Technical Article

National Electrical Code 2023 Basics: Grounding and Bonding Part 4

November 29, 2022 by Lorenzo Mari

Learn how to make ground connections on the supply side of the service disconnect, determine which conductor to ground, and measure the main and system bonding jumpers.

To catch up on Lorenzo Mari’s series on grounding and bonding, please follow these links:

Section 250.25 imparts rules to ground those systems joined to the supply side of the service. Section 250.26 specifies which conductor must be grounded, depending on the transformer connection. Main and system bonding jumpers are vital components of the ground-fault current path and must have the correct size per Section 250.28.

 

Image used courtesy of Pixabay

 

Section 250.25. This section deals with the grounding of supply-side disconnects that are not considered a service–located in casings segregated from the service equipment enclosure.

Section 230.82 lists the equipment that can be connected to the supply side of a service. Some of them are:

Most of this equipment will join the service conductors and terminate in a disconnect switch suitable for use as service equipment–but not officially considered a service or a service disconnect.

Section 250.25 directs the reader to other sections to find the systems' grounding requirements.

Section 250.25(A) Grounded systems. If the supply system is grounded, the equipment grounding will follow sections 250.24(A) through (D).

Section 250.25(B) Ungrounded systems. If the supply system is ungrounded, the equipment grounding will follow section 250.24(F).

Figure 1 is an example of how to ground equipment connected on the supply side of the service disconnect.

 

 

Figure 1. Grounding for the supply side of the service disconnect. Image used courtesy of Lorenzo Mari

 

Section 250.26. Depending on the transformer connection, this section designates the conductor to be grounded.

Section 250.26(1). In single-phase, 2-wire, ground either conductor. See Figure 2.

 

Figure 2. Grounded conductor in single-phase, 2-wire (Typical). Image used courtesy of Lorenzo Mari

 

Section 250.26(2). In single-phase, 3-wire, ground the neutral. See Figure 3.

 

Figure 3. Neutral grounded in single-phase, 3-wire (Typical). Image used courtesy of Lorenzo Mari

 

Section 250.26(3). Ground the neutral in multiphase systems with one common wire. See Figure 4.

 

Figure 4. Neutral grounded in a multiphase system (Typical). Image used courtesy of Lorenzo Mari

 

Section 250.26(4). In multiphase systems with one phase grounded, ground that phase conductor. See Figure 5.

 

Figure 5. Phase conductor grounded in a multiphase system (Typical). Image used courtesy of Lorenzo Mari

 

Section 250.26(5). In multiphase systems using one phase, as in Section 250.26(2), ground the neutral. See Figure 6.

 

Figure 6. Neutral grounded in a multiphase system (Typical). Image used courtesy of Lorenzo Mari

 

Section 250.28. This section rules the installation of main and system bonding jumpers in grounded systems.

The main and system bonding jumpers perform the same electrical function in a grounded AC system–ensuring an effective ground-fault current path. The bonding jumpers connect the equipment grounding conductor(s) and the grounded circuit conductor.

The main bonding jumper provides the connection only to the service equipment. In a  separately derived system, the system bonding jumper wires up either at the first disconnecting means supplied by the source or at the source. The main bonding jumper belongs to the service, and the system bonding jumper is to the separately derived system. See Figure 7.

 

 

Figure 7. Main bonding jumper and system bonding jumper. Image used courtesy of Lorenzo Mari

 

Section 250.28(A). The materials permitted for the main and system bonding jumpers are copper, aluminum, copper-clad aluminum, or other corrosion-resistant substances. They may be a wire, bus, screw, or similar conductor.

Section 250.28(B). Where the main or system bonding jumper is just a screw, it must have a green finish visible with the screw installed.

Section 250.28(C). Attach the main and system bonding jumper in compliance with the methods provided in section 250.8. The chosen method must be suitable for the bonding jumper and enclosure materials.

Section 250.28(D). Size the main bonding jumper and system bonding jumper in compliance with 250.28(D)(1) through 250.28(D)(3).

250.28(D)(1). The size is not smaller than in Table 250.102(C)(1). The basis of this table is the size of the largest conductor–ungrounded–or equivalent cross-sectional area for parallel conductors.

For ungrounded supply conductors above 1 100 kcmil copper or 1 750 kcmil aluminum, the size is not smaller than 12½% of the cross-sectional area of the largest supply conductor or equivalent cross-sectional area for parallel conductors (Note 1 to Table 250.102(C)(1)).

Using the bonding jumper supplied in an apparatus listed as service equipment is acceptable with no further calculations.

250.28(D)(2). When the service consists of more than a single enclosure, size the main bonding jumper for each enclosure according to Section 250.28(D)(1).

Using the bonding jumper supplied in an apparatus listed as service equipment is acceptable with no further calculations.

250.28(D)(3). When a separately derived system supplies more than one enclosure, install the system bonding jumper at either the source of the separately derived system or the first system disconnecting means for each enclosure.

  • If installed at the source, size according to Table 250.102(C)(1) entering the table with the equivalent conductor size resulting from adding the cross-sectional areas of the largest ungrounded conductors, in one phase, for all enclosures.
  • If installed at the first disconnecting means size according to Table 250.102(C)(1). Enter the table with the largest ungrounded conductor feeding each enclosure–or use the system bonding jumper supplied with listed enclosures.

Example 1. Assume copper conductors. Find the minimum size of the system bonding jumper connected at the source of the separately derived system shown in Figure 8.

 

Figure 8. Arrangement for Example 1. Image used courtesy of Lorenzo Mari

 

Answer: Table 1 shows the calculations performed to find the minimum size of the system bonding jumper fitted at the source.

Recall Note 1 to Table 250.102(C)(1) with the 12.5% rule for copper conductors larger than 1 100 kcmil and aluminum conductors larger than 1 700 kcmil.

The minimum conductor size comes from data in Table 8, Chapter 9.

Note the insulated neutral bar.

 

Table 1. Computation of the minimum size of the system bonding jumper at the source.

Enclosure

Conductor size (kcmil)

Conductors per phase

Total cross-sectional area (kcmil)

1

700

1

700

2

400

2

800

Total cross-sectional area to size jumper

1 500

12.5% Rule (From Table 250.102(C)(1))

187.5

Minimum system bonding jumper size at the source (from Table 8, Chapter 9)

4/0

 

Example 2. Repeat example 1 for system bonding jumpers installed at each enclosure. See Figure 9.

 

Figure 9. Arrangement for Example 2. Image used courtesy of Lorenzo Mari

 

Answer:

Enclosure N° 1. Enter Table 250.102(C)(1) with 700 kcmil; the minimum size is N° 2/0 copper.

Enclosure N° 2. Enter Table 250.102(C)(1) with 800 kcmil; the minimum size is N° 2/0 copper.

Note the grounded neutral bar.

 

Key Takeaways of Supply-Side Ground Connections

  • The systems inside enclosures, separated from the service equipment and connected on the service disconnect's supply side, must be grounded by Section 250.24.
  • The conductor attached to the ground is a function of the transformer connection.
  • The main and system bonding jumpers are vital to complete the ground-fault current path. In this way, the overcurrent devices will open under fault conditions.