In order for planning to proceed the design engineer must be able to confidently produce load estimates.
An evaluation of the main electrical services requirements in a building should begin with an estimate of the likely load requirements. This is usually based upon a unit loading on a square metre basis.
When applying unit loads on a W/m2 basis for initial design calculations, it is suggested that the gross area of the building be utilised, subtracting the known areas of lifts, shaft ways... Reducing areas to take account of the thicknesses of exterior walls or columns is an unnecessary level of precision for estimating loads.
The electrical load within most commercial buildings can be arranged into the following broad categories:
- Lighting.
- Small power.
- HVAC equipment.
- Lifts and escalators.
Lighting: The design of lighting systems is provided in the CIBSE Code for lighting and CIBSE/SLL Lighting Guides but for broad planning purposes, taken over the whole of a large office building,a unit load of 12-20 W/m2 is reasonable.
Small power: Usually consists of items which are plugged into socket outlets or permanently connected. The small power requirements vary widely throughout a building, from some areas having virtually no small power loads to other areas, such as computer rooms, which have a relatively high unit loading. It is of extreme importance that the engineer obtains details of all the connected equipment and includes an appropriate allowance for future expansion or load increase.
HVAC equipment: In modern buildings the load required for HVAC systems can represent 40-50% of the total building load. Such loads are affected by the nature of the building fabric, fresh air requirements and internal heat gains from lights, people and equipment. Typically, the electrical load resulting from a mechanically ventilated and cooled building could be in order of 40-50 W/m2, but requirements must be assessed for each project.
Lifts: The evaluation of lift requirements must be undertaken by a lift specialist. A large, tall building may have several hundred kilowatts of lift equipment installed. Of particular interest to the electrical engineer is the fact that peak lift loads can be considered short time loads and their impact on overall building demand discounted but not ignored.
It must be taken in consideration when designing, the harmonics that can be generated in an electrical system.
Harmonics are typically generated by inductive luminaries, computers, rotating machines, electronic speed controllers and uninterruptible power supplies.
Harmonic currents may cause distortion of the voltage wave form (notching) and high neutral currents. The notching effect on the voltage wave form can be particularly troublesome on small distribution systems and this can cause equipment malfunction due to line voltage conditions being outside accepted tolerances.
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