By Mike Holt, NEC Consultant

When making dwelling unit load calculations, use the correct demand factors.

Before you can size the branch circuits for a dwelling, you must calculate the loads. Article 220 allows two distinctly different calculation methods: the standard method in Part III and the optional method in Part IV. They typically give different results.

The standard method

Follow these steps to determine the feeder or service size for a dwelling unit using the standard method [Article 220, Part III]:

Use these six steps:

Step 1: General Lighting and General-Use Receptacles, Small-Appliance, and Laundry Circuits [Table 220.42]. The NEC recognizes that general lighting and receptacle, small-appliance, and laundry circuits won’t all operate at full load at the same time. So you can apply a demand factor to the total connected load [220.52].

Substep a: Determine the total connected load for:

(1) General lighting and receptacles at 3 VA per sq ft [220.12],

(2) Two small-appliance circuits each at 1,500 VA [220.52(A)], and

(3) One laundry circuit at 1,500 VA [220.52(B)].

Substep b: Apply the Table 220.42 demand factors to the total connected load.

Substep c: First 3,000 VA at 100 percent demand; remaining VA at 35 percent demand.

Step 2: Appliances [220.53]. This doesn’t apply to space-heating equipment [220.51], clothes dryers [220.54], cooking appliances [220.55], or air-conditioning equipment. You can apply a 75 percent demand factor when four or more appliances are fastened in place and are on the same feeder. A typical scenario is you have a sink disposal, dishwasher, trash compactor, and water heater. Apply the demand factor to the nameplate rating of the appliances.

Step 3: Clothes Dryer [220.54]. The feeder or service load for clothes dryers in a dwelling unit must be at least 5,000W (or the nameplate rating if greater than 5,000W). If there are more than four dryers in the dwelling unit, you can apply the demand factors from Table 220.54. A feeder or service dryer load isn’t required if the dwelling unit doesn’t have an electric dryer.

Step 4: Cooking Equipment [220.55]. Household cooking appliances rated over 1¾ kW can have the feeder and service calculated per the demand factors of Table 220.55, including Notes 1, 2, and 3.

Step 5: Air-Conditioning versus Heat. Because the air-conditioning and heating loads don’t run at the same time, you can omit the smaller of the two loads [220.60]. Calculate the air-conditioning load and the fixed electric heating load at 100 percent [220.50, 220.51].

Step 6: Feeder and Service Conductor Size. For one-family dwellings, service and feeder conductors supplied by a single-phase, 120/240V system can be sized using 310.15(B)(7).

General lighting and receptacle demand

The NEC recognizes that the general lighting and receptacle, small-appliance, and laundry circuits won’t be operating at full load at the same time. So it allows you to apply a demand factor to the total connected load [220.52]. To determine the feeder demand load for these circuits, use three steps:

Substep a: Determine the total connected load for:

(1) General lighting and receptacles at 3 VA per sq ft [220.12],

(2) Two small-appliance circuits each at 1,500 VA [220.52(A)], and

(3) One laundry circuit at 1,500 VA [220.52(B)].

Substep b: Apply the Table 220.42 demand factors to the total connected load.

Substep c: First 3,000 VA at 100 percent demand; remaining VA at 35 percent demand.

The NEC requires a minimum of 3 VA per sq ft for general lighting and general-use receptacles when doing branch circuit and feeder/service calculations. When determining the area, use the outside dimensions of the building; don’t include open porches, garages, or spaces not adaptable for future use.

The 3 VA per sq ft rule for general lighting includes all 15A and 20A general-use receptacles, but it doesn’t include the small-appliance or laundry circuit receptacles. See 220.14(J) for details.

Demand loads

You can apply demand factors to three types of loads:

1. Add the nameplate ratings of all appliances fastened in place. If there are four or more, apply a 75 percent demand factor [220.53]. This demand factor doesn’t apply to space-heating equipment [220.51], clothes dryers [220.54], electric ranges [220.55], or air-conditioning equipment.
2. When using Table 220.55 (cooking equipment), note that columns A, B, and C apply to different kW ranges (1¾ kW but less than 3½ kW, 3½ kW to 8¾ kW, and 8¾ kW to 12 kW respectively. Use these columns with Notes 1, 2, and 3 when determining demand loads.
3. Compare the air-conditioning load at 100 percent [220.50] to the heating load at 100 percent [220.51]. Use the larger of the two loads [220.60].

