How to calculate the service for a single-family or a multifamily dwelling
The NEC defines a dwelling unit as a single unit that provides complete and independent living facilities for one or more persons that must include permanent provisions for living, sleeping, cooking, and sanitation (Fig. 1 on page xx). A dwelling becomes "multifamily" when it contains three or more dwelling units [Art. 100 Definitions] (Fig. 2 on page xx).
When you size the service for a single-family dwelling, you calculate the load and apply the appropriate demand factors. For a multifamily dwelling, you do the same thing, except you apply the appropriate demand factors to the sum of the individual dwelling units of that multifamily dwelling. You are allowed to use the standard method from Part III of Art. 220 or the optional method from Part IV of Art. 220.
Standard method. The same method used for single dwellings can be applied to multifamily dwellings. The NEC allows some additional demand factors for multifamily dwellings, on the presumption that there will be diversity of usage between the various units. For example, it's very unlikely that four families will run their clothes dryers, ranges, and small appliances at exactly the same time.
The following steps can be used to determine feeder and service sizes for a multifamily dwelling using the standard method contained in Art. 220, Part III:
Step 1: General Lighting, Small Appliance, and Laundry Demand [Table 220.42]
Step 2: Air-Conditioning versus Heat [220.51]
The larger of the air-conditioning load or the space-heating load.
Step 3: Appliance Calculated Load [220.53]
Nameplate ratings of all appliances (except heating, air-conditioning, cooking equipment, and dryers) are taken times a 75% multiplier if there are four or more on the feeder.
Step 4: Household Dryer Calculated Load [220.54]
Dryers are allowed the demand factors of Table 220.54, but this table does not allow less than 100% demand until there are five units or more. The 5kW minimum per dryer applies to all dwelling units [220.54]. A laundry circuit isn't required for an individual dwelling unit if the multifamily unit has common laundry facilities.
Step 5: Household Cooking Equipment Calculated Load [220.55]
Perhaps one of the most confusing tables in the NEC is Table 220.55 for household ranges. This table is confusing because the first two columns are percentage multipliers, while the third column is a final kVA value. The notes to this table further complicate matters. Be sure you study this table carefully and pay close attention to how to properly apply each column.
Step 6: Service Conductor Size [Table 310.16]
When sizing the service or feeder conductors for a single-family dwelling, you can use Table 310.15(B)(6), but that is not the case when sizing conductors for the service or feeder to a two-family or multifamily dwelling. For sizing those conductors, use Table 310.16 instead. Use Table 310.15(B)(6) for the feeders to an individual dwelling unit within the building.
Optional method. When should you use the optional method instead of the standard one? If you have the necessary information, you'll probably want to use the optional method, because it's faster and easier to calculate.
The optional method for multifamily dwellings is different from the one for single-family dwellings. That's because with multifamily dwellings, you apply demand factors in recognition of the diversity of usage of all the loads in all the separate units.
Let's make sure this is clear. In a single family unit, you have diversity among the various types of loads. Although you have that in multifamily units as well, you have diversity among the units that make up the multifamily dwelling — all of the families in a multifamily dwelling aren't using identical loads at identical times.
You can use the optional method [220.84] for multifamily dwelling unit feeder and service calculations only if each dwelling unit is equipped with electric cooking equipment and electric heating and/or air-conditioning, and is supplied by no more than one feeder. Follow these rules:
House loads are those not directly associated with the individual dwelling units of a multifamily dwelling. Some examples include landscape and parking lot lighting, hall and stairway lighting, common recreation areas, and common laundry facilities.
Follow these steps:
Let's look at these three steps in a bit more detail by walking through an example. In practice, you may see NEC-compliant variations of executing these steps.
Step 1: Determine total connected load [220.84(C)].
Add the following loads (from all the dwelling units) together, then apply the Table 220.84 demand factor:
A laundry circuit isn't required for an individual dwelling unit if the multifamily unit has common laundry facilities.
Step 2: Calculate the load.
Apply the demand factor from Table 220.84 to the total connected load (Step 1). You can convert the calculated load (kVA) to amperes by:
Single-Phase Formula: I = VA ÷ E
Three-Phase Formula: I = VA ÷ (1.732 x E)
Step 3: Size feeder and service conductors.
Size the ungrounded conductors per Table 310.16, based on the calculated load.
A 120/240V, single-phase system supplies a 12-unit multifamily building (Fig. 3 on page xx). Each 1,500 sq ft unit contains:
Dishwasher — 1.5kVA
Water Heater — 4.0kVA
Washing Machine — 1.2kVA
Dryer — 4.5kVA
Range — 14.4kVA
A/C (230V x 17A) — 3.91kVA
Electric Space Heating — 5.0kVA
Question: What size conductor is required if the service is rated 120/240V, single-phase and the conductors are installed in parallel in two separate raceways?
