4.4 Grounding Electrode System & GEC Sizing

Grounding Electrode System & GEC Sizing

The grounding electrode system serves as the physical interface between an electrical wiring system and the earth. NEC Article 250 Part III establishes strict requirements for the selection, installation, and interconnection of grounding electrodes. Furthermore, NEC Section 250.66 governs the sizing of the Grounding Electrode Conductor (GEC) that connects the premises wiring system to these electrodes. Mastery of electrode specifications, Table 250.66 sizing, and the famous "sole-connection" exceptions is mandatory for any electrician preparing for the Journeyman licensing exam.

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The Grounding Electrode System Concept (NEC 250.50)

NEC Section 250.50 mandates that all grounding electrodes described in NEC 250.52(A)(1) through (A)(8) that are present at each building or structure served shall be bonded together to form the grounding electrode system.

If none of these electrodes are present, one or more of the electrodes described in 250.52(A)(4) through (A)(8) (such as driven ground rods) must be installed and used.

                     [Service Equipment Enclosure]
                                  │
                 Grounding Electrode Conductor (GEC)
                                  │
  ┌───────────────┬───────────────┴───────────────┬───────────────┐
  ▼               ▼                               ▼               ▼
Metal Water     Concrete-Encased               Ground          Driven Ground
Pipe (10 ft)    Electrode (Ufer)                Ring            Rods (2x)
(Supp. req'd)   (20 ft rebar or 4 AWG Cu)     (2 AWG Cu)      (8 ft long, 6 ft apart)

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Permissible Grounding Electrodes (NEC 250.52(A))

NEC 250.52(A) recognizes eight grounding electrode types:

1. metal underground water pipe;
2. metal in-ground support structures (at least 10 ft vertically in contact with the earth);
3. concrete-encased electrodes;
4. ground rings;
5. rod and pipe electrodes;
6. other listed electrodes;
7. plate electrodes; and
8. other local metal underground systems or structures, such as piping systems, underground tanks, and underground metal well casings not bonded to a metal water pipe.

The most-tested types are detailed below.

1. Metal Underground Water Pipe (NEC 250.52(A)(1))

• Specification: A metal underground water pipe in direct contact with the earth for 10 feet (3.0 m) or more (including any metal well casing bonded to the pipe).
• Interior Piping Limitation (NEC 250.68(C)(1)): The connection to the metal water pipe must be made within the first 5 feet (1.5 m) from where the pipe enters the building. Interior metal water piping beyond 5 feet cannot be utilized as a conductor to interconnect other electrodes.
• Mandatory Supplemental Electrode (NEC 250.53(D)(2)): A metal underground water pipe must always be supplemented by an additional electrode (such as a concrete-encased electrode, ground ring, or driven ground rod). If the water company replaces the exterior metal pipe with nonmetallic plastic (such as PVC or PEX), the electrical system would otherwise lose its earth connection.

2. Concrete-Encased Electrode ("Ufer" Ground) (NEC 250.52(A)(3))

Named after electrical engineer Herbert G. Ufer, who developed the system for the U.S. military during World War II, the concrete-encased electrode is one of the most effective grounding electrodes available because concrete has hygroscopic properties that retain moisture in the soil.

• Conductor Option: Not less than 20 feet (6.0 m) of bare copper conductor not smaller than 4 AWG.
• Steel Rebar Option: Not less than 20 feet (6.0 m) of one or more bare or zinc-galvanized or other electrically conductive coated steel reinforcing bars or rods not less than 1/2 inch (13 mm) in diameter. The rebar may be in one continuous 20-foot length or multiple sections tied together using standard steel tie wires.
• Installation Requirements: Must be encased by at least 2 inches (50 mm) of concrete. It may lie horizontally within the part of a concrete foundation or footing in direct contact with the earth, or vertically within foundations or structural members in direct contact with the earth. Concrete separated from the soil by insulation or a vapor barrier is not in direct contact. If a building has several concrete-encased electrodes, only one needs to be bonded into the system.
• Mandatory Inclusion: If a concrete foundation or footing containing 20 feet of 1/2 in rebar is poured for a new building, it must be included in the grounding electrode system. Electricians cannot ignore an available Ufer ground in favor of ground rods.

3. Ground Ring (NEC 250.52(A)(4))

• Specification: Encircling the building or structure, in direct contact with the earth, consisting of at least 20 feet (6.0 m) of bare copper conductor not smaller than 2 AWG.
• Burial Depth: Must be buried at a depth below the earth's surface of not less than 30 inches (750 mm).

4. Rod and Pipe Electrodes (NEC 250.52(A)(5))

• Length: Not less than 8 feet (2.44 m) in length.
• Diameter: Pipe or conduit electrodes must be not smaller than trade size 3/4 inch and must be galvanized or metal-coated. Ground rods of iron or steel must have a diameter of at least 5/8 inch (15.87 mm), unless listed (listed stainless steel and copper-coated steel rods may be 1/2 inch).
• Installation (NEC 250.53(A)(4)): Driven vertically to a depth of not less than 8 feet. Where rock bottom is encountered, the rod may be driven at an oblique angle not exceeding 45 degrees from vertical, or buried horizontally in a trench at least 30 inches (750 mm) deep.
• The 25-Ohm Rule & Supplemental Rod (NEC 250.53(A)(2)): A single rod electrode that does not have a resistance to ground of 25 ohms or less must be augmented by an additional electrode. If an electrician does not perform a certified ground resistance fall-of-potential test, a second supplemental rod is mandatory.
• No Third Rod Required: If the second supplemental rod is driven, no further resistance testing is required by the NEC, regardless of resistance.
• Spacing Between Rods (NEC 250.53(A)(3)): Multiple ground rods must be spaced not less than 6 feet (1.8 m) apart. (Best engineering practice recommends spacing them twice their length—16 feet apart—to prevent overlapping resistance spheres of influence in the soil).

