2011 Changes to the NEC – 250.24

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250.24(C) Grounded Conductors Brought to Service Equipment

Several editorial revisions were made to this section.

250.24 Service Equipment—Grounding and Bonding.

(C) Grounded Conductor Brought to Service Equipment. A service neutral conductor from the electric utility must be routed with the ungrounded conductors and terminate to the service disconnecting means via a main bonding jumper [250.24(B)] that’s installed between the service neutral conductor and the service disconnecting means enclosure [250.28]. Figures 250–20 and 250–21

Figure 250-20 (Click on image to enlarge)

Figure 250-21 (Click on image to enlarge)

Author’s Comment: The service neutral conductor provides the effective ground-fault current path to the power supply to ensure that dangerous voltage from a ground fault will be quickly removed by opening the overcurrent device [250.4(A)(3) and 250.4(A)(5)]. Figure 250–22

Figure 250-22 (Click on image to enlarge)

Danger: Dangerous voltage from a ground fault won’t be removed from metal parts, metal piping, and structural steel if the service disconnecting means enclosure isn’t connected to the service neutral conductor. This is because the contact resistance of a grounding electrode to the earth is so great that insufficient fault current returns to the power supply if the earth is the only fault current return path to open the circuit overcurrent device. Figure 250–23

Figure 250-23 (Click on image to enlarge)

Author’s Comment: For example, if the neutral conductor is opened, dangerous voltage will be present on metal parts under normal conditions, providing the potential for electric shock. If the earth’s ground resistance is 25 ohms and the load’s resistance is 25 ohms, the voltage drop across each of these resistors will be half of the voltage source. Since the neutral is connected to the service disconnect, all metal parts will be elevated to 60V above the earth’s potential for a 120/240V system. Figure 250–24

Figure 250-24 (Click on image to enlarge)

To determine the actual voltage on the metal parts from an open service neutral conductor, you need to do some complex math calculations. Visit www.MikeHolt.com and go to the “Free Stuff” link to download a spreadsheet for this purpose.

(1) Single Raceway. Because the service neutral conductor serves as the effective ground-fault current path to the source for ground faults, the neutral conductor must be sized so it can safely carry the maximum fault current likely to be imposed on it [110.10 and 250.4(A)(5)]. This is accomplished by sizing the neutral conductor not smaller than specified in Table 250.66, based on the cross-sectional area of the largest ungrounded service conductor. Figure 250–25

Figure 250-25 (Click on image to enlarge)

Author’s Comment: In addition, the neutral conductors must have the capacity to carry the maximum unbalanced neutral current in accordance with 220.61.

Question: What’s the minimum size service neutral conductor required for a 480V, three-phase service installed in one raceway where the ungrounded service conductors are 500 kcmil and the maximum unbalanced load is 100A? Figure 250–26
(a) 3 AWG   (b) 2 AWG     (c) 1 AWG      (d) 1/0 AWG

Figure 250-26 (Click on image to enlarge)

Answer: (d) 1/0 AWG [Table 250.66]
The unbalanced load of 100A requires an 3 AWG service neutral conductor, which is rated 100A at 75ºC in accordance with Table 310.15(B)(16) [220.61]. Table 250.66 requires a minimum of 1/0 AWG based on 500 kcmil ungrounded conductors.

(2) Parallel Conductors in Two or More Raceways. If service conductors are paralleled in two or more raceways, a neutral conductor must be installed in each of the parallel raceways. The size of the neutral conductor in each raceway must not be smaller than specified in Table 250.66, based on the cross-sectional area of the largest ungrounded service conductor in each raceway. In no case can the neutral conductor in each parallel set be sized smaller than 1/0 AWG [310.10(H)(1)].

Author’s Comment: In addition, the neutral conductors must have the capacity to carry the maximum unbalanced neutral current in accordance with 220.61.

Question: What’s the minimum size service neutral conductor required for a 480V, three-phase service installed in two raceways where the ungrounded service conductors in each of the raceways are 350 kcmil and the maximum unbalanced load is 100A? Figure 250–27
(a) 3 AWG   (b) 2 AWG     (c) 1 AWG      (d) 1/0 AWG

Figure 250-27 (Click on image to enlarge)

Answer: (d) 1/0 AWG per raceway [Table 250.66 and 310.10(H)]
The unbalanced load of 50A in each raceway requires an 8 AWG service neutral conductor, which is rated 50A at 75ºC in accordance with Table 310.15(B)(16) [220.61]. Table 250.66 would require a minimum of 2 AWG, however, the smallest service neutral conductor permitted to be installed in parallel in each raceway must not be smaller than 1/0 AWG [310.10(H) and Table 310.15(B)(16)].

ANALYSIS:Several revisions were made to this section to make it an easier section to navigate. The Code now spells out what the rules are for sizing the grounded conductor when dealing with service conductors in a single raceway, as well as when parallel service conductors are installed in multiple raceways. A new subsection (3) was also added to clarify that the grounded conductor of a delta-connected, corner-grounded three-phase, 3-wire service must have an ampacity of no less than that of the ungrounded conductors.

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