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).

Using the connected loads in Figure 1, what is the optional method service load calculation for this 24-unit multifamily dwelling? For this example, the house load will not be included. Start by calculating the general lighting and general-use receptacle load at 3 VA per square foot [220.84(C)(1)]. The general lighting and general-use receptacle load for each unit is 3,150 VA (1,050 3 = 3,150). The general lighting and general-use receptacle load for all 24 units is 75,600 VA (3,150 24 = 75,600). Next, calculate the small-appliance and laundry branch circuit load at 1,500 VA for each circuit [220.84(C)(2)]. The small-appliance and laundry branch circuit load for each unit is 4,500 VA (1,500 3 = 4,500). The small-appliance and laundry branch circuit load for all 24 units is 108,000 VA (4,500 24 = 108,000). Now, calculate the load for the fastened-in-place appliances, ranges and clothes dryers. In accordance with 220.84(C)(3), use the nameplate ratings. The calculated load for the fastened-in-place appliances, ranges and clothes dryers in each unit is 23,600 VA (6,600 + 12,000 + 5,000 = 23,600). The load for the fastened-in-place appliances, ranges and clothes dryers for all 24 units is 566,400 VA (23,600 24 = 566,400) (see Figure 2).

In accordance with 220.84(C)(5), add to the load calculation the larger of the air conditioning load or the fixed electric space heating load. The air conditioning load for each unit is 5,400 VA (4,200 + 840 + 360 = 5,400). Since the blower motor also works with the heat, add the load of the blower motor (840 VA) to the heat load of 5,000 VA. The heating load for each unit is 5,840 VA (840 + 5,000 = 5,840). In this example, the heating load is greater than the air conditioning load; therefore, omit the air conditioning load. The heating load for all 24 units is 140,160 VA (5,840 24 = 140,160) (see Figure 3).

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).

Unless every unit in a multifamily dwelling is supplied by a separate service, it will be necessary to perform more than one load calculation. In fact, it may be necessary to perform three or more load calculations for the same multifamily dwelling. Multiple feeders may be installed to supply power to different floors or different buildings. The calculation will depend on the number of units on the feeder. If the number of units on the feeder is not the same as the number on the service, the multifamily dwelling must be recalculated. Do not just divide the service load calculation by the number of feeders. For example, four feeders will be used to supply power to the 24-unit multifamily dwelling in Figure 1. Each feeder will supply power to six units. Using the same connected loads from Figure 1, what is the optional method feeder load calculation for six units? The general lighting and general-use receptacle load for six units is 18,900 VA (3,150 6 = 18,900). The small-appliance and laundry branch circuit load for six units is 27,000 VA (4,500 6 = 27,000). The calculated load for the fastened-in-place appliances, ranges and clothes dryers for six units is 141,600 VA (23,600 6 = 141,600). The fixed electric space-heating load for six units is 35,040 VA (5,840 6 = 35,040). The air conditioning load is omitted because the heating load was larger. The total connected loads for a feeder supplying six units is 222,540 VA (18,900 + 27,000 + 141,600 + 35,040 = 222,540). The Table 220.84 demand factor for six units is 44 percent. The calculated load after applying the demand factor is 97,918 VA (222,540 44% = 97,917.6 = 97,918). Not considering any house loads, the optional method feeder load calculation for six units is 97,918 VA (see Figure 5).

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, [email protected] and www.charlesRmiller.com.