The first edition of the National Electrical Code (NEC) was published in 1897 by the National Board of Fire Underwriters, which is now the American Insurance Association. Safety was the reason a group of 23 people met to develop a national set of rules for electrical construction and operation. In 1911, the National Fire Protection Association (NFPA) assumed sponsorship and control of the NEC.

The NEC is revised every three years, and since its inception, there have been 51 editions. Regulations have been added to the NEC over the years in an effort to minimize the risk of electrical shock and the potential ignition source of fires and explosions in electrical installations. As stipulated in the 2008 edition, the purpose of the Code is the “practical safeguarding of persons and property from hazards arising from the use of electricity” [90.1(A)]. Electrical hazards can occur because of overloading of wiring systems. Therefore, it is imperative to design and install electrical systems in accordance with requirements in the NEC. The design of the electrical system starts with branch circuits, feeders and services. Article 220 provides requirements for calculating branch-circuit, feeder and service loads.

Last month’s column concluded by covering the prohibited reductions for feeder and service neutral loads. This month, the discussion continues with optional feeder or service load calculations as specified in 220.80.

Parts III and IV of Article 220 provide calculation requirements for feeders and services. Although it is not officially titled as such, Part III is sometimes referred to as the standard method for feeder and service load calculations. Part IV, “Optional Feeder and Service Load Calculations,” contains optional or alternative load-calculation specifications. As stated in 220.80, optional feeder and service load calculations shall be permitted in accordance with Part IV. Sections in this part include 220.80 through 220.88. These sections provide optional calculation procedures for a dwelling unit (new and existing), two dwelling units supplied by a single feeder, a multifamily dwelling, a school, and a new restaurant.

220.80 Optional Method Dwelling Unit

Section 220.82(A) through (C) contains provisions for calculating feeder and service loads in dwelling units. While it is permissible to calculate a 120/240-volt or 208Y/120-volt, 3-wire feeder or service by the optional method, the result must be at least 100 amperes. If the result of the load calculation is not at least 100 amperes, the feeder or service load must be calculated in accordance with the method specified in Part III of Article 220. For example, a load calculation is needed to determine the size of a dwelling unit service. The minimum size as calculated by the optional method is 90 amperes. Since the result of the optional method calculation was not at least 100 amperes, the service must be calculated in accordance with the standard method. Using the same data, the minimum size as calculated by the standard method is 100 amperes. Therefore, the minimum size service for this dwelling is 100 amperes (see Figure 1).

The wording in this section does not mean that a service or feeder cannot be smaller than 100 amperes. It just means that if the feeder or service is less than 100 amperes, the load calculation must be performed in accordance with Part III of Article 220. For example, a 200-ampere service that was calculated by the optional method will be installed on the outside of a dwelling unit. A feeder supplying a panelboard will be installed inside the dwelling unit. The feeder size as calculated by the standard method is 90 amperes. The panelboard installed inside this dwelling can be supplied from a 90-ampere feeder.

There are two elements of optional feeder and service load calculations. The first element includes general loads specified in 220.82(B)(1) through (4). The second element includes heating and air-conditioning loads specified in 220.82(C)(1) through (6). Find the total calculated load by adding the loads from these two elements. There is no optional method for calculating neutral loads for feeder and service-entrance conductors. After calculating the service or feeder by the optional method, calculate the neutral load in accordance with the provisions in 220.61(A) through (C).

The first step of the optional-method load calculation is to multiply the calculated floor area of the dwelling by 3 volt-amperes per square foot. This will be the general lighting and general-use receptacle load. The floor area for each floor shall be calculated from the outside dimensions of the dwelling unit [220.82(B)(1)]. Regardless of the wall thickness, the floor area must be calculated from the outside dimensions. For example, the general lighting and general-use receptacle load is needed for a dwelling unit that will be calculated by the optional method. The inside dimensions, measured from inside wall to inside wall, are 25 by 40 feet. Each exterior wall measures 6 inches deep. Because the exterior walls must be included, the outside dimensions are 26 (25 + 0.5 + 0.5 = 26) by 41 (40 + 0.5 + 0.5 = 41) feet. The calculated floor area is 1,066 square feet (26 41 = 1,066). In accordance with 220.82(B)(1), multiply the calculated floor area by 3 volt-amperes per square foot. The minimum general lighting and general-use receptacle load for the dwelling in this example is 3,198 volt-amperes (1,066 3 = 3,198) (see Figure 2).

A few specified areas are not included in the calculated floor area. It is not necessary to include open porches, garages, or unused or unfinished spaces not adaptable for future use. For example, the general lighting and general-use receptacle load is needed for a dwelling unit that will be calculated by the optional method. The outside dimensions are 30 by 50 feet. This dwelling includes an open front porch that is within the boundaries of the outside dimensions. The front porch measures 5 by 8 feet. The total area, including the open porch, is 1,500 square feet (30 50 = 1,500). The porch’s area is 40 square feet (5 8 = 40). After deducting the area of the open porch, the calculated area is 1,460 square feet (1,500 – 40 = 1,460). After multiplying by 3 volt-amperes per square foot, the minimum general lighting and general-use receptacle load for this dwelling is 4,380 volt-amperes (1,460 3 = 4,380) (see Figure 3).

If a dwelling has more than one story, find the floor area of each floor separately and then add them together. Finish by multiplying the total floor area by 3 volt-amperes per square foot. For example, the service for a two-story dwelling will be calculated by the optional method. The outside dimensions of each floor are 30 by 40 feet. The outside dimensions of the garage are 18 by 28 feet. What is the general lighting and general-use receptacle load? The calculated floor area for the first floor is 1,200 square feet (30 40 = 1,200). Since the second floor is the same as the first floor, the total calculated floor area is 2,400 square feet (1,200 + 1,200 = 2,400). Although it is provided, do not include the area of the garage. After multiplying by 3 volt-amperes per square foot, the minimum general lighting and general-use receptacle load for this dwelling is 7,200 volt-amperes (2,400 3 = 7,200) (see Figure 4).

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, [email protected] and www.charlesRmiller.com.