Calculating Floor Area (Electrical)

The general lighting load for a dwelling is based on the square footage of the dwelling. Here is how this is done.

First Floor Area

To estimate the total load for a dwelling, the occupied floor area of the dwelling must be calculated. Note in the residence plans that the first floor area has an irregular shape. In this case, the simplest method of calculating the occupied floor area is to determine the total floor area using the outside dimensions of the dwelling. Then, the areas of the following spaces are subtracted from the total area: open porches, garages, or other unfinished or unused spaces if they are not adaptable for future use, NEC 220.12.

Many open porches, terraces, patios, and similar areas are commonly used as recreation and entertainment areas. Adequate lighting and receptacle outlets should be provided for these areas.

For practicality, we had chosen to round up dimensions for the determining of total square footage and to round down dimensions for those areas (garage, porch, and portions of the inset at the front of the house) not to be included in the computation of the general lighting load. This produces a slightly larger result as opposed to being on the conservative side. Don’t be miserly with your measurements; rather, be generous. Figure 3-2 shows the procedure for calculating the total square footage of this residence.

Basement Area

Although the NEC in 220.12 tells us that unused or unfinished spaces not adaptable for future use do not have to be included in calculating the square footage of a dwelling, it makes sense to include some of these spaces.

Nearly all basements in homes today certainly could be considered as being adaptable for future use. A crawl space and most attics would not normally be considered as being adaptable for future use. This is a judgment call based on a close examination of the Plans and Specifications. In this residence, more than half of the total basement area is finished off as a recreation room, which certainly is considered a living area. The workshop area also is intended to be used.

To simplify the calculation for this residence, we will consider the entire basement as usable space and figure the basement square footage area as being the same as the area of the first floor.

The combined occupied area of the dwelling is found by adding the first floor and basement areas together:

First Floor     1616 ft2 (149 m2)
Basement     1616 ft2 (149 m2)
Total              3232 ft2 (298 m2)

Calculating Residential Loads (Basics)

When wiring a house, it is all but impossible to know which appliances, lighting, heating, and other loads will be turned on at the same time. Different families lead different lifestyles. There is tremendous diversity. There is a big difference between “connected load” and “actual load.” Who knows what will be plugged into a wall receptacle, now or in the future? It’s a guess at best. Over the years, the NEC has developed procedures for calculating loads in typical one- and two-family homes.

The rules for doing the calculations are found in Article 220. For lighting and receptacles, the computations are based on volt-amperes per square foot. For the small-appliance circuits in kitchens and dining rooms, the basis is 1500 volt-amperes per circuit. For large appliances such as dryers, electric ranges, ovens, cooktops, water heaters, air conditioners, heat pumps, and so on, which are not all used continuously or at the same time, there are demand factors to be used in the calculations. Following the requirements in the NEC, the various calculations roll together in steps that result in the proper sizing of branch circuits, feeders, and service equipment.

Inch-pounds versus Metrics When Calculating Loads

Converting inch-pound measurements to metric measurements and vice versa results in odd fractional results. Adding further to this problem are the values rounded off when the NEC Code-Making Panels did the metric conversions. When square feet are converted to square meters and the unit loads are calculated for each, the end results are different—close, but nevertheless different. To show both calculations would be confusing as well as space consuming. Many of the measurements in this text are shown in both inch-pound units and metric units. Load calculations throughout this text use inch-pound values only, which is in agreement with the Examples given in Annex D of the 2011 NEC.

Basics of Wire and Loading

The NEC establishes some very important fundamentals that weave their way through the decisionmaking process for an electrical installation. They are presented here in brief form, and are covered in detail as required throughout this text.

The NEC defines a branch circuit as The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).* See Figure 3-1. In the residence discussed in this text, the wiring to wall outlets, the dryer, the range, and so on, are all examples of a branch circuit.

The NEC defines a feeder as All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch-circuit overcurrent device.* In the residence discussed in this text, the wiring between Main Panel A and Subpanel B is a feeder.

