Load calculation requirements are in Article 220 of the National Electrical Code (NEC). This article contains provisions for calculating branch-circuit, feeder and service loads. Article 220 does not contain all the provisions that must be considered when calculating branch-circuits, feeders and services. Other references found elsewhere in the Code must be considered.
Thanks to Table 220.3, finding references for specialized applications is not difficult. This table provides the article as well as the section number (or part number) containing additional load calculation references. For example, while there is no specific provision in Part II of Article 220 for sizing a branch circuit that will feed fixed electric space-heating equipment, there is a reference in Table 220.3. The section shown in Table 220.3 for sizing the branch circuit for fixed electric space-heating equipment is 424.3. When sizing the branch circuit for fixed electric space-heating equipment, the load shall be considered a continuous load [424.3(B)]. In accordance with 210.19(A)(1), branch-circuit conductors supplying continuous loads shall have an allowable ampacity not less than 125 percent of continuous loads. Continuous loads also are important when sizing branch-circuit overcurrent-protective devices. In accordance with 210.20(A), where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.
Last month’s Code in Focus concluded by covering optional feeder and service load calculation requirements for existing dwellings in 220.83. This month, the discussion continues with optional feeder or service load calculations for multifamily dwelling units as specified in 220.84.
Part IV of Article 220 provides optional or alternative methods for calculating feeder and service loads. It is permissible to use load calculation provisions in Part IV instead of provisions in Part III. Section 220.84, in Part IV of Article 220, provides requirements for calculating the load of a feeder or service that supplies three or more dwelling units of a multifamily dwelling. As defined in Article 100, a multifamily dwelling is a building that contains three or more dwelling units. In order to fully understand the term multi-family dwelling, the term dwelling unit must be defined. Dwelling unit also is defined in Article 100. A dwelling unit is a single unit, providing complete and independent living facilities for one or more people, including permanent provisions for living, sleeping, cooking and sanitation (see Figure 1).
There are three conditions that must be met in order to use the optional method load calculation for multi-family dwellings. The first condition in 220.84(A) states that no dwelling unit can be supplied by more than one feeder. The second condition states that each dwelling unit must be equipped with electric cooking equipment. The third condition states that each dwelling unit must be equipped with either electric space heating or air conditioning, or both. If all three conditions are not met, do not calculate the multifamily dwelling by the optional method. If all three conditions are met, it is permissible to use the optional method load calculation procedures in Part III of Article 220 (see Figure 2).
Although not official, Part III is commonly referred to as the “standard method load calculation.”
There is an exception to the second condition in 220.84(A). By following the guidelines of this exception, calculating by the optional method is permissible even though each unit is not equipped with electric cooking equipment. Two calculations are required when calculating in accordance with this exception. First, use the method provided in Part III of Article 220 to calculate the multifamily dwelling without electric cooking equipment. Next, insert a virtual electric cooking equipment load of 8 kW per unit for each unit without electric cooking equipment and calculate by the optional method provided in Part IV. After calculating the multifamily dwelling by both methods, compare the results, and it will be permissible to select the lesser of the two loads (see Figure 3).
While there are two methods for calculating loads for ungrounded (hot) feeder and service conductors, there is only one method for calculating neutral loads. Since there is no optional method load calculation for sizing neutral conductors, calculate the neutral load in accordance with provisions in 220.61. Neutral load calculations were previously covered in Code in Focus. See the September and October 2009 Code in Focus columns for a detailed explanation of 220.61.
There are certain loads associated with a multifamily dwelling that cannot be calculated in accordance with the optional method. House loads shall be calculated in accordance with Part III of Article 220 and shall be in addition to the dwelling unit loads calculated in accordance with Table 220.84 [220.84(B)]. House loads include areas such as office, clubhouse, meeting rooms and fitness rooms and common areas. House loads also include loads such as swimming pool and spa equipment, hallway and stairway lighting, perimeter lighting for common areas, and parking lot lighting. When calculating a multifamily dwelling by the optional method, separate the house loads from the dwelling--unit loads. Calculate the dwelling units in accordance with 220.84 and calculate the house load in accordance with Part III of Article 220. After calculating both the house loads and the dwelling-unit loads, add the two together (see Figure 4).
Table 220.84 contains demand factors for three or more multifamily dwelling units. Sometimes demand factors from this table are needed during master and journeyman electrical exams (but not always) to calculate the total load of a multifamily dwelling. Sometimes demand factors from Table 220.84 are needed during electrical exams to calculate the load by the optional method for a certain number of electric clothes dryers. For example, what is the service demand load for a 15-unit multifamily dwelling where each unit will contain a 5-kW electric clothes dryer? Calculate the service demand load by the optional method. First, find the total kilowatt load for the 15 5-kW clothes dryers. Before applying the demand factor, the load is 75 kW (5 15 = 75). Because this question specifies the optional method, use Table 220.84. The demand factor for 15 units (or in this case, clothes dryers) is 40 percent. Finally, multiply the clothes dryer load by the demand factor (75 40 percent = 30). The answer to this sample electrical exam question is 30 kW (see Figure 5).
Next month’s Code in Focus continues 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.