Every three years, the National Electrical Code (NEC) is revised, but the Code has only been on a three-year cycle since 1975. During the NEC’s 114-year existance, revision cycles have ranged from one year to four years. The first edition of the Code book was published in 1897. The newest NEC, the 2011 edition, is the 52nd edition. Look in the first few pages of the NEC for a complete list of all 52 editions. A lot has changed since the first edition, both in the electrical industry as well as in the Code book.
Protecting people and property from dangers that can come from the use of electricity has been the purpose of the Code from the beginning. Although the purpose was not stated in the beginning, it is in the Code now and has been for many years and many editions: “The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity [90.1(A)].” The wording of this first sentence in the NEC has remained exactly as it is today since the 1975 edition. When the wording was revised 36 years ago, the revision did not change the meaning. In accordance with 90-1(a) of the 1971 edition, “the purpose of this Code is the practical safeguarding of persons and of buildings and their contents from hazards arising from the use of electricity for light, heat, power, radio, signaling and for other purposes.” Although the first sentence in the 2011 edition did not change, there are a considerable number of changes. Starting this month, this series will reference the 2011 edition.
Last month’s Code in Focus concluded by covering feeder and service load calculations for multifamily-dwellings in accordance with the optional method. This month, the discussion continues with an optional feeder and service load calculation for two dwelling units that are supplied by a single feeder.
As with dwellings, there are two methods for calculating services or feeders for schools. Instead of using the provisions in Part III of Article 220, the calculation of feeder or service loads for schools shall be permitted in accordance with Table 220.86. While this optional load calculation method for schools is permitted, there are some stipulations. In accordance with 220.86, using the optional calculation method is only permissible if the school is equipped with electric space heating, air conditioning or both. Where the building or structure load is calculated by the optional method in 220.86, calculating feeders within the building or structure by the optional method shall not be permitted. If the building or structure load has been calculated by the optional method, it will be necessary to calculate the loads of feeders within the building or structure in accordance with requirements in Part III of Article 220. If, by using the provisions in Part III, the ampacity of an individual feeder is greater than the ampacity required for the entire building, it shall not be required that the ampere rating of the feeder be larger than the ampere rating of the service. Although many schools have portable classroom buildings, using the optional method to calculate loads to those portable classroom buildings shall not be permitted. Use requirements in Part III of Article 220 to calculate feeders to portable classroom buildings. There is no optional method for calculating neutral loads; therefore, neutral loads must be determined in accordance with 220.61.
This optional method calculation procedure for schools can be divided into four easy steps. The first step is to find the total connected load. The second step is to find the average volt-ampere (VA) per square foot. The third step is to apply the Table 220.86 demand factors. The fourth and final step is to multiply the derated volt-ampere per square foot by the square-foot area of the school. The result of fourth step will be the new derated feeder or service load. The connected load to which the demand factors of Table 220.86 apply shall include all of the interior and exterior lighting, power, water heating, cooking, other loads and the larger of the air conditioning load or space heating load within the building or structure. When calculating the total load of a school, use only 100 percent of the load. Do not multiply continuous loads by 125 percent. Do not derate receptacle loads or cooking equipment. Do not increase the largest motor by 25 percent. For example, using the optional method, what is the service load calculation for a school with a total connected load of 750 kilovolt-amperes (kVA) and a floor area of 30,000 square feet? This total connected load only includes the larger of the air conditioning or heating load within the building. In this example, the total connected load is provided and is 750 kVA (see Figure 1).
The second step is to find the average volt-amperes per square foot. To convert kilovolt-amperes to volt-amperes (VA), multiply kilovolt-amperes by 1,000; therefore, 750 kVA is 750,000 VA (750 1,000 = 750,000). To find volt-amperes per square foot, divide the total connected load by the total square-foot area of the school. In this example, the average volt-amperes per square foot is 25 (750,000 ÷ 30,000 = 25) (see Figure 2).
The third step is to apply the Table 220.86 demand factors to the average volt-amperes per square foot number that was calculated in the second step. Multiply the first 3 (volt-amperes per square foot) by 100 percent (3 100% = 3). In this example, multiply the next 17 by 75 percent (17 75% = 12.75). Note, if the average volt-amperes per square foot number is at least 20, these first two numbers will always be 3 and 12.75 respectively. The remainder is 5 (25 – 20 = 5). Next, multiply the remainder over 20 by 25 percent (5 25% = 1.25). After applying the demand factors in Table 220.86, the volt-ampere per square foot is 17 (3 + 12.75 + 1.25 = 17) (see Figure 3).
The fourth step is to multiply the derated volt-amperes per square foot by the square-foot area of the school. Multiply 17 by 30,000 to finish the calculation (17 30,000 = 510,000). Now, convert 510,000 VA to kilovolt-amperes (510,000 ÷ 1,000 = 510). The optional method service load calculation for this school is 510 kVA (see Figure 4).
Next month’s Code in Focus continues the discussion of optional 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.