A large number of calculation provisions are in the National -Electrical Code (NEC). Knowing where to find those calculation provisions and how to perform them is essential to electricians, engineers, inspectors and others involved in the electrical industry. Knowing how to execute calculations in the NEC could even be the difference between pass and fail on a journeyman or master electrician’s exam. Article 220 contains a large number of calculation provisions and provides requirements for calculating branch-circuit, feeder and service loads.
Last month’s article concluded by covering multifamily-dwelling house loads as calculated in accordance with optional method. This month, the discussion continues with the connected loads for multifamily dwellings as specified in 220.84(C).
Section 220.84(A) through (C) contains provisions for calculating feeder and service loads in multifamily dwelling units by the optional method. Where meeting the three conditions in 220.84(A), it is permissible to calculate the load of a feeder or service that supplies three or more dwelling units of a multifamily dwelling. The first step is to multiply the calculated floor area of the multifamily dwelling by 3 volt-amperes (VA) per square foot [220.84(C)(1)]. This will be the general lighting and general-use receptacle load. Although not specifically stated as such in this section, calculate the floor area for each floor from the outside dimensions of each dwelling unit that will be included in the multifamily dwelling calculation. After determining the total floor area for all the units, multiply by three. For example, an apartment building will contain 32 units. Each unit will have exactly the same floor area and the same connected loads. The calculated floor area for each unit is 1,050 square feet.
Using the optional method, what is the service load (before applying the demand factor) for general lighting and general-use receptacles? Because there are 32 units, multiply the floor area of each unit by 32 (1,050 32 = 33,600). The total calculated floor area is 33,600 square feet. Now, multiply the total calculated floor area by 3 VA per square foot (33,600 3 = 100,800). The connected load (before applying the demand factor in Table 220.84) for general lighting and general-use receptacles in this multifamily dwelling is 100,800 VA (see Figure 1).
The next provision covers small--appliance branch circuits and laundry branch circuits. This provision is usually broken down into two steps. In accordance with 220.84(C)(2), each 2-wire, 20A small-appliance branch circuit covered in 210.11(C)(1) must be included into the calculation at 1,500 VA each. Although there is no maximum number of small--appliance branch circuits, there is a minimum. In accordance with 210.11(C)(1), at least two 20A small-appliance branch circuits must be provided for all receptacle outlets required by 210.52(B).
In the kitchen, pantry, breakfast room, dining room, or similar area of a dwelling unit, the two or more 20A small--appliance branch circuits required by 210.11(C)(1) shall serve all wall and floor receptacle outlets covered by 210.52(A), all countertop outlets covered by 210.52(C), and receptacle outlets for refrigeration equipment [210.52(B)(1)]. Therefore, the minimum number of 2-wire, 20A small-appliance branch circuits is two. Either or both of these circuits also shall be permitted to supply receptacle outlets in the same kitchen and in other rooms specified in 210.52(B)(1). For example, each unit in a 32-unit apartment building will have two 20A small-appliance branch circuits. Using the optional method, what is the service load (before applying the demand factor) for small-appliance branch circuits? Since there are two small-appliance branch circuits and each must be included into the calculation at 1,500 VA, multiply 1,500 by 2 (1,500 2 = 3,000). The small-appliance branch-circuit load of each unit is 3,000 VA. Because there are 32 units, multiply the small-appliance branch-circuit load of each unit by 32 (3,000 32 = 96,000). Before applying the demand factor in Table 220.84, the small-appliance branch-circuit load for this multifamily dwelling is 96,000 VA (see Figure 2).
Also in accordance with 220.84(C)(2), each 2-wire, 20A laundry branch circuit covered in 210.11(C)(2) must be included into the calculation at 1,500 VA each. As stipulated in 210.11(C)(2), at least one additional 20A branch circuit shall be provided to supply the laundry receptacle outlet(s) required by 210.52(F). In dwelling units, at least one receptacle outlet shall be installed for the laundry [210.52(F)]. For example, each unit in a 32-unit apartment building will have one 20A laundry branch circuit. Using the optional method, what is the service load (before applying the demand factor) for laundry branch circuits? Because each 20A laundry branch circuit must be included into the calculation at 1,500 VA, multiply 1,500 by the number of units in the load calculation (1,500 32 = 48,000). Before applying the demand factor in Table 220.84, the laundry branch-circuit load for this multifamily dwelling is 48,000 VA (see Figure 3).
A laundry branch circuit will be installed in each unit of the apartment building in the last example, but this is not always required. There are two exceptions to the provision requiring a laundry receptacle. The first exception is only for multifamily dwellings. In a dwelling unit that is an apartment or living area in a multifamily building where laundry facilities are provided on the premises and are available to all building occupants, a laundry receptacle shall not be required [210.52(F) Exception No. 1]. The other exception pertains to two-family and multifamily dwellings. In other than one-family where laundry facilities are not to be installed or permitted, a laundry receptacle shall not be required [210.52(F) Exception No. 2]. In some multifamily dwellings, laundry facilities are provided on the premises and laundry branch circuits are provided in some, if not all, units. Where a 2-wire, 20A laundry branch circuit is installed, it must be included into the calculation at 1,500 VA each (see Figure 4).
Next month’s article 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.