Technical Article

National Electrical Code 2023 Basics: Grounding and Bonding Part 12

March 10, 2023 by Lorenzo Mari

Learn about the rules for installing grounding electrode systems.

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

Section 250.53 rules the installation of two or more grounding electrodes described in Section 250.52 to create a grounding electrode system as required by Section 250.50. This section also adds requirements, conditions, and restrictions to such installations.

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National Electrical Code Section 250.53 Installation of Grounding Electrode Systems

Section 250.53(A) Rod, Pipe, and Plate Electrodes

Rod, pipe, and plate grounding electrodes must meet the requisites of sections 250.53 (A)(1) through (3) and be free from nonconductive coatings.

250.53(A)(1) Below Constant Moisture Level

Bury the rod, pipe, or plate’s upper end in a soil stratum with permanent moisture, if attainable, and submerge the electrode entirely unless the soil condition averts such installation – apply Section 250.53(A)(4) for options. See Figure 1.

Moisture content is a typical approach to controlling soil resistivity. The electrode’s ground resistance, up to infinity, is substantial. The requirements in this section lessen the ground resistance by reducing the soil resistivity and increasing the electrode’s surface contacting the earth.

250.53(A)(2) Supplemental Electrode Required

Add an electrode–selected from the types depicted in sections 250.52(A)(2) through (8)–to single rods, pipes, or plates unless they have a measured resistance to the ground not higher than 25 Ω (see Figure 1).

Figure 1. A single electrode located below the permanent moisture level and connected to a supplemental electrode. Image used courtesy of Lorenzo Mari

Bond the additional electrode to one of the following:

Directly to the rod, pipe, or plate electrode
Grounding electrode conductor
Grounded service-entrance conductor
Nonflexible grounded service raceway
Any grounded service enclosure

The NEC does not require obtaining a measured resistance lower than 25 Ω after connecting the supplemental electrode.

250.53(A)(3) Supplemental Electrode

Set apart the supplemental rod, pipe, or plate a minimum of 1.8 m.

According to the Informational Note in this section, the paralleling efficiency of ground rods increases by separating them twice the length of the most extended rod.

Figure 2 shows a ground rod supplying current to the earth.

Figure 2. A rod supplying current to the earth layers. Image used courtesy of Lorenzo Mari

There are infinite imaginary layers around the rod, each with a specific resistance value. The layer closest to the rod has the highest resistance due to its smaller surface. The next layer has a lower resistance, and so on.

The entire assembly’s resistance to the infinite equals the sum of the resistances of all layers connected in series.

The critical layers are those close to the rod since they have higher resistance values. Make efforts to keep the resistivity of the nearby layers low to obtain a minimum total ground resistance.

Figure 3 shows two nearby connected rods.

Figure 3. Two rods inserted into the earth. Image used courtesy of Lorenzo Mari

A separation of 1.8 m minimum helps to keep only the outermost layers overlapping and interfering, reducing the impact on the total ground resistance–the farthest layers have the lowest contribution to the total resistance to the ground.

Increasing the separation improves the arrangement efficiency. Placing the original and supplemental rods close would be of little benefit.

250.53(A)(4) Rod and Pipe Electrodes

The electrode must be installed straight down for at least 2.44 m in length, contacting the soil. Connecting a 2.44-long rod above the ground surface will not comply with the rule.

Where encountering rock bottom, the electrode may be pushed at an oblique angle not to exceed 45° from a vertical line–keeping at least 2.44 m of its length inside the ground.

When encountering rock bottom at an angle up to 45°–making it impossible to keep 2.44 m of electrode inside the ground–the electrode is permitted to be buried horizontally in a trench at least 0.75 m deep.

Use ground rod clamps marked as suitable for direct burial in these three options.

The upper end of the ground rod must be even with or below ground level unless the aboveground edge and the grounding electrode conductor add-on are protected against physical damage as required by Section 250.10 or placed where damage is unlikely. This requirement applies to a 3 m long rod, as a 2.44 m long rod complying with the NEC cannot have an aboveground connection.

Figure 4 shows the three rod and pipe electrode installation options.

Figure 4. Three options for installing rod and pipe electrodes. Image used courtesy of Lorenzo Mari

250.53(A)(5) Plate Electrode

Install plate electrodes at a minimum depth of 0.75 m.

Section 250.53(B) Electrode Spacing

Maintain a minimum separation of 1.83 m between any electrodes of the types specified in sections 250.52(A)(5) or (7)–rod, pipe, or plate electrodes–when used on different grounding systems. An example is a grounding electrode for power and light separated from a grounding electrode connected to a strike termination device (see Figure 5).

Figure 5. Minimum spacing between electrodes of different grounding systems. Image used courtesy of Lorenzo Mari

In practice, a separation of 1.83 m introduces sufficient resistance between two systems. Increasing the distance does not provide additional benefits.

One condition to avoid is having low resistance between electrodes of different systems and individual electrodes with high resistance to the ground.

Bond the electrodes if the minimum separation is impracticable.

Section 250.53(C) Bonding Jumper

The bonding jumpers connect the grounding electrodes to form the grounding electrode system. The rules are:

Install by sections 250.64(A), (B), and (E)
Size by Section 250.66
Connect as specified in Section 250.70
Do not use rebar as a conductor to interconnect the electrodes

Example 1. Determine the bonding jumper sizes in Figure 6. Use copper conductors.