5. Plate Electrodes (NEC 250.52(A)(7))

• Surface Area: Must expose not less than 2 square feet (0.186 sq m) of surface area to exterior soil.
• Thickness: Iron or steel plates must be at least 1/4 inch (6.4 mm) thick. Nonferrous (copper) plates must be at least 0.06 inch (1.5 mm) thick.
• Burial Depth: Buried not less than 30 inches (750 mm) below the surface of the earth.

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Summary of Grounding Electrode Specifications

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Electrodes NOT Permitted for Grounding (NEC 250.52(B))

The following items are strictly prohibited from being used as grounding electrodes:

1. Metal Underground Gas Piping Systems: Prohibited under all conditions (NEC 250.52(B)(1)). Stray electrical currents or lightning surges could ignite gas or arc through thin-walled pipe joints.
2. Aluminum Electrodes: Prohibited under all conditions (NEC 250.52(B)(2)). Aluminum corrodes rapidly when in direct contact with soil due to galvanic action, destroying electrical continuity.
3. Swimming pool steel: The structural reinforcing steel and other bonded parts of a pool described in 680.26(B)(1) and (B)(2) may not serve as a grounding electrode.

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Grounding Electrode Conductor (GEC) Installation (NEC 250.64)

The GEC connects the system grounded conductor (at the service disconnect) to the grounding electrode system.

• Material: Copper, aluminum, or copper-clad aluminum.
• Protection from Physical Damage (NEC 250.64(B)):
  • Not exposed to damage: The GEC (or its enclosure) must be securely fastened to the surface it runs on, and may run on or through framing members.
  • 6 AWG or larger, exposed to damage: Must be protected in RMC, IMC, Schedule 80 PVC, RTRC-XW, EMT, or cable armor.
  • Smaller than 6 AWG (e.g., 8 AWG): Must always be enclosed in RMC, IMC, Schedule 80 PVC, RTRC-XW, EMT, or cable armor.
  • Aluminum or copper-clad aluminum GECs may not be in direct contact with masonry or the earth. Outdoors, they may not terminate within 18 inches of the earth (250.64(A)).
• Continuous Without Splices (NEC 250.64(C)): The GEC must be one continuous length. Splices are permitted only by:
  • irreversible compression-type connectors listed as grounding and bonding equipment, or
  • the exothermic welding process. Sections of busbar may be joined to form a GEC. Separately, 250.64(D) lets tap conductors from several service disconnects join a common GEC at a copper or aluminum busbar at least $1/4\text{ in} \times 2\text{ in}$.
• Ferrous Metal Raceways Enclosing GECs (NEC 250.64(E)): If a ferrous metal conduit (such as steel RMC or EMT) encloses a GEC, the raceway must be bonded at both ends to the GEC. Failure to bond both ends creates an "inductive choke" during lightning strikes: the high-frequency surge current creates an intense magnetic field in the steel pipe that chokes off current flow, preventing the surge from reaching earth and causing dangerous flashovers.

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Sizing the Grounding Electrode Conductor: Table 250.66

Under standard rules, the GEC is sized based on the cross-sectional area of the largest ungrounded service-entrance conductor (or equivalent area for parallel conductors) supplying the facility:

Table 250.66: Grounding Electrode Conductor for Alternating-Current Systems

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Sole-Connection Maximum GEC Sizing Caps (NEC 250.66(A), (B), (C))

One of the most heavily tested areas on the Journeyman Electrician exam is the set of sole-connection maximum sizing limits. When a GEC connects solely to specific individual electrodes, it is never required to be larger than the specified cap, regardless of how massive the service conductors are!

1. Rod, Pipe, or Plate Electrodes (NEC 250.66(A))

Where the grounding electrode conductor connects solely to a rod, pipe, or plate electrode, that portion of the conductor that is the sole connection is not required to be larger than 6 AWG copper (or 4 AWG aluminum).

• Example: A 2,000-ampere commercial service is fed with four sets of 500 kcmil copper conductors in parallel ($2{,}000\text{ kcmil}$ total copper). Table 250.66 would normally dictate a 3/0 AWG copper GEC. However, the GEC tap that connects solely to the driven ground rods is capped at 6 AWG copper!

2. Concrete-Encased Electrodes (NEC 250.66(B))

Where the grounding electrode conductor connects solely to a concrete-encased electrode (Ufer), that portion of the conductor that is the sole connection is not required to be larger than 4 AWG copper.

• Example: A 400-ampere service uses two parallel sets of 3/0 AWG copper, an equivalent area of $2 \times 167{,}800 = 335{,}600$ cmil (about 336 kcmil). Table 250.66 calls for a 2 AWG copper GEC to a water pipe. The portion connecting solely to the concrete-encased electrode need not exceed 4 AWG copper.

3. Ground Rings (NEC 250.66(C))

Where the grounding electrode conductor connects solely to a ground ring, that portion of the conductor that is the sole connection is not required to be larger than the conductor used for the ground ring (which is minimum 2 AWG copper).

Why Do Sole-Connection Caps Exist?

The contact resistance between a ground rod and the surrounding earth is typically between 15 and 25 ohms. Sizing a GEC larger than 6 AWG copper (about $0.0005~\Omega$ per foot, from Chapter 9 Table 8) provides virtually zero measurable decrease in total grounding impedance. The soil resistance is the bottleneck, not the wire.

Exam Trap — Water Pipe Does NOT Have a Cap: Metal underground water pipes have enormous surface contact with the earth. Therefore, connections to metal underground water pipes are never capped! They must always be sized to the full requirements of Table 250.66.