The National Electrical Code (NEC) contains many provisions where calculations are involved. Understanding the purpose of these calculations, as well as how to perform them, is essential to individuals who are in the electrical industry. Passing a journeyman’s, master’s or electrical contractor’s exam may depend on a thorough knowledge of NEC calculations. While many articles have some provisions with calculations, Article 220, which is divided into five parts, is full of them.
Part I covers general requirements for load-calculation procedures. Calculation methods for branch-circuit loads are in Part II. Part III contains calculation methods for feeders and services. Although not official, Part III is sometimes referred to as the standard method. Part IV also contains load-calculation methods for feeders and services. This part contains optional or alternative load-calculation procedures for one-family dwellings, existing dwelling units, multifamily dwellings, two dwelling units supplied by a single feeder, schools, feeder or service loads for existing installations, and new restaurants. Although Part V provides calculation methods for farm loads, the feeder or service load of farm dwelling units must be calculated in accordance with the provisions for dwellings in Part III or IV of Article 220.
Last month’s column concluded by covering some of the prohibited reductions for feeder and service-neutral loads. This month, the discussion continues with feeder or service-neutral loads as specified in 220.61.
If the feeder or service is calculated by the standard method (Part III), the neutral load must be calculated in accordance with provisions in 220.61. Even if the feeder or service is calculated in accordance with the optional method (Part IV), the neutral load must be calculated in accordance with provisions in 220.61. This is because there is no optional method for calculating neutral loads.
Requirements in 220.61 are divided into three sections: basic calculations, permitted reductions and prohibited reductions. The first prohibited reduction (covered last month) applies to circuit arrangement or type of electrical system. The second applies to specific types of loads in a specific electrical system. The types of loads in this second prohibition are nonlinear loads. The term “nonlinear load” is defined in the NEC, and since it is used in more than one article, the fine print note (FPN) under the definition provides a little more insight; electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads (see Figure 1).
As previously discussed, where the feeder or service-neutral load exceeds 200 amperes, it is permissible to apply an additional demand factor of 70 percent to that portion of the unbalanced load in excess of 200 amperes. But, applying the 70 percent demand factor to reduce the neutral or grounded conductor’s capacity is not permissible for that portion consisting of nonlinear loads supplied from three-phase, 4-wire, wye-connected systems [220.61(C)(2)]. For example, after calculating the service load for an office building by the basic calculation, the neutral load is 216,000 volt-amperes. The major portion of the neutral load is fluorescent luminaires and information technology equipment. The electrical service will be supplied by a 208Y/120 volt, three-phase, 4-wire system.
After demand factors, what is the neutral load in amperes? First, convert volt-amperes to amperes. Since this is a 208-volt, three-phase system, the total voltage is 360 volts (208 × 1.732 = 360.256 = 360). Divide 216,000 volt-amperes by the total voltage of 360 volts (216,000 ÷ 360 = 600). The neutral load is 600 amperes before the application of demand factors. Since the major portion of the neutral load consists of nonlinear loads, applying the additional demand factor of 70 percent is not permissible. This office building has a neutral demand load of 600 amperes (see Figure 2).
As discussed last month, these prohibited reduction provisions are closely related to the neutral-conductor provisions in 310.15(B)(4)(b) and (c). The provisions in this section pertain to neutral conductors that must be counted as current-carrying conductors when applying the provisions of 310.15(B)(2)(a). On a 208Y/120 volt, three-phase, 4-wire circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral conductor shall, therefore, be considered a current-carrying conductor [310.15(B)(4)(c)]. For example, a raceway containing nine conductors will supply power to fluorescent luminaires that are rated 120 volts. Two three-phase multiwire branch circuits will supply power to the luminaires. There also is an equipment-grounding conductor in the raceway. A three-phase, 4-wire, wye-connected panelboard is supplying power to the luminaires. What is the Table 310.15(B)(2)(a) adjustment factor for these nine conductors? The six ungrounded (hot) conductors feeding the luminaires count as current-carrying conductors. Because of 310.15(B)(5), the grounding conductor does not count. Because the fluorescent luminaires are nonlinear loads on a 208Y/120 volt, three-phase, 4-wire circuit, the neutrals must be counted as current-carrying conductors. Because the neutral must be counted, there are eight current-carrying conductors. The Table 310.15(B)(2)(a) adjustment factor for eight current-carrying conductors is 70 percent (see Figure 3).
If the calculated load (in accordance with the provisions in 220.61) is less than the calculated load of the ungrounded (hot) conductors, it might be possible to reduce the size of the feeder or service-neutral conductor. While it may be permitted to install a neutral conductor that is smaller than the ungrounded conductors, it is not required. The installed neutral conductor can be the same size as the ungrounded conductors.
Caution is advised if the feeder or service has a small neutral load. If the conductor size is based on the calculated neutral load only, it could be smaller than the minimum required size. In accordance with 215.2(A), the size of the feeder-circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel. Before selecting the feeder-circuit grounded conductor, compare the minimum size required for the calculated load with the minimum size equipment-grounding conductor; then, select the larger of the two. For example, a 400-ampere panelboard (protected by a 400-ampere breaker) will be installed in an area within an industrial plant. The panelboard will be supplied by a 208Y/120 volt, three-phase, 4-wire system. The size of the ungrounded feeder conductors will be 500 kcmil copper. The calculated neutral load is 67 amperes. What is the minimum size conductor required for the neutral? Based on the calculated neutral load only, the minimum size conductor from Table 310.16 is 4 AWG. But, the size of this conductor is too small. The size equipment-grounding conductor specified in Table 250.122 for a 400-ampere overcurrent device is 3 AWG copper. Therefore, the minimum size neutral conductor is 3 AWG copper (see Figure 4).
There is a similar requirement for grounded conductors brought to service equipment. In accordance with 250.24(C)(1), the neutral conductor shall not be smaller than the required grounding-electrode conductor specified in Table 250.66. In addition, for service-entrance phase conductors larger than 1,100 kcmil copper or 1,750 kcmil aluminum, the grounded conductor shall not be smaller than 12½ percent of the area of the largest service-entrance phase conductor.
Next month’s column will continue the discussion of feeder and service load calculations.
MILLER, owner of Lighthouse Educational Services, teaches classes and seminars on the electrical industry. He is the author of “Illustrated Guide to the National Electrical Code” and “The Electrician’s Exam Prep Manual.” He can be reached at 615.333.3336, charles@charlesRmiller.com and www.charlesRmiller.com.