Figure 6. Set-up for Example 1. Image used courtesy of Lorenzo Mari

a. Grounding electrode conductor. Enter Table 250.66 with conductor 1 000 kcmil Cu and read conductor size N° 2/0 AWG.

b. Water pipe to the building frame. Enter Table 250.66 with conductor 1 000 kcmil Cu and read conductor size N° 2/0 AWG.

c. Building frame to the concrete-encased electrode. Section 250.66(B) does not require a size larger than N° 4 AWG. However, the concrete-encased electrode sized N° 2 AWG, and the bonding jumper to a ground ring need the increase of the bonding jumper to size N° 2 AWG.

d. Concrete-encased electrode to ground ring. Section 250.66(C) does not require a conductor size larger than N° 2 AWG – the ground ring conductor size.

e. Ground ring to the ground rod. Section 250.66(A) does not require a conductor size larger than N° 6 AWG.

Table 1 summarizes the results.

Table 1. Sizes for bonding jumpers in Figure 6.

BONDING JUMPER	CONDUCTOR SIZE (AWG) (Cu)	PER NEC
Grounding electrode conductor	2/0	Table 250.66
Water pipe to the building frame	2/0	Table 250.66
Building frame to the concrete-encased electrode	2	250.66(B)
Concrete-encased electrode to the ground ring	2	250.66(C)
Ground ring to the ground rod	6	250.66(A)

Example 2. Determine the bonding jumper sizes in Figure 7. Use copper conductors.

Figure 7. Set-up for Example 2. Image used courtesy of Lorenzo Mari

Service equipment to the water pipe. Enter Table 250.66 with conductor 3/0 AWG Cu and read conductor size N° 4 AWG.

Service equipment to the concrete-encased electrode. Section 250.66(B) does not require a size larger than N° 4 AWG.

Service equipment ground ring. Section 250.66(C) does not require a conductor size larger than N° 2 AWG.

Service equipment to the ground rod. Section 250.66(A) does not require a conductor size larger than N° 6 AWG.

Table 2 summarizes the results.

Table 2. Sizes for bonding jumpers in Figure 7.

BONDING JUMPER	CONDUCTOR SIZE (AWG) (Cu)	PER NEC
Service equipment to water pipe	4	Table 250.66
Service equipment to concrete-encased electrode	4	250.66(B)
Service equipment to the ground ring	2	250.66(C)
Service equipment to the ground rod	6	250.66(A)

Section 250.53(D) Metal Underground Water Pipe

The underground metallic water pipe used as a grounding electrode should meet the following:

Ensure the continuity of the grounding path or the bonding connection to the interior piping does not depend on meters, filters, or the like.

Figure 8 shows a water meter and filter bypassed via bonding jumpers.

Figure 8. The bonding jumpers ensure the grounding path continuity. Image used courtesy of Lorenzo Mari

This rule avoids the loss of grounding if the water meter, filter, or similar equipment is withdrawn for service or substituted with nonmetallic units.

Although this section does not state a rule to size the bonding jumpers, Section 250.53(C) should apply–size by Section 250.66.

Install a supplemental electrode of the type specified in 250.52(A)(2) through (8).

Bond to the grounding electrode conductor, grounded service-entrance conductor, nonflexible grounded service raceway, or grounded service enclosure.

Figure 9 shows a supplemental ground rod bonded to the grounding electrode conductor.

Figure 9. Supplemental ground rod bonded to the grounding electrode conductor. Image used courtesy of Lorenzo Mari

The need for supporting metal water piping with supplemental electrodes comes from replacing metal pipes with plastic parts–leaving the electric system without a grounding electrode.

The exception to this section permits bonding the supplemental electrode to the interior metal water piping following Section 250.68(C)(1). This section requires the internal metal water piping to be electrically continuous with a metal underground water pipe electrode and locate the connection at not more than 1.52 m from the entrance to the building. See Figure 10.

Figure 10. Supplemental electrode bonding per 250.53(D)(2) Exception. Image used courtesy of Lorenzo Mari

The piping at this point extends the bonding conductor connections to the grounding electrode – recall from Section 250.52(A)(1)] that only the underground portion is a grounding electrode.

The exception to Section 250.68(C)(1) permits the connection to a point farther than 1.52 m from the entrance in industrial, commercial, and institutional buildings if qualified persons supervise the installations. Additionally, interior metal water piping must be visible except for short sections through walls, floors, or ceilings.

Section 250.53(E) Bonding Jumper Size for the Supplemental Grounding Electrode

When the supplemental electrode is a rod, pipe, or plate, the size of the bonding jumper dedicated solely to the supplemental electrode does not need to be larger than 6 AWG in copper or 4 AWG in aluminum. See the bonding jumper in Figure 9.

Section 250.53(F) Ground Ring

Install the ground ring at a depth of not less than 0.75 m. See the ground ring in Figure 7.

Takeaways of Grounding Electrode System Installation

Grounding electrodes must be free from nonconductive coatings.
Bury rods, pipes, or plates below the permanent moisture level.
Separate pipes, rods, or plates a minimum of 1.8 m when employing some of them to lower the ground resistance. A slight separation will bring a little upgrade.
Install the electrodes straight down for at least 2.44 m in length contacting the soil, as a first option. Follow Section 250.53(A)(4) for choices when facing rock bottom.
Keep a minimum separation of 1.83 m between electrodes serving different systems.
Size the bonding jumpers per Section 250.66, using Table 250.66 and the individual stipulations for particular electrodes.
Do not rely on water meters, filters, and similar devices for the continuity of the grounding path. Use bonding jumpers.
Bond the internal metallic water piping to a supplemental electrode—no need for a bonding jumper size larger than N° 6 copper or N° 4 aluminum.
Install ground rings at a minimum depth of 0.75 m.

Featured image used courtesy of Adobe Stock