The electrical load within most commercial buildings can be arranged into the following broad categories:
Whilst the design of lighting systems is outside the scope of this Guide, it should be noted that average lighting levels of approximately 500 lux would usually be considered satisfactory in a general office environment. Given the efficacy of modern fluorescent lamps, it can be shown that an average level of 500 lux can be achieved with an electrical input of approximately 18 w/m².
In some areas of a building are likely to have higher load requirements. For example speciality lighting such as:
The impact of these should be evaluated on a case by base basis.
These areas, when part of a large building, often represent a small fraction of the total area and therefore have a small or negligible effect on the overall electrical capacity required from the lighting system.
For broad planning purposes, taken over the whole of a large building a unit load of 18-20 watts/m² is quite reasonable.
Small power and user equipment would generally consist of items which would be plugged into socket outlets or may be permanently connected. Most of which is equipment not usually selected by the Building Services Designer but brought into the building by the occupants.
In modern office environments this is often a very controversial and exaggerated figure. Many users have an expectation of load which rarely, if ever, materialises.
The actual requirements vary widely throughout a building. From some areas having virtually no small power to other areas such as Financial Trading Rooms or Computer Rooms which have relatively high unit loading.
Across the whole of a typical commercial office building however, a design allowance for a connected load of 40 w/m² would generally be more than adequate for small power. If special areas exist with known loads, the higher load should be used for the areas affected.
For example, a large computer room a unit load of 450-500 watts/m² may be required and a dealing area in a financial organisation may require 90-100 watts/m².
Ventilating and air conditioning apparatus can represent 40-50% of the total building load in modern air conditioned buildings.
These loads should be determined by the project mechanical engineer and are affected by:
Typically, the cooling requirement is approximately 3 tons of refrigeration per 11000 ft². This would usually represent an electrical load of approximately 45-50 w/m² but must be checked for each project.
In electrically heated buildings the required load must similarly be calculated by the project mechanical engineer. Since the heating and cooling loads are not usually considered only the larger of the two should be used for system calculation.
The evaluation of lift requirements must be performed by a lift specialist who, on the basis of the building population, should determine the number, speed and capacity of lifts.
A large tall building may have several hundred kilowatts of lift equipment installed. Of particular interest to the electrical engineer however, is the fact that peak lift loads can be considered short time loads and their impact on overall building demand discounted (but not ignored).
The starting current of a large lift motor must be evaluated for potential effects on system voltage. Additionally, the nature of solid state speed controllers commonly used today may create disturbances on the distribution system, particularly when the lift load is a large percentage of system capacity as in a small building or when operating on a standby generator.
The figures quoted are based on a traditional services building.
It should be noted that applying an intelligent building methology and associated systems can reduced these figures considerably. For simplicity these are not detailed within this section.
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