Step 1: Total Connected Load
Step 1a: Determine the General Lighting Load:
General Lighting
(1,500 sq ft x 3VA = 4,500VA)
Small-Appliance Circuits
(2 circuits x 1,500VA = 3,000VA)
Laundry Circuit (1,500VA)
4,500 + 3,000 + 1,500 = 9,000
9,000VA x 12 units = 108,000VA
Step 1b: Determine the Appliance Calculated Load:
Dishwasher (1,500VA)
Water Heater (4,000VA)
Dryer [nameplate] (4,500VA)
Range [nameplate] (14,400VA)
1,500 + 4,000 +4,500 + 14,400 = 24,400
24,400VA x 12 units = 292,800VA
Step 1c: Compare the Air-Conditioning versus Heat Load:
A/C = 3,910VA (omit)
Heat = 5,000VA x 12 units = 60,000VA
Step 2: Total Connected Loads
General Lighting, Receptacles (108,000VA)
Appliances Connected Load (292,800VA)
Heat (60,000VA)
108,000 + 292,800 + 60,000 = 460,800
Total Connected Load = 460,800VA
Total Calculated Load = Total Connected Load x Demand Factor [Table 220.84]
Total Calculated Load = 460,800VA x 0.41 [Table 220.84]
Total Calculated Load = 188,928VA
Step 3: Service Conductor Size (Fig. 4 on page xx)
I = VA ÷ E
I = 188,928VA ÷ 240V
I = 787A
Conductor size if paralleled in two raceways [240.4(B)]:
787A ÷ 2 raceways = 393A per conductor
Feeder/Service Conductors:
Parallel 600kcmil conductors rated 420A at 75ºC [Table 310.16] would meet these load requirements.
Grounding electrode conductor sizing. What size grounding electrode conductor is required if the service ungrounded conductors are 600kcmil with two conductors in parallel in two separate raceways?
600kcmil x 2 = 1,200kcmil (equivalent area of the ungrounded conductors) [Table 250.66, Note 1].
Over 1,100kcmil for ungrounded conductors requires a grounding electrode conductor of 3/0 AWG [Table 250.66].
Feeder installation. When installing feeders, include an equipment grounding conductor in each raceway. Section 250.118 lists allowable equipment grounding conductors.
To size this conductor using a wire-type equipment grounding conductor, go to Table 250.122 and select the equipment grounding conductor based on the overcurrent device protecting the conductors in the raceway [250.122(F)]. For instance, the equipment grounding conductor in each raceway of an 800A feeder, which is paralleled using two 600kcmil conductors per phase, will require a 1/0 AWG equipment grounding conductor in each raceway.
Two more samples. Working these two additional sample problems will reinforce what we've learned thus far.
Size the grounding electrode conductor.
Question: What size grounding electrode conductor is required if the service ungrounded conductors are 300kcmil with three conductors in parallel in three separate raceways?
300kcmil x 3 = 900kcmil equivalent area of the ungrounded conductors [Table 250.66, note 1]
900kcmil for ungrounded conductors requires a grounding electrode conductor of 2/0 AWG [Table 250.66]
Size the parallel service conductor.
Question: What size conductor is required if the service with a calculated load of 787A is rated 120/240V, single-phase and the conductors are installed in parallel in four separate raceways?
787A ÷ 4 raceways = 196.75A
3/0 AWG copper is rated 200A at 75ºC [Table 310.16], so four 3/0 AWG conductors can be paralleled for this service.
A word about two-family dwellings. The feeder for a two-family dwelling unit is calculated using the standard method in Part III of Art. 220. When that calculated load exceeds the calculation for three identical units using the optional method of 220.84, the lesser of the two calculations is permitted to be used [220.85].
Avoiding confusion. The sizing of branch circuits, feeders, and service conductors for multifamily dwellings is similar to the sizing for single-family dwellings. You size the feeders to individual dwelling units in the same manner, whether that dwelling unit is a single-family dwelling or an individual unit of an apartment building.
The NEC allows the use of Table 310.15(B)(6) for sizing the feeders or service conductors to an individual dwelling unit. However, to size the conductors that provide the service to a two-family or multifamily dwelling you must use Table 310.16.
Whether calculating the service for a single-family or a multifamily dwelling, be sure to follow the Code rules for the specific calculation you are working on and do not intermix the standard method with the optional method. Follow the steps outlined in this article and apply the demand factors allowed for each method carefully, and you will be successful.