrance conductors are paralleled, the equivalent size is determined by the sum of the circular mil areas of the parallel conductors of one phase.

The Three Sole-Connection Caps — 250.66(A), (B), (C)

These caps override the table and are among the most frequently tested items in Article 250:

250.66(A) Connections to Rod, Pipe, or Plate Electrodes. Where the grounding electrode conductor or bonding jumper connected to a single or multiple rod, pipe, or plate electrodes, as described in 250.52(A)(5) or (A)(7), does not extend on to other types of electrodes that require a larger size conductor, the grounding electrode conductor shall not be required to be larger than 6 AWG copper wire or 4 AWG aluminum wire.

250.66(B) Connections to Concrete-Encased Electrodes. Where the grounding electrode conductor or bonding jumper connected to a single or multiple concrete-encased electrodes, as described in 250.52(A)(3), does not extend on to other types of electrodes that require a larger size conductor, the grounding electrode conductor shall not be required to be larger than 4 AWG copper wire.

250.66(C) Connections to Ground Rings. Where the grounding electrode conductor or bonding jumper connected to a ground ring, as described in 250.52(A)(4), does not extend on to other types of electrodes that require a larger size conductor, the grounding electrode conductor shall not be required to be larger than the conductor used for the ground ring.

The common phrase in all three is "does not extend on to other types of electrodes." The cap applies only to the portion that terminates at that electrode type. A GEC that runs from the service to a rebar footing and then continues to a driven rod is sized by the table for the footing portion; only the final leg to the rod may drop to 6 AWG.

Worked Examples

The fourth row is worth staring at. A 2000 kcmil industrial service connected solely to driven ground rods needs only a 6 AWG grounding electrode conductor. That looks wrong to intuition, and it is a favorite exam question. It is correct because a driven rod cannot carry more current than the soil will allow regardless of how big the conductor to it is.

Installation — 250.64

250.64(A) Aluminum or Copper-Clad Aluminum Conductors. Bare, covered, or insulated aluminum or copper-clad aluminum grounding electrode conductors shall not be used where in direct contact with masonry or the earth or where subject to corrosive conditions. Where used outside, aluminum or copper-clad aluminum grounding electrode conductors shall not be terminated within 18 inches (450 mm) of the earth unless terminated within a listed wire connector system rated for direct burial or encasement in concrete.

250.64(B) Securing and Protection Against Physical Damage. Where exposed, a grounding electrode conductor or its enclosure shall be securely fastened to the surface on which it is carried.

• 4 AWG or larger copper or aluminum: shall be protected where exposed to physical damage.
• 6 AWG grounding electrode conductor that is free from exposure to physical damage: shall be permitted to be run along the surface of the building construction without metal covering or protection if securely fastened to the construction; otherwise it shall be protected in RMC, IMC, PVC, RTRC, EMT, or cable armor.
• Grounding electrode conductors smaller than 6 AWG: shall be protected in RMC, IMC, PVC, RTRC, EMT, or cable armor.

So 6 AWG is the smallest conductor that may be run exposed on a surface, and only where free from exposure to physical damage.

250.64(C) Continuous. Except as provided in 250.30(A)(5) and (A)(6), 250.30(B)(1), and 250.68(C), grounding electrode conductor(s) shall be installed without a splice or joint except by one of the following:

1. Irreversible compression-type connectors listed as grounding and bonding equipment or the exothermic welding process;
2. Sections of busbars connected together;
3. Bolted, riveted, or welded connections of structural metal frames of buildings;
4. Threaded, welded, brazed, soldered, or bolted-flange connections of metal water piping.

A wire nut, a split bolt, or a set-screw lug is not an acceptable GEC splice. This is a common inspection failure.

