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9.3 Grounding Electrode Conductor Sizing and Installation

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Alaska Journeyman Electrician Study Guide Exam

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1 Chapter 1: Alaska Certificates of Fitness, Law & Exam Logistics

1.1 The Alaska Certificate of Fitness System 1.2 Journeyman Eligibility: 8,000 Hours and How Alaska Counts Them 1.3 Trainee, Residential Wireman, Lineman, Provisional & Reciprocal Certificates 1.4 The Journeyman Examination: Format, Scheduling, Open-Book Rules & Retakes 1.5 Adopted Codes, Fees, Renewal, Inspection & Enforcement

2 Chapter 2: Using the 2020 NEC

2.1 Article 90: Purpose, Scope, Arrangement & Enforcement 2.2 Navigating the Code Book: Structure, Index & Tabbing Strategy 2.3 Article 100 Definitions You Must Know Cold 2.4 Electrical Theory for the Exam: Ohm's Law, Power & Three-Phase

3 Chapter 3: General Requirements — Article 110

3.1 Approval, Listing, Interrupting Rating & Terminations 3.2 Temperature Limitations of Terminations — 110.14(C) 3.3 Working Space, Access, Egress & Illumination — 110.26(A) through (D) 3.4 Dedicated Equipment Space, Guarding & Over-1000-Volt Installations

4 Chapter 4: Conductors and Ampacity — Article 310

4.1 Conductor Types, Insulation Markings & Table 310.16 4.2 Ambient Temperature Correction and Alaska's Cold 4.3 Adjustment for More Than Three Current-Carrying Conductors 4.4 Putting It Together: Full Conductor-Sizing Problems 4.5 Parallel Conductors, Dwelling Service Sizing & Conditions of Use

5 Chapter 5: Wiring Methods and Materials — NEC Chapter 3

5.1 Article 300 General Requirements and Underground Cover 5.2 Thermal Movement: Expansion Fittings and Table 352.44 5.3 Boxes and Enclosures: Box Fill and Pull Box Sizing 5.4 Cable Assemblies: NM, MC, AC, SE, UF and TC 5.5 Raceways: RMC, IMC, EMT, FMC, LFMC, PVC and ENT 5.6 Conduit Fill: Chapter 9 Tables and Annex C 5.7 Cable Trays, Wireways, Busways, Gutters and Surface Raceways

6 Chapter 6: Branch Circuits and Feeders

6.1 Branch-Circuit Ratings, Conductor Sizing & Multiwire Circuits 6.2 Required Outlets and Receptacle Spacing in Dwellings 6.3 GFCI Protection — Article 210.8 6.4 AFCI Protection and Receptacle Requirements 6.5 Feeders and Outside Branch Circuits — Articles 215 and 225 6.6 Grounded Conductor Identification, Switches & Panelboards

7 Chapter 7: Overcurrent Protection — Articles 240 and 242

7.1 Article 240 Fundamentals: Ratings, Locations & Conductor Protection 7.2 Feeder Tap Rules — 240.21(B) 7.3 Transformer Secondary Conductors — 240.21(C) 7.4 Fuses, Circuit Breakers, Series Ratings & Article 242 Surge Protection

8 Chapter 8: Services and Load Calculations

8.1 Service Conductors, Clearances and Service Equipment 8.2 Service Disconnects, the 2020 Rule Change, SPDs and Service GFPE 8.3 Article 220 Part II: General Lighting, Receptacles and Demand Factors 8.4 The Standard Dwelling Calculation — Article 220 Part III 8.5 Optional Dwelling and Multifamily Calculations 8.6 Commercial and Industrial Load Calculations 8.7 Neutral Load Calculations and Harmonics — 220.61

9 Chapter 9: Grounding and Bonding — Article 250

9.1 System Grounding, 250.24 and the Main Bonding Jumper 9.2 The Grounding Electrode System — 250.50 through 250.53 9.3 Grounding Electrode Conductor Sizing and Installation 9.4 Bonding: Services, Metal Piping, Structural Metal and Other Systems 9.5 Equipment Grounding Conductors: Types, Sizing and Continuity 9.6 Separately Derived Systems and Separate Buildings 9.7 Ground-Fault Protection of Equipment and Grounding of Specific Equipment

