Last month’s article concluded with calculation requirements for new restaurants in accordance with the optional method. This month, the discussion continues with calculating farm loads in accordance with Article 220 in the National Electrical Code (NEC).

Although the farm load calculations are located after the optional method load calculations, farm load calculations are not part of the optional method load calculations. Farm loads shall be calculated in accordance with Part V of Article 220 [220.100]. Farms usually have a dwelling unit and one or more additional buildings or structures. Requirements for calculating dwelling units on farms are covered in 220.102(A). There are two methods for calculating services and feeders for one-family dwellings, multifamily dwellings and certain farm dwelling units. Procedures for the standard method are in Part III, and procedures for the optional method are in Part IV of Article 220. Optional method load calculation requirements for a dwelling unit are located 220.82. If the dwelling and farm loads are supplied by separate services, it is permissible to calculate the dwelling by either Part III or IV of Article 220. It is permissible to use either the standard method or the optional method unless the dwelling has electric heat, the farm has electric grain-drying systems, and both are on the same service. In accordance with 220.102(A), the dwelling unit must be calculated in accordance with the standard method located in Part III of Article 220 if the farm dwelling unit has electric heat and the farm has electric grain-drying systems where the dwelling and farm loads are supplied by a common service (see Figure 1).

 

Requirements for calculating farm loads, other than the dwelling, are covered in 220.102(B). Where a feeder or service supplies a farm building or other load that has two or more separate branch circuits, the load for feeders, service conductors and service equipment shall be calculated in accordance with demand factors not less than indicated in Table 220.102. There are many types of installations on farms where this section would apply. If the farm dwelling and farm loads are supplied by separate services, use this section to calculate the minimum size service for the farm loads. For example, a farm dwelling and farm building will be supplied by separate services. What is the minimum size service for the farm loads? The total ampere rating for all the farm loads (including simultaneously operated loads) will be 385 amperes (A). All the loads that are expected to operate simultaneously will have a total rating of 65A. The largest motor will be a 10 horsepower (hp), 240-volt (V), single-phase motor. The farm service will be supplied by a 120/240V, single-phase system. Calculate the service for this farm equipment in accordance with Table 220.102. The first step is to compare three loads and select the largest. All the loads that are expected to operate simultaneously will be 65A. In accordance with Table 220.102, multiply the full-load current of the largest motor by 125 percent. The full-load current for the 10-hp motor (from Table 430.248) is 50A. After multiplying by 125 percent, the rating is 62.5A (50 125% = 62.5). The third load is the first 60A of the total load. After comparing the three, the largest is 65A. This load is added to the calculation at 100 percent. The remaining farm loads have a rating of 320A (385 – 65 = 320). Multiply the next 60A by 50 percent. Also add 30A to the calculation (60 50% = 30). The remaining farm loads have a rating of 260A (320 – 60 = 260). Multiply the remaining loads by 25 percent. The remainder of all other loads multiplied by the demand factor is 65A (260 25% = 65). The total rating after applying the demand factors is 160A (65 + 30 + 65 = 160); this rating is not a standard rating. In accordance with 240.6(A), the next standard ampere rating higher than 160 is 200. The minimum size service for the farm loads in this example is 200A (see Figure 2).

 

If a feeder is needed to supply power to farm loads, use Table 220.102 to calculate the minimum size. If the feeder is fed from a service installed just for farm loads, use this section. Likewise, if the feeder is fed from a common service (farm dwelling and farm loads), use this section. For example, the farm equipment loads and the farm dwelling will be supplied by a common service. A feeder is needed to supply power to the farm equipment. What is the minimum size feeder for the farm loads? The total ampere rating for all the farm loads (including simultaneously operated loads) will be 140A. All the loads that are expected to operate simultaneously will have a total rating of 47A. The largest motor will be a 5-hp, 240V, single-phase motor. The farm service will be supplied by a 120/240V, single-phase system. Calculate the feeder for this farm equipment in accordance with Table 220.102. The first step is to compare three loads and select the largest. All the loads that are expected to operate simultaneously will be 47A. The full-load current for the 5-hp motor (from Table 430.248) is 28A. After multiplying by 125 percent, the rating is 35A (28 125% = 35). The third load is the first 60A of the total load. After comparing the three, the largest is the first 60A of the total load. This load must be added to the calculation at 100 percent. The remaining farm loads have a rating of 80A (140 – 60 = 80). Multiply the next 60A by 50 percent. Also add 30A to the calculation (60 50% = 30). The remaining farm loads have a rating of 20A (80 – 60 = 20). Multiply the remaining loads by 25 percent. The remainder of all other loads multiplied by the demand factor is 5A (20 25% = 5). The total rating after applying the demand factors is 95A (60 + 30 + 5 = 95). This rating is not a standard rating. In accordance with 240.6(A), the next standard ampere rating higher than 95 is 100. The minimum size feeder for the farm loads in this example is 100A (see Figure 3).

 

Next month’s column will continue the discussion of farm 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.