250.64(D) Service with Multiple Disconnecting Means Enclosures. Where a service consists of more than a single enclosure, grounding electrode connections shall be made by one of three methods:

• (D)(1) Common Grounding Electrode Conductor and Grounding Electrode Conductor Taps. A common grounding electrode conductor and grounding electrode conductor taps shall be permitted. The common grounding electrode conductor shall be sized in accordance with 250.66 based on the sum of the circular mil area of the largest ungrounded conductor of each set. Each tap shall be sized in accordance with 250.66 based on the largest ungrounded conductor serving the individual enclosure. The tap connection to the common grounding electrode conductor shall be made using a connector listed as grounding and bonding equipment, listed connections to aluminum or copper busbars not less than 1/4 in. x 2 in., or by the exothermic welding process. Grounding electrode conductor taps shall be connected to the common grounding electrode conductor in such a manner that the common grounding electrode conductor remains without a splice or joint.
• (D)(2) Individual Grounding Electrode Conductors. A grounding electrode conductor shall be connected between the grounded conductor in each service equipment disconnecting means enclosure and the grounding electrode system.
• (D)(3) Common Location. A grounding electrode conductor shall be connected in a wireway or other accessible enclosure on the supply side of the disconnecting means to the grounded service conductor.

250.64(E) Raceways and Enclosures for Grounding Electrode Conductors. Ferrous metal raceways, enclosures, and cable armor enclosing a grounding electrode conductor shall be made electrically continuous from the point of attachment to cabinets or equipment to the grounding electrode and shall be securely fastened to the ground clamp or fitting. Ferrous metal raceways and enclosures that are not physically continuous from cabinets or equipment to the grounding electrode shall be made electrically continuous by bonding each end of the raceway or enclosure to the grounding electrode conductor. Nonferrous metal raceways and enclosures are not required to be electrically continuous.

The physics: a grounding electrode conductor running through a steel conduit that is bonded at only one end forms a choke. The changing magnetic field from a lightning surge induces opposing currents in the steel, dramatically increasing the effective impedance of the path. Bonding both ends parallels the steel with the conductor and cancels the effect.

250.64(F) Installation to Electrode(s). A grounding electrode conductor shall be permitted to be run to any convenient grounding electrode available in the grounding electrode system, or to one or more grounding electrodes individually. The grounding electrode conductor shall be sized for the largest grounding electrode conductor required among all the electrodes connected to it.

Connections to Electrodes — 250.68 and 250.70

250.68(A) Accessibility. All mechanical elements used to terminate a grounding electrode conductor or bonding jumper to a grounding electrode shall be accessible. Exception No. 1: an encased or buried connection to a concrete-encased, driven, or buried grounding electrode shall not be required to be accessible. Exception No. 2: exothermic or irreversible compression connections, together with the mechanical means used to attach such fittings to fireproofed structural metal, shall not be required to be accessible.

250.68(B) Effective Grounding Path. The connection of a grounding electrode conductor or bonding jumper to a grounding electrode shall be made in a manner that will ensure an effective grounding path. Where necessary to ensure the grounding path for a metal piping system used as a grounding electrode, bonding shall be provided around insulated joints and around any equipment likely to be disconnected for repairs or replacement.

250.68(C) permits grounding electrode conductors and bonding jumpers to be connected at: interior metal water piping located not more than 5 feet from the point of entrance to the building; the metal structural frame of a building; or a concrete-encased electrode extended from its location within the concrete to an accessible location above the concrete.

250.70 Methods of Grounding and Bonding Conductor Connection to Electrodes. The grounding or bonding conductor shall be connected to the grounding electrode by exothermic welding, listed lugs, listed pressure connectors, listed clamps, or other listed means. Connections depending on solder shall not be used. Ground clamps shall be listed for the materials of the grounding electrode and the grounding electrode conductor and, where used on pipe, rod, or other buried electrodes, shall also be listed for direct soil burial or concrete encasement. Not more than one conductor shall be connected to the grounding electrode by a single clamp or fitting unless the clamp or fitting is listed for multiple conductors.