Technical Article

National Electrical Code 2023 Basics: Grounding and Bonding Part 14

April 04, 2023 by Lorenzo Mari

Learn about the rules for installing grounding electrode conductors.

To catch up on Lorenzo Mari’s series on National Electrical Code 2023 Basics: Grounding and Bonding, follow these links:

Section 250.64 rules the installation of grounding electrode conductors for services or for buildings or structures supplied with feeders or branch circuits–per Section 250.32–or separately derived AC systems–per Section 250.30.

Image used courtesy of Pixabay

National Electrical Code Section 250.64 Installation of Grounding Electrode Conductors

Sections 250.64(A) to (G) rule the grounding electrode conductors installation.

Section 250.64(A) Aluminum or Copper-Clad Aluminum Conductors

This section partially limits the use of aluminum or copper-clad aluminum conductors. Abide by the following rules:

Do not install bare or covered conductors without an extruded polymeric covering, in contact with concrete, or where subject to corrosion.

The reason for this rule is to avoid a chemical reaction between the concrete and the oxide layer that covers and protects the aluminum conductor–losing the ability of the oxide layer to shelter the metal. See Figure 1.

Figure 1. Grounding electrode conductor separated from the concrete wall. Image used courtesy of Lorenzo Mari

In outdoor applications employing enclosures listed and identified for the environment, aluminum or copper-clad aluminum conductors may be terminated within 450 mm of the bottom of the enclosure.

Terminate aluminum or copper-clad aluminum conductors external to buildings or equipment at least 450 mm above the earth.

Figure 2 shows a bare aluminum or copper-clad aluminum conductor terminated at a grounding electrode. Although the limitation applies only to the termination, not the path, a bare conductor cannot contact the concrete wall or earth.

Figure 2. Aluminum or copper-clad aluminum conductor terminated at least 450 mm above the earth. Image used courtesy of Lorenzo Mari

The restrictions placed on aluminum or copper-clad aluminum conductors are one of the reasons for favoring copper grounding electrode conductors. Copper is a corrosion-resistant metal suitable for most locations.

Section 250.64(B) Securing and Protecting From Physical Damage

This section imparts rules to limit the exposure of the grounding electrode conductor to physical damage. Securely fasten exposed grounding electrode conductors or their enclosures to the surfaces supporting them. The type of protection depends on the conductor size.

250.64(B)(1) Not Subjected to Physical Damage

Conductors size N° 6 AWG or larger not exposed to physical damage may run along the surface of the construction without metal covering or protection. See Figure 3.

Figure 3. A grounding electrode conductor not exposed to physical damage. Image used courtesy of Lorenzo Mari

250.64(B)(2) Exposed to Physical Damage

Protect grounding electrode conductors size N° 6 AWG or larger exposed to physical damage with:

Rigid metal conduit
Intermediate metal conduit
Schedule 80 rigid PVC conduit
Reinforced thermosetting resin conduit
Electric metallic tubing
Cable armor

Figure 4 shows a grounding electrode conductor located outdoors protected against physical damage.

Figure 4. A grounding electrode conductor exposed to physical damage. Image used courtesy of Lorenzo Mari

250.64(B)(3) Conductors Smaller Than N° 6 AWG

Protect grounding electrode conductors smaller than size N° 6 AWG with the same enclosures listed in Section 250.64(B)(2).

Note that these conductor sizes require enclosures regardless of whether or not they are exposed to physical damage.

Protect the smallest size allowed in Table 250.66 (N° 8 AWG). The grounding electrode conductor shown in Figure 3 would require protection if it were size No. 8 AWG.

250.64(B)(4) In Contact with the Earth

Grounding electrode conductors and bonding jumpers in contact with the earth do not need to comply with the rules for underground installations in sections 300.5 (burial depths) or 305.15 (identification). However, they shall be buried for protection if subject to physical damage. See Figure 5.