10 Chapter 10: Equipment for General Use — NEC Chapter 4

10.1 Flexible Cords, Cables and Fixture Wires 10.2 Switches, Receptacles and Industrial Control Panels 10.3 Luminaires, Lampholders and Recessed Installations 10.4 Appliances and Fixed Electric Space Heating 10.5 Deicing, Snow Melting, Pipeline and Vessel Heating 10.6 Motors Part I: Full-Load Current, Conductors and Overload Protection 10.7 Motors Part II: Short-Circuit Protection, Controllers, Disconnects and HVAC

11 Chapter 11: Transformers, Generators and Special Occupancies

11.1 Transformers — Article 450 11.2 Generators, Storage Batteries and Energy Storage Systems 11.3 Hazardous (Classified) Locations — Articles 500 through 506 11.4 Garages, Hangars, Fuel Dispensing, Bulk Storage and Spray Application 11.5 Health Care Facilities — Article 517 11.6 Agricultural Buildings, Manufactured Homes, RV Parks and Marinas 11.7 Assembly, Theaters, Carnivals, Temporary Installations and Pools

12 Chapter 12: Special Equipment, Special Conditions and Exam Strategy

12.1 Signs, Cranes, Elevators, Electric Vehicle Charging and Welders 12.2 Solar Photovoltaic, Interconnection and Stand-Alone Systems 12.3 Wind Electric Systems and Fire Pumps 12.4 Emergency, Legally Required Standby, Optional Standby and COPS 12.5 Class 1, 2 and 3 Circuits, Fire Alarm, Optical Fiber and Communications 12.6 Cold-Climate Practice in Alaska: Code Basis and Engineering Judgment 12.7 Exam-Day Strategy: Navigation, Calculation Workflow and Time Management

9.3 Grounding Electrode Conductor Sizing and Installation

Key Takeaways Explain This Topic AI

Table 250.66 sizes the grounding electrode conductor from the largest ungrounded service-entrance conductor: 8 AWG copper for 2 AWG and smaller, 6 AWG for 1 or 1/0, 4 AWG for 2/0 or 3/0, and 2 AWG for over 3/0 through 350 kcmil.

NEC 250.66(A) caps the grounding electrode conductor at 6 AWG copper where it is the sole connection to a rod, pipe, or plate electrode.

NEC 250.66(B) caps it at 4 AWG copper where it is the sole connection to a concrete-encased electrode, and 250.66(C) caps it at the size of the ground ring conductor.

NEC 250.64(C) requires the grounding electrode conductor to be installed without a splice or joint except by irreversible compression connectors listed for grounding, exothermic welding, sections of busbar, or bolted bus connections.

NEC 250.64(E) requires ferrous metal raceways and enclosures for grounding electrode conductors to be electrically continuous and bonded at each end to the grounding electrode conductor.

Last updated: August 2026

Table 250.66

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

  • Largest ungrounded service-entrance conductor (copper) Copper GEC Aluminum or copper-clad aluminum GEC
  • 2 AWG or smaller 8 AWG 6 AWG
  • 1 or 1/0 AWG 6 AWG 4 AWG
  • 2/0 or 3/0 AWG 4 AWG 2 AWG
  • Over 3/0 through 350 kcmil 2 AWG 1/0 AWG
  • Over 350 through 600 kcmil 1/0 AWG 3/0 AWG
  • Over 600 through 1100 kcmil 2/0 AWG 4/0 AWG
  • Over 1100 kcmil 3/0 AWG 250 kcmil

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

  • Service conductors Electrode Required GEC
  • 350 kcmil copper Driven rods only 6 AWG copper — 250.66(A) cap
  • 350 kcmil copper Concrete-encased only 4 AWG copper — 250.66(B) cap
  • 350 kcmil copper Metal underground water pipe 2 AWG copper — Table 250.66, no cap applies
  • 2000 kcmil copper Driven rods only 6 AWG copper — 250.66(A) cap
  • 2000 kcmil copper Water pipe 3/0 AWG copper — table maximum

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:

Irreversible compression-type connectors listed as grounding and bonding equipment or the exothermic welding process;

  • Sections of busbars connected together;
  • Bolted, riveted, or welded connections of structural metal frames of buildings;

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.