The ampacity (current-carrying capacity) of a conductor must not be less than the rating of the overcurrent device protecting that conductor, NEC 210.19 and NEC 210.20. A common exception to this is a motor branch circuit, where it is quite common to have overcurrent devices (fuses or breakers) sized larger than the ampacity of the conductor. Motors and motor circuits are covered specifically in NEC Article 430. The ampere rating of the branchcircuit overcurrent protective device (fuse or circuit breaker) determines the rating of the branch circuit. For example, if a 20-ampere conductor is protected by a 15-ampere fuse, the circuit is considered to be a 15-ampere branch circuit, NEC 210.3.

Standard branch circuits that serve more than one receptacle outlet or more than one lighting outlet are rated 15, 20, 30, 40, and 50 amperes. A branch circuit that supplies an individual load can be of any ampere rating, NEC 210.3.

If the ampacity of the conductor does not match up with a standard rating of a fuse or breaker, the next higher standard size overcurrent device may be used, provided the overcurrent device does not exceed 800 amperes, NEC 240.4(B). This deviation is not permitted if the circuit supplies receptacles where “plug-connected” appliances, and so on, could be used, because too many “plug-in” loads could result in an overload condition, NEC 240.4(B)(1). You may go to the next standard size overcurrent device only when the circuit supplies other than receptacles for cord-and-plug-connected portable loads.

For instance, when a conductor having an allowable ampacity of 25 amperes [see NEC Table 310.15(B) (16 )] 14 AWG Type THHN is derated to 70%:

25 x 0.70 = 17.5 amperes

It is permitted to use a 20-ampere overcurrent device if the circuit supplies only fixed lighting outlets or other fixed loads.

If the previous example were to supply receptacle outlets, then the rating of the overcurrent device would have to be dropped to 15 amperes; otherwise it is possible to overload the conductors by plugging in more load than the conductors can safely carry.

The allowable ampacity of conductors commonly used in residential occupancies is found in NEC Table 310.15(B)(16). This includes Type NM cable. It is required to be manufactured with 90°C insulated conductors. Typically, the insulation is Type THHN. As a result, the cable is limited to use in dry locations. See NEC 334.10(A)(1). NEC 334.80 allows the 90°C ampacity to be used for derating purposes so long as the final ampacity is selected from the 60°C column of NEC Table 310.15(B)(16).

The ampacities in Table 310.15(B)(16) are subject to correction factors that must be applied if high ambient temperatures are encountered—for example, in attics; see NEC Table 310.15(B)(2)(a).

Conductor ampacities are also subject to a derating factor if more than three current-carrying conductors are installed in a single raceway or cable; see NEC Table 310.15(B)(3)(a). See Chapter 18 for complete coverage of correction and derating factors.

Most general-use receptacle outlets in a residence are included in the general lighting load calculations, NEC220.14(J).

Receptacle outlets connected to the 20-ampere small-appliance branch circuits in the kitchen, dining room, laundry, and workshop are not considered part of the general lighting load. Additional load values must be added into the calculations for these receptacle outlets. This is discussed later in this text.

The minimum lighting load for dwellings is 3 volt-amperes per square foot. See NEC 220.12 and Table 220.12.

Continuous Loads

The NEC defines continuous load in Article 100 as a load where the maximum current is expected to continue for three hours or more.* Continuous loads shall not exceed 80% of the rating of the branch circuit. General lighting outlets and receptacle outlets in residences are not considered to be continuous loads.

Certain loads in homes are considered to be continuous and must be treated accordingly. Examples are electric water heaters (422.13), central electric heating [424.3(B)], snow-melting cables (426.4), and airconditioning equipment (440.32). For these loads, the branch-circuit rating, the conductors, and the overcurrent device shall not be less than 125% of the rating of the equipment. Mathematically, sizing the conductors and overcurrent device at 125% of the load is the same as loading the conductors and overcurrent device to 80%.

For example, an electric furnace with a nameplate rating of 40 amperes would require the supply conductors and overcurrent protection to be not less than

40 x 1.25 = 50 amperes