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)