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).

Using the connected loads in Figure 2, what is the optional method feeder load calculation for this two-family dwelling? Start by calculating the load under Part III of Article 220. Calculate the general lighting and general use receptacle load at 3 VA per square foot [220.12]. The general lighting and general use receptacle load for each unit is 5,400 VA (1,800 3 = 5,400). The general lighting and general use receptacle load for both units is 10,800 VA (5,400 2 = 10,800). Next, calculate the small-appliance and laundry branch-circuit load at 1,500 VA for each circuit [220.52(A) and (B)]. 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 both units is 9,000 VA (4,500 2 = 9,000). The total general lighting load, including small-appliance and laundry branch circuits, is 19,800 VA (10,800 + 9,000 = 19,800). Next apply the Table 220.42 demand factors to the general lighting load. The first 3,000 VA remain at 3,000 (3,000 100% = 3,000). This leaves 16,800 VA (19,800 – 3,000 = 16,800). Multiply the remaining 16,800 VA by 35 percent (16,800 35% = 5,880). The general lighting load for both units is 8,880 VA (3,000 + 5,880 = 8,880). The total fastened-in-place appliance load for both units is 18,312 VA (9,156 2 = 18,312). Because there are more than three fastened-in-place appliances, it is permissible to apply a demand factor of 75 percent to this load. After applying the Section 220.53 demand factor, the fastened-in-place appliance load is 13,734 VA (18,312 75% = 13,734). In accordance with 220.54, the electric clothes dryer load is 10,000 VA (2 5,000 100% = 10,000). The maximum demand for two 12,000 VA ranges from Table 220.55 is 11,000 VA. The heating load, with the blower motor, is 11,176 VA (10,000 + 1,176 = 11,176). This load is larger than the air conditioning load, and because of 220.60, it is permissible to use only the larger of the noncoincident loads. Therefore, the total heating load for both units is 22,352 VA (11,176 2 = 22,352). As required by 220.50 and 430.24, this calculation must include 25 percent of the largest motor. Since the compressor was omitted, the largest motor is the blower motor. Twenty-five percent of the largest motor is 294 VA (1,176 25% = 294). After applying all demand factors, the standard method load calculation for these two dwelling units is 66,180 VA (8,880 + 13,734 + 10,000 + 11,000 + 22,352 + 294 = 66,180) (see Figure 3).

The optional method load calculation for two dwelling units involves using the optional method load calculation procedures for multifamily dwellings, but not for two dwelling units. Perform the load calculation procedures in 220.84, but calculate these two dwelling units as if there were three identical units. 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 5,400 VA (1,800 3 = 5,400). The general lighting and general use receptacle load for three units is 16,200 VA (5,400 3 = 16,200). Remember, this optional calculation is based on three units, not two. 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 three units is 13,500 VA (4,500 3 = 13,500). In accordance with 220.84(C)(3), use the nameplate ratings for the fastened-in-place appliances, ranges and clothes dryers. The load for the fastened-in-place appliances, ranges and clothes dryers in each unit is 26,156 VA (9,156 + 12,000 + 5,000 = 26,156). The load for the fastened-in-place appliances, ranges and clothes dryers for three units is 78,468 VA (26,156 3 = 78,468). 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. In this example, the heating load is larger than the air conditioning load, therefore omit the air conditioning load. The heating load for each unit is 11,176 VA (10,000 + 1,176 = 11,176). The heating load for three units is 33,528 VA (11,176 3 = 33,528) (see Figure 4).

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