The actual operating load rarely, if ever, equals the sum of all loads installed. The maximum operating load can be related to connected loads or to the sum of demand loads by the demand factor or the diversity factor.
To avoid confusion designers must always keep in mind actual operating conditions in an installation. An electrical distribution system can be broken down into groups of smaller systems or branches, successively connected together forming the whole network or tree. Each of these branches can contain smaller branches and ultimately, at the smallest branch, individual items of equipment.
The operating load existing at any location in a system at a given point in time is the sum of the loads downstream existing at that point in time.
Diversity occurs in an operating system because not all loads connected are operating simultaneously or are not simultaneously operating at their maximum rating.
The inverse of diversity factor is known as the coincident factor.
Examples of this occur in a building in the following manner:
The load is time dependant as well as being dependant upon equipment characteristics. The diversity factor recognises that the whole load does not equal the sum of its parts due to this time Interdependence (i.e. diverseness).
An example would be a conveyor belt made up of six sections, each driven by a 2 kW motor. As material is transported along this belt, it is first carried by section 1, then each section in succession until the final section is reached. In this simple example only one section of conveyor is carrying material at any point in time. Therefore five motors are only handling no-load mechanical losses (say .1 kW) keeping the belts moving whilst one motor is handling the load (say 1 kW). The demand presented by each motor when it is carrying its load is 1 kW, the sum of the demand loads is 6 kW but the maximum load presented by the system at any time is only 1.5 kW.
The diversity factor for this system is:
|∑Demand Loads||6 kW|
|Maximum Demand||1.5 kW|
The demand factor for this system is:
|Maximum Demand||1.5 kW|
|∑Connected Load||12 kW|
In commercial or industrial buildings lighting demand is often assumed to equal 100% of connected load.
However, some luminaires will not be operated. In installations provided with local switching (or occupancy sensing) lights in unoccupied areas may not be on. The extend of this diversity may be very small but nonetheless it exists.
For several years there has been a trend to anticipate the use of large quantities of electronic equipment in general office spaces. As a result design levels for small power loads have been rising.
Experience has shown however, that demand loads have not grown as rapidly as connected loads. This may be due to several factors, among which are:
Here again the design engineer must have an appreciation of realistic values for operating demand as a percentage of connected loads. Good record keeping and obtaining operational data from completed installations are useful tools to aid in gaining such an appreciation.
The capacity of air conditioning plant can be significantly affected by the small power loads. Reasonably accurate assessment of operating demand loads will enable an HVAC design engineer to properly select equipment for a project. Oversized equipment can be extremely difficult to control when operating at very small percentages of rated capacity. In some cases, as in Variable Air Volume systems, occupied spaces may be seriously overcooled even at minimum control settings.
In an operating system various loads comprising the system may have differing power factors.
When totalling loads at any point in the system this must be considered and loads added algebraically. It is often convenient for initial analysis to work in real load (kW) only.
When a building is electrically heated the heating load must be considered independently from any unit load design guidelines and generally added to the maximum demand calculations.
In an air conditioned building however, it is likely that maximum heating load will not occur simultaneously with cooling loads. Therefore, this diversity must be considered in determining maximum demand.
In general, all load evaluation is based upon nominal system voltage.
The designer should always be aware of the effects of voltage drop or supply variations.
Voltage drops within the range recommended by various codes and standards or Supply Company regulations will not usually present any problems.