Branch-Circuit, Feeder and Service Calculations, Part XLII

As stated in the introduction, the purpose of the National Electrical Code (NEC) is the practical safeguarding of people and property from hazards arising from the use of electricity [90.1(A)]. Ensuring branch circuits, feeders and services are not undersized is one aspect of practical safeguarding. Requirements for calculating branch-circuit, feeder and service loads are provided in Article 220. Results from these calculations are then used to size conductors and overcurrent protective devices. While the size of the grounded or neutral conductor could be the same as the ungrounded (hot) conductors, it may be possible to reduce the size and still be in compliance with the National Electrical Code (NEC). Depending on the system and the load, the current draw on the neutral or grounded conductor could be significantly less than the current draw on the ungrounded conductors. While most of the requirements in Article 220 pertain to calculating loads on ungrounded conductors, one section pertains to calculating neutral loads. Provisions in 220.61 cover basic calculations, permitted reductions and prohibited reductions for feeder or service neutral loads. Because there are no neutral load calculation procedures in Part IV, Optional Feeder and Service Load Calculations, all neutral loads must be calculated in accordance with 220.61. Therefore, calculate the neutral load in accordance with 220.61 regardless of whether the feeder or service calculation was performed in accordance with the standard method or optional method.

Last month’s column concluded by covering grounded-conductor identification. This month, the discussion continues with feeder or service neutral loads as specified in 220.61.

The feeder or service neutral load is the maximum unbalance of the load determined by Article 220. As stated in 220.61(A), the maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor. When calculating the neutral load, it is not necessary to include loads that do not contribute to the neutral current. For example, a nondwelling occupancy has a calculated service load of 92 amperes on one phase and 88 amperes on the other phase. The loads have been balanced as evenly as possible. This calculated load already includes the noncontinuous and continuous loads at 100 percent and 125 percent, respectively. An electric water heater is one of the loads included in the load calculation. The water heater has a current draw of 19 amperes at 240 volts. The voltage at this panelboard will be single-phase, 120/240 volts. Since the water heater is 240 volts, it will not contribute to the neutral current. Therefore, 19 amperes can be omitted from the neutral load calculation. Subtract 19 amperes from the highest calculated load (92 – 19 = 73 amperes). Although the feeder conductors must have an ampere rating of at least 92 amperes, the conductor feeding the neutral must have an ampacity of at least 73 amperes (see Figure 1).

While the neutral conductor in the last example could be the same size as the ungrounded conductors, it could be smaller and still be Code-compliant. For example, what minimum size THHN copper conductors are required for the service in the last example? All conductor terminations are rated 75°C. Because of the 75°C terminations, select the allowable ampacity in the 75°C column of Table 310.16. The ungrounded conductors have a calculated load of 92 amperes. Therefore, the minimum size is 3 AWG copper for the ungrounded conductors. The neutral conductor must have an allowable ampacity of only 73 amperes. The minimum size required for the neutral conductor is 4 AWG (see Figure 2).

When calculating the feeder or service neutral load, it is permissible to apply a demand factor of 70 percent to household electric ranges, wall-mounted ovens, counter-mounted cooking units and electric dryers. A service or feeder supplying loads specified in 220.61(B)(1) or (2) shall be permitted to have an additional demand factor of 70 percent as determined by the basic calculation. For example, a clothes dryer will be installed in a one-family dwelling. What is the minimum neutral load on the service for a 5,500-watt clothes dryer? When calculating feeder or service neutral loads for clothes dryers, start by finding the feeder or service load. Because there is only one dryer, the minimum service load (at 100 percent) is 5,500 volt-amperes [Table 220.54]. Now, multiply the service load by 70 percent (5,500 × 70% = 3,850). The minimum neutral load on the service for a 5,500-watt clothes dryer is 3,850 volt-amperes (see Figure 3).

The demand factor for the neutral load may be in addition to any demand factors that may have already been applied. Demand factors for household electric clothes dryers are in Table 220.54, and demand factors for household electric cooking equipment are in Table 220.55. For example, a 10-unit apartment building will have a 12-kW range in each apartment. What load will these ranges add to a service neutral calculation? Start by looking in the left column of Table 220.55 for 10 appliances. The maximum demand for 10 12-kW ranges is 25 kW. The neutral load can be calculated by multiplying the maximum demand by 70 percent (25 × 70% = 17.5 kW). The neutral load for 10 12-kW household electric ranges is 17.5 kW (see Figure 4).

Where the calculated neutral current is more than 200 amperes, another reduction is permitted. Where the feeder or service is supplied from a three-wire DC or single-phase AC system; a four-wire, three-phase, three-wire, two-phase system; or a five-wire, two-phase system, it is permissible to apply a demand factor of 70 percent to that portion of the unbalanced load in excess of 200 amperes [220.61(B)(2)]. This demand factor is also in addition to any demand factors that may have already been applied. For example, after calculating the service load for an office building by the basic calculation, the neutral load is 216,000 volt-amperes. What is the neutral load after applying the demand factor from 220.61(B)(2)? The electrical service will be supplied by a 208Y/120-volt, three-phase, four-wire system. 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 (before the additional 70 percent demand factor) is 600 amperes. Find the amperes in excess of 200 amperes (600 – 200 = 400). Multiply 400 amperes by the 70 percent demand factor (400 × 70% = 280). Add 280 back to the original 200-ampere load (280 + 200 = 480). This office building has a neutral demand load of 480 amperes (see Figure 5).

Next month, the discussion of feeder and service load calculations continues.

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, and

About the Author

Charles R. Miller

Code Contributor

Charles R. Miller, owner of Lighthouse Educational Services, teaches custom-tailored seminars on the National Electrical Code and NFPA 70E. He is the author of “Illustrated Guide to the National Electrical Code” and “Electrician's Exam Prep Manual.”...

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