# Branch-Circuit, Feeder and Service Calculations, Part LVIII

This month, Code in Focus covers an optional feeder and service load calculation for two dwelling units that are supplied by a single feeder. There are two methods for calculating services and feeders for one-family dwellings and multifamily dwellings, and either is permissible. In Article 220, the procedures are covered in Part III for the standard method and in Part IV for the optional method. There also are two methods permitted for calculating loads in two dwelling units.

In accordance with 220.85, where two dwelling units are supplied by a single feeder and the calculated load under Part III of Article 220 exceeds that for three identical units calculated in accordance the provisions in 220.84, the lesser of the two loads shall be permitted (see Figure 1).

In accordance with 220.85, it is necessary to calculate by both methods and then select the lesser of the two loads. For example, a single feeder will supply two dwelling units. Each unit in this two-family dwelling will have 1,800 square feet of floor area, two 20-ampere (A) small-appliance branch circuits, one 20A laundry branch circuit, four fastened-in-place appliances with a total rating of 9,156 volt-amperes (VA), a range rated 12,000 VA and an electric clothes dryer rated 5,000 VA. The heating and air conditioning system in each unit will consist of a compressor rated 4,200 VA, a blower motor rated 1,176 VA, a condenser fan motor rated 360 VA and electric heat rated 10,000 VA (see Figure 2).

The total connected load for three units is 141,696 VA (16,200 + 13,500 + 78,468 + 33,528 = 141,696). After finding the total connected loads, apply the Table 220.84 demand factor for the number of dwelling units. The Table 220.84 demand factor for three units is 45 percent. The calculated load after applying the demand factor is 63,763 VA (141,696 45% = 63,763). As shown in Figure 3, the result of the standard load calculation was 66,180 VA. Since 66,180 exceeds that for three identical units calculated in accordance the provisions in 220.84, the calculated feeder load for these two dwellings is 63,763 VA (see Figure 5).

Performing the optional method load calculation for two dwelling units without performing the standard method load calculation is permissible, but the result could be larger than the standard method load calculation.

Next month’s column will continue 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.