For calculations required in Article 220, the total area of occupancy is determined from which dimensions of the building?

The correct answer is based on the need to assess the total area for electrical load calculations accurately according to Article 220 of the National Electrical Code (NEC). When determining the total area of occupancy, it is essential to use the exterior dimensions of the building. This approach ensures that all aspects of the building's footprint are accounted for, including areas where electrical systems may be needed but are not necessarily included in the usable or interior measurements.

Using the exterior dimensions captures the full extent of the structure, allowing for a comprehensive assessment of potential electrical demand and ensuring compliance with code requirements. This methodology helps in accurately calculating feeder and service requirements, which are critical for maintaining safety and efficiency.

Other options, like interior or usable dimensions, may omit certain areas critical to the overall load calculations, while focusing solely on the floor might lead to a misrepresentation of the total area that requires electrical services. Hence, utilizing the exterior dimensions facilitates a clearer understanding of the entire scope of occupancy that needs to be serviced.