National Electrical Code 2023 Basics: Grounding and Bonding Part 3

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

• National Electrical Code 2023 Basics: Grounding and Bonding Part 1
• National Electrical Code 2023 Basics: Grounding and Bonding Part 2

The correct grounding of the AC service limits the voltage to ground inflicted on the electric system by lightning, line surges, and accidental high-voltage crossovers. It also controls the voltage to the ground during system operation. The grounded service conductor is vital in closing the ground-fault current path.

 

Image used courtesy of Pixabay

 

Section 250.24.

This section–divided into six essential subsections–deals with the grounding of service-supplied alternating current systems. The subsections are:

   (A) System grounding connections

   (B) Load-side grounding connections

   (C) Main bonding jumper

   (D) Grounded conductor brought to service equipment

   (E) Grounding electrode conductor

   (F) Ungrounded system grounding connections

Section 250.24(A) A premises electrical installation, fed by a grounded alternating current system, must have a grounding electrode conductor attached to the grounded service conductor at each service, complying with the following provisions:

250.24(A)(1) Connect the grounding electrode conductor at any reachable place from the load end of the overhead or underground service conductors, service drop, or service lateral to the terminal or bus–at the service disconnecting means–to which the grounded conductor is connected. Figure 1 shows three places permitted for the connection.

 

 

Figure 1. Three places permitted to connect the grounding electrode conductor and the grounded service conductor. Image used courtesy of Lorenzo Mari

 

250.24(A)(2) Outdoor transformers supplying the services require a minimum of one additional grounding connection, to a grounding electrode, from the grounded service conductor–either at the transformer or elsewhere outside the building.

Figure 2 shows an additional grounding connection for the outdoor transformer.

 

 

Figure 2. Additional grounding connection. Image used courtesy of Lorenzo Mari

 

The exception to this section prohibits the additional grounding for systems with an impedance grounded neutral.

250.24(A)(3) This section permits using a single grounding electrode conductor at the tie point of the grounded conductors from each source in dual-fed services–either in a shared enclosure or grouped in separate compartments. See Figure 3.

 

 

Figure 3. Grounding of dual-fed services. Image used courtesy of Lorenzo Mari

 

250.24(A)(4) According to this rule, the grounding electrode conductor can be connected to the equipment grounding terminal only if the main bonding jumper is a wire or bus bar linking the grounded conductor terminal bar or bus to the equipment grounding bar or bus. The rule does not apply if the main bonding jumper connects the grounded conductor terminal bar or bus to the service equipment enclosure. See Figure 4.

 

 

Figure 4. Main bonding jumper as wire or busbar. Image used courtesy of Lorenzo Mari

 

Section 250.24(B) This section prohibits grounding the conductor on the load flank of the service disconnecting means –  except as permitted in other sections of Article 250.

Section 250.24(C) When the system is grounded, and the service equipment comprises several disconnecting means. This section requires an unspliced main bonding jumper connecting–inside the enclosure–the grounded service conductor, the equipment grounding conductor(s), and the service-disconnect enclosure for each service disconnect. See Figure 5.

The main bonding jumpers are sized by 250.28(D)(1) through 250.28(D)(3).

 

 

Figure 5. Main bonding jumper. Image used courtesy of Lorenzo Mari

 

The exception N° 1 to this rule allows using only one unspliced main bonding jumper when there are several service disconnecting means in an assembly listed for use as service equipment. Typically, the manufacturer furnishes the bonding jumper. See Figure 6.

 

 

Figure 6. Bonding jumper in an assembly listed for use as service equipment. Image used courtesy of Lorenzo Mari

 

Section 250.24(D) This section requires, in grounded AC systems operating at less than 1 kV, routing the grounded conductors with the ungrounded conductors to every service disconnecting means and connecting the former to each grounded conductor terminal or bus. Additionally, it requires connecting the main bonding jumper between the grounded conductor(s) and each enclosure. The grounded conductor(s) installation shall comply with 250.24(D)(1) through 250.24 (D)(4). See Figure 7.

