Underlined text indicates a Code change.
Q. The neutral of a grounded electric utility service must be installed and terminated at what location in accordance with the NEC?
A. A service neutral conductor must be run from the electric utility power supply with the ungrounded conductors and terminate to the service disconnect neutral terminal [Sec. 250.24(C)]. A main bonding jumper [Sec. 250.24(B)] must be installed between the service neutral terminal and the service disconnect enclosure [Sec. 250.28].
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 protection device [Sec. 250.4(A)(3) and Sec. 250.4(A)(5)].
Even if the service equipment is only supplying loads with no neutral conductor, such as a three-phase motor, a service neutral conductor between the electric utility and service is part of the effective ground fault current path needed in case there is a ground fault condition.
Warning! Dangerous voltage from a ground fault won’t be removed from metal parts, metal piping, and structural steel if the service disconnect 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 protection device.
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 voltage for a 120/240V system.
Q. What does the Code require as a minimum size for the service neutral conductor?
A. Neutral Sizing for Single Raceway or Cable. [Sec. 250.24(C)(1)] 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 [Sec. 110.10 and Sec. 250.4(A)(5)]. This is accomplished by sizing the neutral conductor not smaller than specified in Table 250.102(C)(1), based on the cross‑sectional area of the largest ungrounded service conductor. In addition, the neutral conductors must have the capacity to carry the maximum unbalanced neutral current in accordance with Sec. 220.61.
Example: What’s the minimum size service neutral conductor required where the ungrounded service conductors are 350kcmil and the maximum unbalanced load is 100A?
Answer: 2 AWG is the minimum size service neutral conductor required [Table 250.102(C)(1)].
The unbalanced load of 100A requires a 3 AWG service neutral conductor, which is rated 100A at 75˚C in accordance with Table 310.15(B)(16) [Sec. 220.61], but the neutral conductor can’t be smaller than 2 AWG to carry fault current, based on the 350kcmil ungrounded conductors in accordance with Table 250.102(C)(1).
Neutral Sizing for Parallel Conductors in Two or More Raceways or Cables. [Sec. 250.24(C)(2)]. If service conductors are paralleled in two or more raceways or cables, a neutral conductor must be installed in each of the parallel raceways or cables. The size of the neutral conductor in each raceway or cable isn’t permitted to be smaller than specified in Table 250.102(C)(1), based on the cross‑sectional area of the largest ungrounded service conductor in each raceway or cable. In no case can the neutral conductor in each parallel set be sized smaller than 1/0 AWG [Sec. 310.10(H)(1)]. In addition, the neutral conductors must have the capacity to carry the maximum unbalanced neutral current in accordance with Sec. 220.61.
Example: What’s the minimum size service neutral conductor required for each of two raceways, where the ungrounded service conductors in each of the raceways are 350kcmil and the maximum unbalanced load is 100A?
Answer: The minimum size service neutral conductor required is 1/0 AWG per raceway [Table 250.102(C)(1) and 310.10(H)].
The unbalanced load of 50A in each raceway requires an 8 AWG service neutral conductor, which is rated 50A at 75˚C in accordance with Table 310.15(B)(16) [Sec. 220.61]. Also, Table 250.102(C)(1) requires a minimum of 2 AWG in each raceway, however, 1/0 AWG is the smallest conductor permitted to be paralleled [Sec. 310.10(H) and Table 310.15(B)(16)].
These materials are provided to us by Mike Holt Enterprises of Leesburg, Fla. To view additional Code training materials offered by this company, visit www.mikeholt.com.
