The National Board of Fire Underwriters, which is now the American Insurance Association, published the first edition of the National Electrical Code (NEC) in 1897. In 1911, the National Fire Protection Association (NFPA) assumed sponsorship and control of the NEC. Regulations have been added in an effort to minimize the risk of electric shock and the potential ignition source of fires and explosions in electrical installations. As the 2011 edition stipulates, the purpose of the Code is the “practical safeguarding of persons and property from hazards arising from the use of electricity” [90.1(A)]. It is crucial to design and install electrical systems in accordance with requirements in the NEC to prevent overloading. 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.
This month’s column continues the discussion of determining existing loads in accordance with 220.87 with calculating feeder and service loads in accordance with the optional method.
As with dwellings and schools, there are two methods for calculating services or feeders for new restaurants. Procedures for the first load calculation method (sometimes referred to as the standard method) can be found in Part III of Article 220. The second method is the optional method, and procedures for this method are in Part IV. As 220.88 states, instead of using the provisions in Part III of Article 220, the calculation of a service or feeder load for a new restaurant shall be permitted in accordance with Table 220.88. This section also states that the overload protection of the service conductors shall be in accordance with 230.90 and 240.4. Table 220.88 is divided into three columns. The left column is the total connected load in kilovolt-amperes (kVA). Once the new restaurant’s total connected load has been calculated, select the row with the range of numbers that includes the total connected load. In accordance with the note under Table 220.88, the total connected load must include all electrical loads, including both heating and cooling loads (see Figure 1).
If the connected load is no more than 200 kVA and the new restaurant is all electric, multiply the calculated load by 80 percent. For example, what is the optional method service load calculation for a new restaurant with a total connected load of 180 kVA? This new restaurant will be all electric. When using the optional method, start by finding all the loads for the new restaurant. Include all loads at 100 percent: do not multiply continuous loads by 125 percent, do not include 25 percent of the largest motor, and do not apply the demand factors in Table 220.56 for commercial kitchen equipment. In this example, the total connected load is given. Because this new restaurant is all electric and the connected load is less than 200 kVA, multiply the connected load by 80 percent (180 80% = 144 kVA). The optional method service load calculation for this new restaurant is 144 kVA (see Figure 2).
If the total connected load is more than 200 kVA, an additional math step is required. For example, what is the optional method service load calculation for a new restaurant with a total connected load of 394 kVA? This new restaurant will be all electric. Start by looking in the “Total Connected Load (kVA)” column, and find the row that includes 394. Since 394 is between 326 and 800 kVA, it is in the third row. Follow this row across to the center column because this restaurant will be all electric. Next, calculate the amount over 325. The amount over 325 is 69 kVA (394 – 325 = 69). Now multiply the amount over 325 by 50 percent (69 50% = 34.5). Finally, add 34.5 to 172.5 kVA (34.5 + 172.5 = 207). The optional method service load calculation for this new restaurant is 207 kVA (see Figure 3).
Don’t make the mistake of thinking the service load calculation for the new restaurant in Figure 3 is 207A. It is 207 kVA. Because the result from Table 220.88 will be kilovolt-amperes, the minimum ampere rating for the service will depend on the voltage. For example, what is the minimum ampere rating for the service in Figure 3? A 208Y/120-volt (V), three-phase, 4-wire system will supply the service. Use Ohm’s Law to find current when volt-amperes (VA) and voltage are known. To find current, divide volt-amperes by the total voltage. In this system, the total voltage is 360V (208 1.732 or 120 3). To convert kilovolt--amperes into volt-amperes, multiply kilovolt-amperes by 1,000. As calculated by the optional method, the load is 207,000 VA (207 1,000 = 207,000). The total current for this new restaurant is 575A (207,000 ÷ 360 = 575). In accordance with 240.6(A), the next standard ampere rating higher than 575 is 600. The minimum size service for this new restaurant is 600A (see Figure 4).
When selecting service conductors for this new restaurant, the total combined ampacity per phase must be at least 575A. Where meeting the requirements in 310.10(H), the conductors can be installed in parallel. For example, two sets of 350 kcmil copper conductors can be installed in parallel. At 75°C, the allowable ampacity of one 350 kcmil copper conductor is 310A. A parallel set of 350 kcmil copper conductors has an allowable ampacity of 620A (310 2 = 620). Another option is to install three sets of 3/0 AWG copper conductors in parallel. At 75°C, the allowable ampacity of one 3/0 AWG copper conductor is 200A. Three sets of 3/0 AWG copper conductors that are in parallel have an allowable ampacity of 600A (200 3 = 600).
Table 220.88 is divided into two types of restaurants: all electric and not all electric. In the first two examples, the restaurants were all electric. Use the far right column if the restaurant is not all electric. For a new restaurant that is not all electric where the connected load is no more than 200 kVA, the calculated load must be taken at 100 percent. If the restaurant is not all electric and the connected load is more than 200 kVA, an additional math step is necessary. For example, what is the optional method service load calculation for a new restaurant with a total connected load of 455 kVA? This new restaurant will be supplied by natural gas and electricity. Because this restaurant is not all electric and the connected load is 455 kVA, the optional method formula is 45% (amount over 325) + 262.5. The connected load over 325 is 130 kVA (455 – 325 = 130). After multiplying 130 by 45 percent, the product is 58.5 kVA (130 45% = 58.5 kVA). After adding the derated kilovolt-amperes to 262.5, the sum is 321 kVA (58.5 + 262.5 = 321 kVA). The optional method service load calculation for this new restaurant is 321 kVA (see Figure 5).
Using this optional method to calculate a feeder is only permitted where the feeder serves the total load for a new restaurant. If using this optional method to calculate a feeder, feeder conductors shall not be required to be of greater ampacity than the service conductors [220.88].
Service or feeder conductors whose calculated load is determined by this optional calculation shall be permitted to have the neutral load determined by 220.61 [220.81].
Next month’s column 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.