Multifamily Dwelling Unit Service and Feeder Calculations

 

By Mike Holt – taken from my NEC Exam Preparation textbook, Unit 10

Get the calculations right for multifamily dwelling units

The NEC defines a dwelling unit as a single unit that provides complete and independent living facilities for one or more persons. It must include permanent provisions for living, sleeping, cooking, and sanitation. A dwelling becomes “multifamily” when it contains three or more dwelling units.

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 Article 220 or the optional method from Part IV of Article 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 Article 220, Part III:

Step 1: General Lighting, Small Appliance, and Laundry Demand [Table 220.42]

• 3 VA per sq ft for general lighting and general-use receptacles [Table 220.12].
• 1,500 VA for each small-appliance circuit (minimum of 2 circuits) [220.52(A)].
• 1,500 VA for each laundry circuit [220.52(B)].

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 is 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 percent demand until there are five units or more. The 5 kW 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 this clear. In a single family unit, you have diversity among the various types of loads. You have that in multifamily units as well, but you also 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:

1. Use the demand factors of Table 220.84, based on the number of dwelling units.
2. Determine the feeder/service neutral calculated load per 220.61.
3. Calculate house loads for common areas per Article 220, Part III and then add them to the Table 220.84 calculated load.

House loads are those not directly associated with the individual dwelling units of a multifamily dwelling. Some examples are landscape and parking lot lighting, hall and stairway lighting, common recreation areas, and common laundry facilities.

Follow these steps:

1. Determine total connected load.
2. Calculate the load.
3. Size feeder and service conductors.

Let’s look at these three steps in a bit more detail, then walk 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:

•           3 VA per sq ft for general lighting and general-use receptacles.

•           1,500 VA for each small-appliance circuit (minimum of 2 circuits).

•           1,500 VA for each laundry circuit.

•           The nameplate rating of all appliances.

•           The nameplate rating of all motors.

•           The larger of the air-conditioning load or the space-heating load.

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:                      Three-Phase Formula:

I = VA/E                                               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.

Example problem

A 120/240V, single-phase system supplies a 12-unit multifamily building. Figure 10-13. Each 1,500 sq ft unit contains:

Dishwasher                             1.50 kVA

Water Heater                          4 kVA

Washing Machine                   1.20 kVA

Dryer                                       4.50 kVA

Range                                   14.40 kVA

A/C (230V)      17A

Electric Space Heating           5 kVA

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?

(a) 350 kcmil   (b) 400 kcmil  (c) 500 kcmil   (d) 600 kcmil

Answer: (d) 600 kcmil

Step 1: Total Connected Load

Step a:  Determine the General Lighting Load:

General Lighting (1,500 sq ft x 3 VA)                                  4,500 VA

Small-Appliance Circuits (2 circuits x 1,500 VA)     3,000 VA

Laundry Circuit                                                      + 1,500 VA

                                                                                    9,000 VAx 12 units =   108,000 VA

Step b:  Determine the Appliance Calculated Load:

Dishwasher                                                              1,500 VA

Water Heater                                                                       4,000 VA

Dryer (nameplate)                                                    4,500 VA

Range (nameplate)                                             + 14,400 VA

                                                                                    24,400 VA x 12 units = 292,800 VA

Step c:  Compare the Air-Conditioning versus Heat Load:

A/C VA = 230V x 17A,                                              3,910 VA (omit)

Heat                                                                         5,000 VA x 12 units =  60,000 VA

Step 2: Total Connected Loads

General Lighting, Receptacles                                   108,000 VA

Appliances Connected Load                                      292,800 VA

Heat                                                                          +  60,000 VA

Total Connected Load                                                460,800 VA

Total Calculated Load = Total Connected Load x Demand Factor [Table 220.84]

Total Calculated Load = 460,800 VA x 0.41 [Table 220.84]

Total Calculated Load = 188,928 VA

Step 3: Service Conductor Size Figure 10-14

I = VA/E

I = 188,928 VA/240V

I = 787A

Conductor size if paralleled in two raceways [240.4(B)]:

787A/2 raceways = 393A per conductor

Feeder/Service Conductors: Parallel 600 kcmil rated 420A at 75ºC [Table 310.16].

 

Grounding electrode conductor sizing

Question: What size grounding electrode conductor is required if the service ungrounded conductors are 600 kcmil with two conductors in parallel in two separate raceways?

(a) 4 AWG       (b) 1 AWG      (c) 3/0 AWG    (d) 4/0 AWG

Answer: (c) 3/0 AWG

600 kcmil x 2 = 1,200 kcmil equivalent area of the ungrounded conductors [Table 250.66, Note 1].

Over 1,100 kcmil 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 600 kcmil 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 300 kcmil with three conductors in parallel in three separate raceways?

(a) 4 AWG       (b) 2/0 AWG   (c) 3/0 AWG    (d) 4/0 AWG

Answer: (b) 2/0 AWG

300 kcmil x 3 = 900 kcmil equivalent area of the ungrounded conductors [Table 250.66, note 1]

900 kcmil 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?

(a) 1/0 AWG    (b) 3/0 AWG   (c) 500 kcmil   (d) 600 kcmil

Answer: (b) 3/0 AWG

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 Article 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.

 

Mike Holt’s Dwelling Unit Calculations textbook is an extraction from his NEC Exam preparation textbook. To purchase, please click here or call our office at 888.632.2633 for more information.