Technical Article

National Electrical Code 2023 Basics: Grounding and Bonding Part 6

December 07, 2022 by Lorenzo Mari

Learn about the rules regarding the supply-side bonding jumper, grounded conductor, grounding electrode, and grounding electrode conductor in separately derived AC systems.

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Image used courtesy of Cooling Power

The correct connection and sizing of the supply-side bonding jumper, grounded conductor, grounding electrode, and grounding electrode conductor in separately derived systems are vital. They reduce overvoltages, inhibit fires, prevent objectionable currents, and carry and provide a low-impedance return path for line-to-ground fault currents.

NEC Section 250.30(A)(2) deals with the supply-side bonding jumpers.

The rules are:

Install a supply-side bonding jumper between the first disconnecting means and the source of a separately derived system when located in separate enclosures.
Install along with the circuit conductors.
Size is only sometimes bigger than the ungrounded conductors.
It may be of the nonflexible metal raceway, wire, or bus types.

◦ Wire type shall observe Section 250.102(C).

◦ The bus type must have a cross-sectional area not smaller than the wire type by Section 250.102(C).

Exception: Not required between enclosures for installations made by Section 250.30(A)(1), Exception N° 2.

Figure 1 shows a wire-type supply-side bonding jumper.

Figure 1. Wire-type supply-side bonding jumper. Image used courtesy of Lorenzo Mari

The supply-side bonding jumper closes the path for the ground-fault current to reach its source.

The wire can be bare, covered, or insulated. Examples of nonflexible metal raceways are rigid metal conduit (RMC), intermediate metal conduit (IMC), and electrical metallic tubing (EMT).

Arrangements with nonmetallic or flexible raceways must contain a wire-type supply-side bonding jumper.

Example 1. Compute the minimum size for a wire-type supply-side bonding jumper in an arrangement consisting of a dry-type transformer connected to a first disconnecting means located in a separate enclosure, with ungrounded conductors size N° 300 kcmil, copper.

Answer: Section 250.102(C) refers to Table 250.102(C)(1). Entering the table with the ungrounded conductor size N° 300 kcmil, copper column, the answer is size N° 2 AWG copper or N° 1/0 AWG aluminum.

Note that the sizing procedure for the supply-side bonding jumper is independent of the system bonding jumper location – at the source enclosure or the disconnecting means enclosure.

Section 250.30(A)(3) covers the grounded conductor. An arrangement comprising a grounded conductor and a system bonding jumper connection not situated at the source must comply with the following:

250.30(A)(3)(a). Size the grounded conductor when in a single raceway, per Table 250.102(C)(1). This rule is the same as sizing the grounded conductor in a service (See Section 250.24(D)(1)).

As a general rule, the grounded conductor does not need to be larger than the derived ungrounded conductor. Correct sizing must also contemplate the calculated load.

Example 2. Size the neutral conductor for a separately derived system employing ungrounded copper conductors size N° 350 kcmil AWG. In typical systems, the neutral is the grounded conductor.

Answer: Enter Table 250.102(C)(1) with N° 350 kcmil AWG, copper column, and read conductor size N° 2 AWG copper or size N° 1/0 AWG aluminum.

250.30(A)(3)(b). An arrangement of parallel ungrounded conductors in several raceways or cables requires grounded conductors installed in parallel in each raceway or cable. Figure 2 shows two sets of parallel conductors in two raceways.

Figure 2. Parallel conductors in two conduits. Image used courtesy of Lorenzo Mari

The size of the grounded conductors in each raceway or cable will follow Table 250.102(C)(1). The basis is the largest derived ungrounded conductor in each raceway or cable or the total cross-sectional area of the largest derived ungrounded conductors from each array connected in parallel in each cable or raceway.

The minimum required size is N° 1/0 AWG.

Example 3. A transformer, qualifying as a separately derived system, employs two sets of secondary ungrounded copper conductors in parallel in two conduits. Compute the minimum conductor size for the grounded conductor if the size of the ungrounded conductors is N° 2/0 AWG.

Answer: Enter Table 250.102(C)(1) with N° 2/0 kcmil AWG, copper column, and read conductor size N° 4 AWG copper or size N° 2 AWG aluminum.

However, this section requires size N° 1/0 AWG as a minimum and does not specify copper or aluminum. Then, use conductor size N° 1/0 AWG, either copper or aluminum, in each conduit.

This rule is the same as sizing parallel grounded conductors in services (See Section 250.24(D)(2)).

Section 310.10(G) presents rules for parallel grounded conductors.

250.30(A)(3)(c). The minimum ampacity of a grounded conductor in a 3-phase, 3-wire delta system shall be that of the ungrounded conductors.

This rule is the same as delta-connected services (See Section 250.24(D)(3)).

250.30(A)(3)(d). Sections 250.36 (480 V to 1 kV) or 250.187 (over 1 kV) rule the grounded conductor for impedance grounded neutral systems.