 

 

Figure 7. Grounded conductor to the service equipment. Image used courtesy of Lorenzo Mari

 

Note in Figure 7 that the grounded conductor must also be run to the service equipment when there is no supply to line-to-neutral loads–for a grounded utility service system.

The grounded conductor is part of the ground-fault current path between the service equipment and the utility source–on the supply edge of the service disconnecting means.

The exception to this section permits joining the grounded conductor(s) to the assembly where two or more than one service disconnecting means are located in one body approved as service equipment. The main bonding jumper, included in the assembly, shall connect the grounded conductor(s) to the enclosure. See Figure 8.

 

 

Figure 8. Example of bonding jumper in an apparatus listed for use as service equipment. Image used courtesy of Lorenzo Mari

 

Section 250.24(D)(1) This section refers to sizing the grounded conductor in a single raceway or cable. It requires the system grounded conductor (regularly a neutral) to be not smaller than the size specified in Table 250.102(C)(1).

Table 250.102(C)(1) lists the minimum sizes for grounded conductors, main bonding jumpers, system bonding jumpers, and supply-side bonding jumpers for AC systems.

Steps to size the system grounded conductor:

   1. Find the size and material of the service-entrance ungrounded conductor(s).

   2. Find the maximum unbalanced load from load calculations by Section 220.61.

   3. Determine the minimum conductor size required to handle the full unbalanced load.

   4. Enter table 250.102(C)(1) with data from step 1 and select the minimum conductor size.

   5. Compare the sizes in steps 3 and 4, and pick the largest of these conductors.

Example 1. A service wiring system employs ungrounded copper conductors size N° 300 kcmil. Load calculations show a maximum unbalanced load of 95 A. Compute the minimum size of the grounded service conductor (copper).

Answer:

   1. N° 300 kcmil copper conductors.

   2. 95 A.

   3. Table 310.16 shows a conductor size N° 3 AWG, in column 75 °C, to handle 95 A.

   4. N° 2 AWG.

   5. Pick size N° 2 AWG.

Another situation arises when installing parallel sets of conductors per 310.10(G) in a single raceway or cable. Section 310.10(G) allows connecting conductors in parallel for sizes 1/0 or larger–with a few exceptions.

The NEC deals with conductors connected in parallel as a single conductor with a total cross-sectional area equivalent to the sum of the cross-sectional areas of the paralleled conductors.

The steps to follow if in one raceway, such as a wireway, are:

   1. Compute the total cross-sectional area of the largest set of conductors in parallel and consider it as one conductor. Also, define the conductor material. Look for the conductor’s cross-sectional area in Table 8 of Chapter 9 (conductor properties).

   2. Find the maximum unbalanced load from load calculations by Section 220.61.

   3. Determine the minimum conductor size required to handle the maximum unbalanced load.

   4. Enter table 250.102(C)(1) with data from step 1 and select the minimum conductor size.

   5. Compare the sizes in steps 3 and 4, and pick the largest of these conductors.

If the cross-sectional area of the ungrounded set of conductors is larger than 1 100 kcmil copper or 1 750 aluminum, apply the 12.5% rule in Note 1 to Table 250.102(C)(1).

Figure 9 shows parallel conductors in one raceway.

 

 

Figure 9. Parallel conductors in a single raceway. Image used courtesy of Lorenzo Mari

 

The following example shows the procedure for using the 12.5% rule.

Example 2. Four sets of 350 kcmil copper conductors supply a 1 200 A service. Compute the minimum size of the grounded conductor.

Answer:

   1. Table 8 of Chapter 9 shows that the cross-sectional area is 350 kcmil each. 4 x 350 kcmil = 1 400 kcmil.

   2. 1 400 kcmil > 1 100 kcmil.

   3. 1 400 kcmil x 12.5% = 175 kcmil = 175 000 circular mils.