Sizing service conductors

For one-family dwellings, service and feeder conductors supplied by a single-phase, 120/240V system can be sized using 310.15(B)(7)(1) through (4).

Note the following:

• Service conductors supplying the entire load of a one-family dwelling can have an ampacity of 83 percent of the service rating.
• 310.15(B)(7)(2) can’t be used to size feeder conductors where a feeder doesn’t carry the entire load of the dwelling unit, except as permitted in 310.15(B)(7)(3).
• 310.15(B)(7) doesn’t apply to 3-wire service or feeder conductors connected to a three-phase, 120/208V system, because the neutral conductor in these systems always carries neutral current, even when the load on the phases is balanced [310.15(B)(5)(b)].

Use 70 percent of the demand load as determined by 220.55 to calculate the neutral load for household cooking appliances (e.g., electric ranges) and for electric clothes dryers [220.61(B)(1)].

Optional method

You can use the optional method [Article 220, Part IV] only for dwelling units served by a single 120/240V or 120/208V, 3-wire set of service or feeder conductors with an ampacity of 100A or larger [220.82].

Instead of six steps, there are only three:

Step 1: General Loads [220.82(B)]

The demand load must not be less than 100 percent for the first 10 kVA, plus 40 percent of the remainder of the following loads:

(1) 3 VA per sq ft

(2) 1,500 VA for each 20A small-appliance and laundry branch circuit specified in 220.52.

(3) The nameplate VA rating of all appliances and motors that are fastened in place (permanently connected) or located on a specific circuit, except heating and air-conditioning.

Be sure to calculate the range and dryer at their nameplate ratings!

Step 2: Larger of Air-Conditioning versus Heating [220.82(C)]

Include the larger of (1) through (6):

(1) 100 percent

(2) 100 percent

(3) 100 percent of the nameplate rating of the heat-pump compressor and 65 percent of the supplemental electric heating for central electric space-heating systems.

If the heat-pump compressor is prevented from operating at the same time as the supplementary heat, you can omit it in the calculation.

(4) 65 percent

(5) 40 percent

(6) 100 percent

Step 3: Service Size [310.15(B)(7)]

For one-family dwellings, service and feeder conductors supplied by a single-phase, 120/240V system can be sized per 310.15(B)(7)(1) through (4).

Existing Dwelling Unit

You can use the optional method to determine if the existing service or feeder is sufficient to serve additional loads.

Here are the three steps:

Step 1: General Loads [220.83(B)] The demand load must not be less than 100 percent for the first 8 kVA, plus 40 percent of the remainder of the following loads: (1) 3 VA per sq ft

(2) 1,500 VA for each 20A small-appliance and laundry branch circuit specified in 220.52.

(3) The nameplate VA rating of all appliances and motors that are fastened in place (permanently connected) or located on a specific circuit, except heating and air-conditioning.

Note: Be sure to calculate the range and dryer at their nameplate ratings!

Step 2: Larger of Air-Conditioning versus Heating [220.83(C)] Include the larger of air-conditioning equipment versus space-heating.

Step 3: Service Size [310.15(B)(7)]

For one-family dwellings, service and feeder conductors supplied by a single-phase, 120/240V system can be sized in accordance with 310.15(B)(7)(1) through (4).

Not so simple

Sizing a residential service isn’t simply a matter of adding up all of the loads or all of the circuit breaker sizes. The NEC provides two different calculation methods for residential service sizing: standard and optional. You should be familiar with both.

The NEC recognizes that typically not all loads will be used simultaneously, so there are several “demand factors” that come into play when sizing an electrical service. It’s important to be familiar with all of the requirements of Article 220 so you apply the proper demand factors to a given installation. There are many tables and requirements to follow, but take them one at a time and you’ll be able to complete these calculations.