Last month’s column covered the first two types of connected loads to which the demand factors in Table 220.84 apply. The third item under connected loads is divided into four parts. The connected loads for multifamily dwellings shall include the nameplate rating of all appliances that are fastened-in-place, permanently connected, or located to be on a specific circuit [220.84(C)(3)].
The optional calculation for a multifamily dwelling starts out the same as the standard method calculation specified in Part III of Article 220. General lighting loads and small-appliance branch circuit loads are calculated the same regardless of the method (standard or optional.) Where meeting Exception No. 1 in 210.52(F), a laundry branch circuit is not required. But if laundry branch circuits are installed, the loads also are calculated the same way. The two methods are not the same when it comes to appliances. With the standard method load calculation, when there are four or more fastened-in-place appliances, it is permissible to apply a 75 percent demand factor [220.53]. When applying the 75 percent demand factor to a multifamily dwelling, the four or more fastened-in-place appliances do not have to be in one unit. For example, a 4.5-kilowatt (kW), 240-volt (V) water heater will be installed in each unit of a four-unit multifamily dwelling. The water heaters are the only fastened-in-place appliance in this multifamily dwelling. One service will supply all four units. Using the standard method, what is the service load (after applying the demand factor) for fastened-in-place appliances? Although each unit has only one fastened-in-place appliance, there will be four fastened-in-place appliances on the service. Therefore, applying the 75 percent demand factor is permissible.
The total before applying the demand factor is 18,000 watts (W) (4,500 4 = 18,000). Next, multiply the load by the 75 percent demand factor in 220.53 (18,000 75 percent = 13,500). The service load (calculated by the standard method) for these appliances is 13,500W (see Figure 1).
With the optional method, do not apply the 75-percent demand factor to fastened-in-place appliances. Include the nameplate rating of all appliances that are fastened in place, permanently connected, or located to be on a specific circuit [220.84(C)(3)(a)]. This means that all fastened-in-place appliances, regardless of the number, must be included at 100 percent. For example, find the optional method service load calculation (before applying the Table 220.84 demand factor) for a 12-unit multifamily dwelling with each unit containing the following fastened-in-place appliances: a dishwasher rated 10 amperes (A) at 120V; a hp, 120V kitchen waste disposer, and a 4.5 kW, 240V water heater. The dishwasher has a load of 1,200 volt-amperes (VA) (10 120 = 1,200). The load of the kitchen waste disposer is 1,176 VA (9.8 120 = 1,176). The load of the water heater is 4,500W (4.5 1,000 = 4,500). The fastened-in-place appliance load for each unit is 6,876 VA (1,200 + 1,176 + 4,500 = 6,876). Since there are 12 units, multiply the appliance load by 12 (6,876 12 = 82,512). Because this is the optional method, do not apply the 75 percent demand factor to these fastened-in-place appliances. Before applying the Table 220.84 demand factor, the optional method service load calculation for these fastened-in-place appliances is 82,512 VA (see Figure 2).
Another difference between the standard and optional method is how household cooking equipment and clothes dryers are calculated. With the standard method, it is permissible to apply Table 220.55 demand factors to household electric ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances individually rated in excess of 1 kW. It is also permissible to apply Table 220.54 demand factors to electric clothes dryers. With the optional method, household cooking equipment and electric clothes dryers—that are not connected to the laundry branch circuit specified in 220.84(C)(2)—are added to the calculation at 100 percent of the nameplate rating [220.84(C)(3)(b) and (c)]. For example, what is the optional method service load calculation (before applying the Table 220.84 demand factor) for a 15-unit multifamily dwelling with each unit containing the following: a 12 kW, 240V household electric range and a 5 kW, 240V electric clothes dryer? The load of the range is 12,000 VA (12 1,000 = 12,000). The load of the dryer is 5,000 VA (5 1,000 = 5,000). The total load in each unit for the range and clothes dryer is 17,000 VA (12,000 + 5,000 = 17,000). Since there are 15 units, multiply the range and dryer load by 15 (17,000 15 = 255,000). Before applying the Table 220.84 demand factor, the optional method service load calculation for these ranges and dryers is 255,000 VA (see Figure 3).
The next item in 220.84(C)(3) falls into more than one category. A water heater must be added to the optional method load calculation at nameplate rating [220.84(C)(3)(d)]. Note that a water heater is listed separately in 220.82(C)(3)(d). But, a water heater is also fastened-in-place, permanently connected, and located on a specific circuit which is covered in 220.82(B)(3)(a). If the multifamily dwelling has water heaters, and they were included with the fastened-in-place appliances, do not include them here. If the water heaters were not included with the fastened-in-place appliances, add the nameplate rating of the water heaters to the general loads covered in 220.82(C).
Next month’s Code in Focus continues 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.