Figure 5. Grounding electrode conductor and bonding jumper buried for protection against physical damage. Image used courtesy of Lorenzo Mari

Section 250.64(C) Continuous

The basic rule affirms that grounding electrode conductors must run in one continuous length–without joint or splice–with the following exceptions:

When spliced by irreversible compression connectors or the exothermic welding process (thermite welding).

The compression connector utilizes a tool that makes the splicing irreversible. See Figures 6 and 7.

Figure 6. Splice made with compression connector and tool. Image used courtesy of Burndy

Figure 7. Exothermic welding connection. Image used courtesy of Erico Cadweld

When busbar segments are bolted together in the field (Busbar-type grounding electrode conductor). See Figure 8.

Figure 8. Bolted busbar connection. Image used courtesy of ResearchGate

Riveted, bolted, or welded connections of the building’s structural metal frames.

See Figure 9.

Figure 9. Welded connection of structural metal frame. Image used courtesy of Avon Lake Sheet Metal

Welded, brazed, threaded, soldered, or bolted-flange connections of water piping. See Figures 10 and 11.

Figure 10. Bolted-flange connection of metal water piping. Image used courtesy of Pixabay

Figure 11. Soldered connection of metal water piping. Image used courtesy of Pixabay

Section 250.64(D) Services with Multiple Enclosures

This section permits three methods to connect the grounding electrode where the service has multiple enclosures.

250.64(D)(1) Tap Method

This rule permits tapping a common grounding electrode conductor (EGC). Keep to the following steps:

1. Install a common grounding electrode conductor. Size the conductor by entering Table 250.66 with the total circular mil area of the biggest ungrounded conductors supplying the disconnecting means.

2. Run grounding electrode conductor taps to each disconnecting means enclosure. Size taps by entering Table 250.66 with the largest service entrance or feeder conductor supplying the individual enclosure.

3. Connect the taps to the grounding electrode conductor by one of the following methods:

Exothermic welding.
Listed connectors.
Connection to an aluminum or copper busbar securely fastened and accessible.

4. Keep the common grounding electrode conductor without a splice or joint.

Example 1. Size the grounding electrode conductor and taps shown in Figure 12. Use copper conductors and find the circular mils from NEC’s Table 8, Chapter 9.

Figure 12 and Table 1 summarize data and results for Example 1.

Figure 12. Setup for Example 1. Image used courtesy of Lorenzo Mari

Table 1. Data and results for Example 1

DISCONNECTING MEANS (A)	UNGROUNDED CONDUCTOR		TAP SIZE (AWG)	EGC SIZE
	SIZE AWG or kcmil	CIRCULAR MILS

100	2	66,360	8
200	3/0	167,800	4
400	500	500,000	1/0
Total circular mils		734,160		2/0

1. 734,160 circular mils lay between conductors sizes 700 and 750 kcmil. Pick 750 kcmil, enter Table 250.66, and get conductor size N° 2/0 AWG.

2. Get the tap sizes directly from Table 250.66, entering with the conductors supplying the individual enclosures.

The tap method eliminates the complications found in looping grounding electrode conductors between enclosures. See Figure 13.

Figure 13. An involved and expensive looping method. Image used courtesy of Lorenzo Mari

250.64(D)(2) Individual Conductors

Run individual grounding electrode conductors from each enclosure, sized per Table 250.66. Input the table with the size of the service entrance or feeder conductor supplying the disconnecting means.

Example 2. Size the grounding electrode conductors shown in Figure 14. Use copper conductors.

Figure 14 and Table 2 summarize the data and results for Example 2.

Figure 14. Setup for Example 2. Image used courtesy of Lorenzo Mari

Table 2. Data and results for Example 2

DISCONNECTING MEANS (A)	UNGROUNDED CONDUCTOR SIZE (AWG or kcmil)		EGC SIZE
100	2	8
200	3/0	4
400	500	1/0

Get the tap sizes directly from Table 250.66, entering with the conductors supplying the individual enclosures.

