250.24 Grounding and Bonding at Service Equipment Continued
(1) Minimum Size Grounded Neutral Conductor. Because the grounded neutral service conductor is required to serve as the effective ground-fault current path, it must be sized so that 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 grounded neutral conductor in accordance with Table 250.66, based on the total area of the largest ungrounded conductor. Figure 250.61
a€¢ A ground-fault cannot be cleared to remove dangerous voltage on the metal parts, metal piping, and structural steel if the service disconnecting means enclosure is not bonded to the grounded neutral service conductor. Figure 250.58
a€¢ If the grounded neutral service conductor is open, a ground fault cannot be cleared and metal parts will become and remain energized. Figure 250.59
In addition, if the grounded neutral conductor is opened, dangerous voltage will be present on metal parts under normal conditions, providing the potential for electric shock. For example: If the earth's ground resistance is 25Ω and the load's resistance is 25Ω, the voltage drop across each of these resistors would be 1/2 of the voltage source. Since the grounded neutral is bonded to the service disconnect, all metal parts will be elevated to 60V above the earth's potential for a 120/240V system. Figure 250.60
This dangerous condition is of particular concern in buildings containing pools, spas, or hot tubs.
To determine the actual voltage on the metal parts from an open grounded neutral service conductor, you need to do some complex math calculations with a spreadsheet to accommodate the variable conditions. Visit www.NECcode.com and go to the Free Stuff link to download a spreadsheet for this purpose.
The unbalanced load requires a 3 AWG grounded neutral service conductor, which is rated 100A at 75AºC in accordance with Table 310.16 [220.61]. However, the grounded neutral service conductor cannot be smaller than 1/0 AWG in accordance with Table 250.66 to ensure that it will accommodate the maximum fault current likely to be imposed on it.
In addition, the grounded neutral conductors must have the capacity to carry the maximum unbalanced neutral current in accordance with 220.61.
Question: What is the minimum size grounded neutral service conductor required for a 480V, three-phase service where the ungrounded service conductors are sized at 500 kcmil and the maximum unbalanced load is 100A? Figure 250.62
(a) 3 AWG (b) 2 AWG (c) 1 AWG (d) 1/0 AWG
Answer: (d) 1/0 AWG [Table 250.66]
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