The evaluation of main electrical service requirements in a building should begin with an estimate of the likely load requirements, usually based upon a unit loading on a square metre basis. The unit loading used should be based upon a reasonable statistically determined figure from similar buildings.
When applying unit loads on a w/m² basis for initial design calculations, it is suggested that building gross area be utilised less known areas for lifts, shaftways, courtyards, etc. Area reduction for exterior wall thickness or columns is an unnecessary level of precision for load estimating. The difference between lettable areas and building gross areas has little impact on equipment selection unless unreasonably large unit loads are used for the calculations.
Minimum Design Load For Lighting & Small Power For Various Types Of Building
| Building | Minimum Capacity (w/m²) |
| Office | 60/40 |
| Schools | 30 |
| Residential | 30 |
| Hospitals | 25 |
| Hotels | 25 |
| Churches | 15 |
This summarises unit load estimates for various buildings for lighting and small power, to this figure should be added an estimate for mechanical apparatus such as lifts, air conditioning equipment, pumps, etc. These loading’s are to be considered recommended minimum requirements.
Measured Total Demand Load For Representative Sample of Large Office Building
| Building | Gross Area (m²) | Maximum Demand (mW) | Maximum Demand (W/m²) |
| A | 35,000 | 2.32 | 67 |
| B | 28,000 | 2.34 | 84 |
| C | 40,000 | 3.28 | 82 |
| D | 46,000 | 2.42 | 53 |
| E | 50,000 | 2.10 | 42 |
This table presents a summary of measured maximum operating
demand loads for modern high-tech
commercial office
buildings. Each building is fully air conditioned and contains
dealing facilities of various sizes some of which are as large
as 250 traders.
It is significant to note that the highest recorded demand is considerably below some recent user expectations of 15-20 watts/ft².