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

Last month’s Code in Focus covered multifamily-dwelling house loads as calculated in accordance with the optional method. This month, the discussion continues with the connected loads for multifamily dwellings as specified in 220.84(C)(4) of the National Electrical Code (NEC).

Where there are three or more units of a multifamily dwelling, it is permissible to calculate the load of a feeder or service by the optional method instead of the standard method covered in Part III of Article 220. If three conditions are met, it is permissible to apply a demand factor from Table 220.84 to the calculated loads covered in 220.84(C). Applying the Table 220.84 demand factor to house loads of multifamily dwellings is not permitted. House loads must be calculated in accordance with Part III of Article 220.

Find the total connected loads in accordance with 220.84(C), and then multiply the total by the demand factor for the number of dwelling units. For example, one service will supply power to a 24-unit multifamily dwelling. Each unit in this multifamily dwelling will have 1,050 square feet of floor area, two 20-ampere (A) small-appliance branch circuits, one 20A laundry branch circuit, fastened-in-place appliances rated 6,600 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 840 VA, a condenser fan motor rated 360 VA and electric heat rated 5,000 VA (see Figure 1).

The total connected loads for the 24-unit multifamily dwelling in Figure 1 is 890,160 VA (75,600 + 108,000 + 566,400 + 140,160 = 890,160). 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 24 units is 35 percent. The calculated load after applying the demand factor is 311,556 VA (890,160 35% = 311,556). Not considering any house loads, the optional method service load calculation for this 24-unit multifamily dwelling is 311,556 VA (see Figure 4).

Do not try to find a shortcut by dividing the service load calculation by the number of feeders. In this example, there are four feeders. By dividing the optional method service load calculation of 311,556 VA by four feeders (six units on each feeder), the load would be 77,889 VA (311,556 ÷ 4 = 77,889). By performing the load calculation with this incorrect method, the result is 20,029 VA (97,918 – 77,889 = 20,029), which is less than the correct load calculation.

Next month’s column will continue the discussion of 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.

#### Charles R. Miller

Code Contributor
Charles R. 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 NFPA’s “Electrical Reference.” He can be reached at 615....