The size of the neutral or grounded conductor must be sufficient for carrying the computed load and the fault current. Follow Section 220.61 to compute the feeder or service neutral loads. Grounded conductors carrying fault currents must meet the requirements of Section 250.102(C).

Section 250.30(A)(4) contains rules for the grounding electrode. The separately derived system must use the same grounding electrode of the building or structure.

Figure 3 shows a separately derived system connected to a metal underground water pipe – an electrode permitted by 250.52(A)(1) for grounding a building or structure.

Figure 3. Separately derived system connected to a permitted grounding electrode. Image used courtesy of Lorenzo Mari

Exception: A separately derived system may use the grounding electrode of the listed service equipment where it originates.

Section 250.104(D) requires bonding between the separately derived system, the structural metal of the building frame, and the water piping in the area supplied by the system.

Apply Section 250.30(C) for outdoor sources.

Section 250.30(A)(5). This section deals with the grounding electrode conductor for a sole separately derived system. This conductor connects the grounded conductor with the grounding electrode. The connection point is the same as the system bonding jumper.

Figures 4 and 5 show two usual locations – at the source and at the first disconnecting means – for connecting the grounding electrode conductor to the grounded conductor.

Figure 4. Grounding electrode conductor connected at the source. Image used courtesy of Lorenzo Mari

Figure 5. Grounding electrode conductor connected at the first disconnecting means. Image used courtesy of Lorenzo Mari

The connections shown keep the neutral current through the grounded conductor, avoiding the flow on parallel paths, and provide a low-impedance return path to line-to-ground fault currents–easing the maneuvering of the overcurrent protective devices.

Size the grounding electrode conductor per Table 250.66 based on the size of the largest derived ungrounded conductor or equivalent cross-sectional area for conductors in parallel.

Example 4. Two paralleled size N° 300 kcmil copper conductors supply a panelboard. Compute the minimum size for the grounding electrode conductor.

Answer: 300 kcmil x 2 = 600 kcmil. Enter Table 250.66, copper column, with size N° 600 kcmil. Minimum size = N° 1/0 AWG copper or N° 3/0 AWG aluminum.

Example 5. Two paralleled size N° 4/0 copper conductors supply a panelboard. Compute the minimum size for the grounding electrode conductor.

Answer: Table 8, Chapter 9, shows a cross-sectional area of 211 600 circular mils for conductor size N° 4/0.

211 600 x 2 = 423 200 circular mils = 423.20 kcmil.

Enter Table 250.66, copper column, with size N° 423.20 kcmil. Minimum size = N° 1/0 AWG copper or N° 3/0 AWG aluminum.

Example 6. Three paralleled size N° 600 kcmil copper conductors supply a panelboard. Compute the minimum size for the grounding electrode conductor.

Answer: 600 kcmil x 3 = 1 800 kcmil. Enter Table 250.66, copper column, with size N° 1 800 kcmil.

1 800 kcmil is over 1 100 kcmil. From this ungrounded conductor size, the minimum size of the grounding electrode conductor is N° 3/0 AWG copper or N° 250 kcmil aluminum.

Example 7. A 3-phase, 4-wire, delta-connected system supplies a panelboard with the midpoint of one phase winding as a circuit conductor (high-leg delta). The high leg employs a conductor size N° 2/0 AWG, the other phases use a conductor size N° 500 kcmil, and the neutral uses a conductor size N° 4/0 AWG. All conductors are copper. Compute the minimum size for the grounding electrode conductor.

Answer: Obtain the minimum grounding electrode conductor size from Table 250.66 based on the largest ungrounded conductor, in this case, 500 kcmil. The minimum conductor size is N° 1/0 AWG copper or N° 3/0 AWG aluminum.

Exception N° 1 permits connecting the grounding electrode conductor and the equipment grounding terminal, bar, or bus if it is of sufficient size and the bonding jumper is a wire or busbar. Figure 6 shows such a connection.

Figure 6. Grounding electrode conductor connected to the equipment grounding terminal. Image used courtesy of Lorenzo Mari

Exception N° 2 permits using the grounding electrode conductor of listed service equipment when the latter contains the source of the separately derived system and the conductor size is enough.

The grounding electrode may be connected to an internal grounding bus of a suitable size. See figure 7.

Figure 7. Source of a separately derived system inside service equipment. Image used courtesy of Lorenzo Mari

Follow the steps below to determine if Exception N° 2 can apply:

1. Compute the minimum grounding electrode conductor size the derived system requires.

2. Compute the equipment grounding bus and grounding electrode conductor sizes on the HV side.

3. Compare steps 1 and 2 and determine if the results in step 2 qualify as grounding electrode conductors.

Key Takeaways of Supply-side Bonding Jumpers, Grounded Conductor, Grounding Electrode, and Grounding Electrode Conductor

A supply-side bonding jumper must connect the source, and the first disconnecting means when in separate enclosures.
There is no requirement for the grounded conductor to be larger than the ungrounded conductors.
An indoor, separately derived system shares the building’s grounding electrode system.
Connecting the grounding electrode conductor and the system bonding jumper on the same point prevents parallel paths for neutral current.