Learn the NEC rules on how to use the grounded neutral conductor as an equipment-grounding conductor.
Under certain conditions, you can ground all metal parts of enclosures used to install service equipment by the grounded neutral conductor on the supply side of the system. When you're using this type of grounding scheme, this includes the service weatherhead, service raceway, service meter base, and service equipment enclosure.
Using grounded neutral conductors on the supply side: Sec. 250 142(a)(1) through (a)(3). You can use the grounded neutral conductor as a current-carrying conductor and grounding means on the supply side of the service disconnecting means and supply side of a separately derived system as follows:
(1) On the supply side of service equipment disconnecting means.
(2) On the supply side of the main disconnecting means for separate buildings and structures, per Sec. 250-32(b).
(3) On the line side of the disconnect or overcurrent protection devices of a separately derived system per Sec. 250-30(a)(1).
Sizing grounded conductor for neutral current: Sec. 220-22. To control the maximum unbalanced load, you must compute the neutral load ampacity. Consider the neutral load (in amps) wherever you use a neutral in conjunction with one or more ungrounded phase conductors. On single-phase circuits using one ungrounded conductor and a neutral, the neutral will carry the same amount of current as the ungrounded phase conductor. However, since a two-wire circuit is rare, let's assume there is a neutral with two or more ungrounded phase conductors. If there are two ungrounded conductors using a shared neutral, the neutral must carry the total current in amps from both phase conductors; which is not an accepted practice.
Let's take a look at an example for clarification. Consider the ungrounded phase conductors supplying the main service panelboard in our example are 600kcmil THWN copper conductors (60 degreesC/75 degreesC terminals). You can determine the neutral conductor size by:
Step 1: Determine the load on each phase conductor. L1 = 112A, L2 = 110A, and L3 = 106A.
Step 2: Calculate the maximum unbalanced load. L1 = 112A.
Step 3: Select the proper size neutral conductor per Table 310-16. Here, 112A requires a No. 2 AWG conductor.
Sizing neutral to carry fault current: Sec. 250-24(b)(1) and (b)(2). When you ground an AC system (operating at less than 1000V), you must run the grounded neutral conductor to each service disconnecting means. The grounded neutral conductor size must not be less than requirements shown in Table 250-66. You must route this grounded neutral conductor with the phase conductors. If service conductors are larger than 1100kcmil (copper) or 1750kcmil (aluminum), you can't use Table 250-66. If the grounded conductor is larger than the above figures, the grounded neutral conductor size can't be smaller than 12.5% of the area of the largest phase conductor. If the conductors are paralleled, you must base the grounded neutral conductor size on the total circular mil area of the paralleled conductors per Sec. 250-24(b)(2) and Sec. 310-4.
Let's look at an example showing how to size an equipment-grounding conductor to clear ground-faults. The ungrounded phase conductors supplying the main service panelboard in our example are 600kcmil THWN copper conductors (60 degreesC/75 degreesC terminals). You determine the neutral conductor size by referring to Table 250-66. Here, No. 1/0 AWG THWN copper conductor was selected to meet Code.
Neutral tip. Sec. 310-15(b)(4)(c) requires you to increase the size of the neutral supplied by a 3-phase, 4-wire, wye-connected system if a major portion of the load consists of nonlinear loads saturated with harmonic currents. FPN 2 of Art. 220-Sec. 22 warns of this problem. Here, you may have to double the neutral in size or harmonic filters installed to correct the problem.