The 2008 National Electrical Code (NEC) has been revised by declaring the appropriate procedure for calculating the load to size the neutral conductor. There has always been a controversy between designers on the application of the NEC concerning this calculation. By revising 220.61, as well as Example D3 in Appendix D, the NEC’s position on how this calculation must be performed has been clarified.
Service equipment supply side
On the supply side of the service equipment, the grounded neutral conductor can carry current during normal use and also be used as an equipment-grounding conductor (EGC) to clear a ground-fault per 250.142(A)(1). Therefore, 220.61 must be adhered to for the grounded (neutral) conductor to have the capacity to carry the neutral current. To also serve as the EGC and have the capability of carrying the available fault-current, Section 250.24(C)(1) must be complied with. Note that the larger of the two calculations must be used to select the appropriate size grounded-neutral conductor.
Calculating neutral load
To control neutral loading, designers must calculate the maximum unbalanced load in amps between the neutral and one or more of the ungrounded (phase) conductors.
Sections 210.19(A)(1), 215.2(A)(1) and 230.42(A)(1) require the ungrounded (phase) conductors per 310.12(C) to be calculated at 125 percent of the continuous load and added to the noncontinuous load. For example, a continuous load of 100 amps must be multiplied by 125 percent, and a 50-amp noncontinuous load will be added at 100 percent (100A × 125 percent + 50A × 100 percent = 175A). However, if a neutral load of 50 amps is present, a neutral load of 50 amps is used to size the neutral load (50A × 100 percent = 50A).
In the Appendix D of the 2008 NEC, the Example D3 (I’m paraphrasing) contains information that states the 125 percent rule is used to derate the loading of the overcurrent protection device by 80 percent of its rating (such as 175A × 80 percent = 140A or 125 percent × 140A = 175A). The busbar that the grounded (neutral) conductor connects to does not require a derating of 80 percent because the busbar is rated for 100 percent loading. Therefore, the application of the 80 percent rule is not necessary when sizing the grounded (neutral) conductor. When the neutral load carried on L1 is 50 amps, L2 is 47 amps, and L3 is 45 amps, the grounded (neutral) conductor must be selected and based on the maximum unbalanced load of 50 amps.
Sizing the neutral conductor
When an AC electrical system is grounded, the grounded (neutral) conductor must be routed to each service-disconnecting means as outlined in 250.24(C). To prevent heating problems, this conductor is run with the ungrounded (phase) conductors as required by 250.24(C)(1).
Section 250.24(C)(1) refers to Table 250.66 for sizing the neutral conductor when it is used as an EGC. For example, if the ungrounded (phase) conductors are 2/0 THWN copper conductors, and the grounded (neutral) conductor is to be used as an EGC, it is required to be a 4 AWG copper conductor per Table 250.66. Should the service conductor makeup exceed 1,100 kcmil for copper and 1,750 kcmil for aluminum, the 12½ percent rule applies, and the grounded (neutral) conductor must not be smaller than 12½ percent of the largest (phase) conductor(s). If the conductors are paralleled, the grounded (neutral) conductor size is based on the total circular mil area of the paralleled conductors as routed per 250.24(C)(2) and 310.4. Paralleled conductors run in two or more raceways must be selected and based on the larger service conductor in each raceway but under no conditions smaller than 1/0 AWG.
A feeder circuit supplying a number of dwelling units in an apartment complex has a neutral of load of 400 amps, and such a load can have a demand factor applied as outlined in 220.61(B)(2). For example, the first 200 amps are calculated at 200 amps (200A × 100 percent = 200A), and the remaining 200 amps are calculated at 140 amps (200A × 70 percent = 140A). This load is totaled to equal 340 amps (200A + 140A = 340A) for sizing the grounded (neutral) conductor. Let’s consider six 10-kW cooking units that are installed in individual dwelling units. Based on six 10-kW units, a demand of 21 kW per 220.61(B)(1) and Table 220.55, Column C can be applied for sizing the grounded (neutral) conductor.
Note that 310.15(B)(4)(c) requires an increase in the sizing of the neutral supplied by a three-phase, four-wire, wye-connected system when the major portion of the load consists of nonlinear loads saturated with harmonic currents. Fine Print Note 2 to 220.61 warns designers of this problem. As you can see, the calculation procedure to determine the size of the neutral depends on how it is used in the electrical system.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206.