Article 220 of the National Electrical Code (NEC) provides requirements for calculating branch-circuit, feeder and service loads and is divided into five parts. Part I provides general requirements for calculation methods. Part II covers requirements for branch-circuit load calculations.

The branch-circuit load calculations can either be used as stand-alone calculations or combined with other load calculations from Article 220. Results from calculations in Part II can be used to size branch-circuit conductors and branch-circuit overcurrent devices. Results from Part II can also be incorporated with results from other sections in Article 220 to size conductors and overcurrent devices for feeders and services.

Feeder and service load calculation methods are in Part III. The load calculation methods in Part III are often referred to as the standard method load calculations. Part IV contains optional or alternative load calculation procedures for one-family dwellings, existing dwelling units, multifamily dwellings, two dwelling units supplied by a single feeder, schools, feeder and service loads for existing installations, and new restaurants. Optional feeder and service load calculations shall be permitted in accordance with Part IV of Article 220 [220.80]. Part V contains calculation methods for farms.

Last month’s column concluded with methods for calculating farm loads for other than dwelling units. This month, the discussion continues with calculating farm loads in accordance with Part V.

Farm load calculations are located after the optional method load calculations in Article 220. Farm loads shall be calculated in accordance with Part V of Article 220 [220.100]. If one service supplies the farm dwelling and a separate service supplies power to the farm equipment building(s), calculate the loads in accordance with 220.102(A) and (B). Where supplied by a common service, the total load of the farm for service conductors and service equipment shall be calculated in accordance with the farm dwelling unit load and demand factors specified in Table 220.103.

Use this calculation method to determine the minimum size for a service supplying power to a farm dwelling and also to the farm equipment. For example, one service will supply both a farm dwelling and farm equipment. What is the minimum size service for the following loads? All the loads that are expected to operate simultaneously will have a total rating of 60 amperes (A). The largest motor will be a 10 horsepower (hp), 240-volt (V), single-phase motor. Another load will be 50A. The remaining loads will be 92A. The total load for the dwelling, calculated in accordance with Part III in Article 220, is 110A. A 120/240V, single-phase system will supply the farm service.

As specified in Table 220.103, individual loads are calculated 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 motor load is the largest load. Therefore, multiply the motor load by 100 percent (62.5 100% = 62.5). The next largest load (all the loads that are expected to operate simultaneously) is 60A. The second largest load after applying the demand factor is 45A (60 75% = 45). The third largest load is 50A. The third largest load after applying the demand factor is 32.5A (50 65% = 32.5). The remaining load, after applying the demand factor, is 46A (92 50% = 46). The total load, after applying the demand factors, is 186A (62.5 + 45 + 32.5 + 46 = 186).

As specified in the note under Table 220.103, add the total farm loads (after demand factors) to the farm dwelling unit load. In this example, the farm dwelling unit was calculated in accordance with Part III in Article 220 and the load is 110A. The minimum ampere rating for the dwelling unit and farm equipment is 296A (186 + 110 = 296). This minimum rating is not a standard rating in accordance with 240.6(A). The next standard ampere rating higher than 296 is 300. The minimum size service for the farm loads and dwelling unit load in this example is 300A (see Figure 1).

Where there is equipment in two or more farm equipment buildings or for loads having the same function, such loads shall be calculated in accordance with Table 220.102 and shall be permitted to be combined as a single load in Table 220.103 for calculating the total load [220.103]. For example, what is the minimum size service for a farm that will have the dwelling unit and four barns supplied by the one service? The total load for the dwelling, calculated in accordance with Part III in Article 220, is 147A. The barn loads are as follows: barn 1 = 103A; barn 2 = 84A; barn 3 = 72A; and barn 4 = 62A. A 120/240V, single-phase system will supply the farm service. Calculate the barn loads separately from the dwelling unit load. Multiply the barn loads by the demand factor percentages in Table 220.103. The largest load is 103A (103 100% = 103). The second largest load is 63A (84 75% = 63). The third largest load is 46.8A (72 65% = 46.8). The fourth largest load is 31A (62 50% = 31). The total load after applying the demand factors is 243.8A (103 + 63 + 46.8 + 31 = 243.8). Now add the total farm loads (after demand factors) to the farm dwelling unit load. The dwelling unit load is 147A. The minimum ampere rating for the dwelling unit and the four barns is 391A (243.8 + 147 = 390.8 = 391). This minimum rating is not a standard rating in accordance with 240.6(A). The next standard ampere rating higher than 391 is 400. The minimum size service for the farm loads in this example is 400A (see Figure 2).

There are two methods for calculating the farm dwelling load. Procedures for the standard method are in Part III and procedures for the optional method are in Part IV of Article 220. Where supplied by a common service, it is permissible to use either the standard method or the optional method for the farm dwelling unit unless the dwelling has electric heat and the farm has electric grain-drying systems. In accordance with the note under Table 220.103, 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.

There are different types of electrical installations on farms: dwelling and farm loads supplied by separate services and dwelling and farm loads supplied by a common service. If the dwelling and farm loads are supplied by separate services, calculate the dwelling by either Part III or IV of Article 220; calculate the farm loads in accordance with 220.102(B) and Table 220.102. If the dwelling and farm loads are supplied by a common service, calculate the loads in accordance with 220.103 and Table 220.103.

Begin by calculating the farm loads in accordance with Table 220.103. To determine the size of the common service, add the dwelling demand load to the farm demand load. Unless the farm dwelling has electric heat, the farm has electric grain-drying systems and the loads are supplied by a common service, it is permissible to use either the standard or optional method to calculate the dwelling. Regardless of whether the dwelling and farm loads are supplied by a common service or not, calculate feeders supplying power to farm buildings or other loads having two or more separate branch circuits in accordance with 220.102(B) and Table 220.102 (see Figure 3).

This concludes this long-running series on branch-circuit, feeder and service 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, [email protected] and www.charlesRmiller.com.