One-Family Dwelling Unit Load Calculations

Determine the total connected load for:

1. General lighting and receptacles at 3VA per sq ft [220.12],
2. Two small-appliance circuits each at 1,500VA [220.52(A)], and
3. One laundry circuit at 1,500VA [220.52(B)].

Next, apply the Table 220.42 demand factors to the total connected load. Since this is a dwelling unit, the first 3,000VA is applied at 100% demand and the remaining VA at 35% demand.

Step 2: Section 220.53 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% 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 (Fig. 2).

Step 3: 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) [220.54]. 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 (Fig. 3).

Step 4: Household cooking appliances rated over 1.75kW can have the feeder and service calculated per the demand factors of Table 220.55, including Notes 1, 2, and 3.

Step 5: 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% [220.50, 220.51].

Step 6: For one-family dwellings, service and feeder conductors supplied by a single-phase, 120/240V system can be sized using Sec. 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] (Fig. 1). To determine the feeder demand load for these circuits, use three steps:

Step 1: Determine the total connected load for:

1. General lighting and receptacles at 3VA per sq ft [220.12],
2. Two small-appliance circuits each at 1,500VA [220.52(A)], and
3. One laundry circuit at 1,500VA [220.52(B)].

Step 2: Apply the Table 220.42 demand factors to the total connected load.

Step 3: First 3,000VA at 100% demand; remaining VA at 35% demand.

The NEC requires a minimum of 3VA 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 (Fig. 4).

The 3VA 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 Sec. 220.14(J) for details.

Demand factors

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% 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 (1¾kW but less than 3½kW, 3½kW to 8¾kW, and 8¾kW to 12kW, respectively. Use these columns with Notes 1, 2, and 3 when determining demand loads.
3. Compare the air-conditioning load at 100% [220.50] to the heating load at 100% [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 Sec. 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% 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 Sec. 310.15(B)(7)(3).
• 310.15(B)(7) doesn’t apply to 3-wire service or feeder conductors connected to a 3-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% of the demand load as determined by Sec. 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 [Art. 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: For general loads [220.82(B)], the demand load must not be less than 100% for the first 10kVA, plus 40% of the remainder of the following loads:

1. 3VA per sq ft
2. 1,500VA 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: For air-conditioning versus heating [220.82(C)], include the larger of (1) through (6):

1. 100% of the nameplate rating of the air-conditioning and cooling unit
2. 100% of the nameplate rating of the heat pump, when it’s not supplemented by electric heating
3. 100% of the nameplate rating of the heat-pump compressor and 65% 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% of the nameplate rating of the electric space heating, if less than four separately-controlled units
5. 40% of the nameplate rating of the electric space heating, if four or more separately-controlled units
6. 100% of the nameplate rating of electric thermal storage and other heating systems

Step 3: For one-family dwellings, service and feeder conductors supplied by a single-phase, 120/240V system can be sized per Sec. 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: For general loads [220.83(B)], the demand load must not be less than 100% for the first 8kVA, plus 40% of the remainder of the following loads:

1. 3VA per sq ft
2. 1,500VA 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: Include the larger of

air-conditioning equipment versus space-heating [220.83(C)].

Step 3: For one-family dwellings, service and feeder conductors supplied by a single-phase, 120/240V system can be sized in accordance with Sec. 310.15(B)(7)(1) through (4).

Not so simple

Sizing a residential service is not 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 familiarize yourself with all of the requirements of Art. 220 so that 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.          

Holt is the owner of Mike Holt Enterprises, Inc. in Leesburg, Fla. He can be reached at www.mikeholt.com.