   4. Table 8 of Chapter 9 shows that the next higher standard is 211 600 circular mils, corresponding to N° 4/0 AWG.

   5. Load calculations might require a larger conductor.

Table 1 shows the minimum size of the grounded conductor for a few services having parallel ungrounded conductors with total cross-sectional areas higher than 1 100 kcmil, copper (12.5% rule).

 

Table 1. Minimum grounded conductor size for ungrounded conductors in parallel in a single raceway.

Service rating (A)	Ungrounded conductor kcmil	Minimum grounded conductor kcmil (12.5%)	Minimum grounded conductor size (AWG or kcmil)
1 000	3 x 400 = 1 200	150	3/0
1 200	4 x 350 = 1 400	175	4/0
1 600	5 x 400 = 2 000	250	250
2 000	6 x 400 = 2 400	300	300

Section 250.24(D)(2) This section refers to sizing the grounded conductors in two or more raceways or cables in parallel. Under this situation, the grounded conductor shall also be in parallel. Conductor size must be N° 1/0 AWG or larger. See also Section 310.10(G).

The grounded conductor size in each raceway, or cable, comes from Table 250.102(C)(1). The input to the table is the total cross-sectional area of the largest parallel ungrounded conductors in each raceway or cable.

Figure 10 shows parallel conductors in two raceways.

 

 

Figure 10. Parallel conductors in two raceways. Image used courtesy of Lorenzo Mari

 

Example 3. A service lateral utilizes two conduits with 1 x N° 250 kcmil aluminum conductor per phase in each one. Determine the minimum size of the grounded service copper conductor in each conduit.

Answer: the procedure is similar to example 1.

   1. N° 250 kcmil, aluminum.

   2. Find the maximum unbalanced load from load calculations in Section 220.61.

   3. Determine the minimum conductor size required to handle the maximum unbalanced load.

   4. Table 250.102(C)(1) shows N° 2 AWG copper or N° 1/0 AWG aluminum. The minimum size must be N° 1/0 AWG for parallel grounded conductor connections.

   5. Compare the sizes in steps 3 and 4, and pick the largest of these conductors.

   6. Connect the largest of these conductors in each conduit.

Section 250.24(D)(3) This section covers the grounded conductors for 3-phase, 3-wire services connected in delta.

This rule applies to corner-grounded systems and requires an ampacity equal to or higher than the ungrounded conductors.

The grounded conductor is treated like a neutral in 120 V and 240 V single-phase systems. See Figure 11.

 

 

Figure 11. 120 V, 3-phase, corner-grounded delta. Image used courtesy of Lorenzo Mari

 

Section 250.24(D)(4) This section covers impedance grounded services. It requires connecting the impedance grounding conductor according to sections 250.36 or 250.187, as applicable.

Section 250.24(E) This section covers the grounding electrode conductor. The grounding electrode conductor connects the equipment grounding conductor, the service-equipment enclosure, and–in grounded systems–the grounded service conductor to the grounding electrode. Size the conductor according to Section 260.66. See the grounding electrode conductor in Figure 4.

Section 250.24(F) This section deals with the grounding connections in ungrounded systems. It requires the connection of a grounding electrode conductor to the grounding electrode at each service. Connect the grounding electrode conductor to a metal cage of the service conductors. This practice results in the grounding of the metal casings.

Use Table 250.66 for sizing the grounding electrode conductor.

 

Key Takeaways of Grounding Service-Supplied Alternating-Current Systems

• NEC Section 250.24 covers the grounding of service-supplied alternating current systems.
• An AC premises wiring must have a grounding electrode conductor linked to the grounded service conductor at each service.
• The grounded conductor cannot be grounded on the load flank of the service disconnecting means.
• In grounded systems with various disconnecting means, an unspliced bonding jumper must connect the grounded service conductor, the equipment grounding conductor(s), and the service-disconnect enclosure for each service disconnect.
• The grounded conductor minimum size comes from Table 250.102(C)(1).
• Ungrounded systems require the grounding of all metal casings.

 

Featured image used courtesy of Adobe Stock