![]() ![]() There are certain loads associated with a multifamily dwelling that cannot be calculated in accordance with the optional method. See the September and October 2009 Code in Focus columns for a detailed explanation of 220.61. Neutral load calculations were previously covered in Code in Focus. Since there is no optional method load calculation for sizing neutral conductors, calculate the neutral load in accordance with provisions in 220.61. While there are two methods for calculating loads for ungrounded (hot) feeder and service conductors, there is only one method for calculating neutral loads. 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). 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. First, use the method provided in Part III of Article 220 to calculate the multifamily dwelling without electric cooking equipment. Two calculations are required when calculating in accordance with this exception. 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. There is an exception to the second condition in 220.84(A). 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).Īlthough not official, Part III is commonly referred to as the “standard method load calculation.” If all three conditions are not met, do not calculate the multifamily dwelling by the optional method. The third condition states that each dwelling unit must be equipped with either electric space heating or air conditioning, or both. The second condition states that each dwelling unit must be equipped with electric cooking equipment. The first condition in 220.84(A) states that no dwelling unit can be supplied by more than one feeder. There are three conditions that must be met in order to use the optional method load calculation for multi-family dwellings. 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). Dwelling unit also is defined in Article 100. In order to fully understand the term multi-family dwelling, the term dwelling unit must be defined. As defined in Article 100, a multifamily dwelling is a building that contains three or more dwelling units. 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. It is permissible to use load calculation provisions in Part IV instead of provisions in Part III. Part IV of Article 220 provides optional or alternative methods for calculating feeder and service loads. This month, the discussion continues with optional feeder or service load calculations for multifamily dwelling units as specified in 220.84. Last month’s Code in Focus concluded by covering optional feeder and service load calculation requirements for existing dwellings in 220.83. ![]() 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. Continuous loads also are important when sizing branch-circuit overcurrent-protective devices. 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. When sizing the branch circuit for fixed electric space-heating equipment, the load shall be considered a continuous load. The section shown in Table 220.3 for sizing the branch circuit for fixed electric space-heating equipment is 424.3. 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. This table provides the article as well as the section number (or part number) containing additional load calculation references. Thanks to Table 220.3, finding references for specialized applications is not difficult. Other references found elsewhere in the Code must be considered. Article 220 does not contain all the provisions that must be considered when calculating branch-circuits, feeders and services. This article contains provisions for calculating branch-circuit, feeder and service loads. ![]() Load calculation requirements are in Article 220 of the National Electrical Code (NEC). ![]()
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