The Wiring Regulations BS 7671 requires that the maximum demand of an installation is assessed. The purpose of the assessment is to ensure that the supply to the installation will be of sufficient current-carrying capacity for the purpose for which the installation is intended. An assessment of maximum demand requires that sufficient information is known about how the installation is likely to be used and any diversity factors that may be applied. Diversity factors, together with knowledge of the connected load, are used to calculate the maximum demand.

Maximum demand
The maximum demand of an installation is the maximum rate of consumption expressed in amperes, kW or kVA. It is generally taken as the average rate of consumption over a period of time, usually 30 minutes but in some cases 15 minutes or even an hour, depending on the tariff and electricity meter. Maximum demand does not include motor starting currents or other transient effects. Fault currents and overload currents are also excluded. Maximum demand in KW is relevant only for metering/tariff purposes. For the design of an installation, kVA or amperes must always be used.

Diversity factor

This is a factor which takes into account the:

• time profiles of electrical loads

• coincidence of electrical loads

• mechanical loading of electric motors

• seasonal changes in loading (particularly heating, lighting and cooling)

• any special requirements for the particular installation

A diversity factor may be described as a factor expressing the maximum demand as a fraction of the connected load.