250.64(D)(3) Common Location

Connecting a grounding electrode conductor in a wireway or auxiliary gutter installed on the supply side of the disconnecting means is valid. Connect the conductor to one of the following:

Grounded service conductor.
Equipment grounding conductor installed with the feeder.
Supply-side bonding jumper.

Size the equipment grounding conductor per Table 250.66 based on the largest ungrounded service-entrance or feeder conductor supplying the common location.

Figure 15 shows a grounding electrode conductor connected to the grounded service conductor at the common location.

Figure 15. Grounding electrode conductor connected at the common location. Image used courtesy of Lorenzo Mari

Section 250.64(E) Cable Armor, Raceways, and Enclosures for Grounding Electrode Conductors

This section copes with the electrical continuity of ferrous enclosures from the origin of the grounding electrode conductor to its connection to the grounding electrode.

250.64(E)(1) General

Keep ferrous metal raceways, cable armor, and enclosures for grounding electrode conductors electrically continuous from the origin to the connection to the grounding electrode.

Create a parallel current path by bonding each end of all ferrous metal raceways, cable armor, and enclosures to the grounding electrode or grounding electrode conductor.

Such bonding connections prevent the ferrous metal raceway, cable armor, and enclosures from creating an inductive choke on the grounding electrode conductor.

The grounding electrode conductor and enclosures share the current when a ground fault occurs. The most significant amount of fault current flows through the enclosures. Interrupting the continuity of the sections made of ferrous materials will send all the ground-fault current through the grounding electrode conductor, seeing a high impedance.

There is no requirement for the electrical continuity of nonferrous metal raceways, cable armor, and enclosures.

Using nonferrous or nonmetallic raceways–such as aluminum rigid metal conduit or PVC conduit–to enclose the grounding electrode conductor will reduce the impedance as compared with the same conductor in a ferrous raceway.

Figure 16 shows a grounding electrode conductor bonded to ferrous enclosures at each end—no need to connect the nonferrous metal raceway.

Figure 16. Grounding electrode conductor bonded at each end to ferrous enclosures only. Image used courtesy of Lorenzo Mari

From a practical and economic standpoint, installing the grounding electrode conductor in nonmetallic raceways appropriate for usage is better.

250.64(E)(2) Methods

Comply with Section 250.92(B) and ensure by one of the methods in sections 250.92(B)(2) through (4).

250.64(E)(3) Sizing

The bonding jumpers shall be equal to, or larger than, the enclosed grounding electrode conductor.

Employing bonding jumpers larger than the grounding electrode conductor may be convenient. Evidence shows that most ground-fault current flows through the enclosure and the bonding jumpers.

Section 250.64(F) Installation to the Electrode(s)

Run grounding electrode conductors to one or more grounding electrodes individually. See Figure 17.

Figure 17. Grounding electrode conductors run individually. Image used courtesy of Lorenzo Mari

Connect bonding jumpers from grounding electrodes and the grounding electrode conductor to a copper or aluminum busbar. See Figure 18.

Figure 18. Bonding jumpers from grounding electrodes and the grounding electrode conductor connected to a busbar. Image used courtesy of Lorenzo Mari

Section 250.64(G) Enclosures with Ventilation Apertures

Do not install the grounding electrode conductors through the ventilation apertures.

Takeaways of Grounding Electrode Conductor Installation

NEC Section 250.64 regulates the installation of grounding electrode conductors.
Take care of the particular limitations when using aluminum or copper-clad aluminum conductors.
Protect the grounding electrode conductors exposed to physical damage and all conductors smaller than N° 6 AWG.
Bury the grounding electrode conductors and bonding jumpers if subject to physical damage.
Run the grounding electrode conductors in one continuous length unless employing one of the exceptions.
Three methods exist to connect the grounding electrode where the service has multiple enclosures.
Bond each end of all ferrous metal raceways, cable armor, and enclosures to the grounding electrode conductor.
Run grounding electrode conductors to the grounding electrodes individually or through a busbar.
Do not mount the grounding electrode conductors through the ventilation openings.