How to calculate diversity and maximum demand
To calculate the maximum demand by applying diversity, first you need to know the total connected load. The total connected load is all of the loads added together. Diversity might also be applied to individual items of equipment such as a cooker.
What is Total Connected Load?
Total connected load (or full load) is the sum of all loads added together, for example:
Load 1 + Load 2 + Load 3 = Total Connected Load.
What is Diversity?
Diversity is a factor applied as an allowance of load not likely to be used at the same time, for example:
If load 1 is only used 50% of the time and load 2 is only used the remaining 50% of the time,
10 Amps (Load 1) + 10 Amps (Load 2) = 20 Amps (Total Connected Load), then
20 Amps (Total Connected Load) x 0.5 (Diversity Factor @ 50%) = 10 Amps (Maximum Demand).
This method may be referred to as a usage factor (UF).
For another example a 32 Amp ring final circuit for standard 13 amp socket-outlets, every socket-outlet on the circuit are very unlikely to all be used at the same time, therefore the On-Site Guide offers a diversity factor calculation which maybe applied to this circuit.
The IET On-Site Guide offers some guidance on applying diversity to different types of loads and varying types of premises.
What is Maximum Demand?
Maximum demand is the load after applying diversity, for example:
Total Connected Load x Diversity = Maximum Demand.
How to Calculate Diversity
There are many ways to apply different diversity factors to different types of loads, fixed equipment or electrical circuits. Calculating diversity is not an exact science since there are multiple varying factors which must be considered when calculating diversity and maximum demand, these factors will vary from premises to premises.
It is also worth considering the life span of the electrical installation as should the way the installation is used change then the diversity factors applied previously may no longer be relevant.
As a very basic example to calculate diversity for a cooker in a dwelling, based on Appendix A of the OSG:
- Cooker = 8kW
- 8kW x 230v = 34.78 Amps
The first 10 Amps + 30% of the remaining load so, 10 Amps + 7.43 Amps = Total (after diversity) 17.43 Amps.
Diversity can be applied to a cooker as it is very unlikely that the oven, grill and all 4 hob rings are going to be heating all at the same time, once the oven or hob ring has reached its target temperature then it will switch itself off until the temperature drops.
Some types are load are not permitted to have diversity to be applied such as electric heaters and showers because it is either on or off.
The designer of an electrical installation might also take into account that certain circuits or loads within the installation are not likely to be used at the exact same time so they might apply a usage factor (UF) to those loads.
An example of this might be in a premises with a master water heater and a backup water heater, the backup water heater can only be used if the master water heater fails.
What diversity factor should i use? Although there is some guidance in the OSG unfortunately there is no single answer to this one, which is why applying diversity to connected loads can be time consuming and complicated, detailed knowledge of the electrical installation in question and experience is required to decide on which diversity factors to use and apply to the connected electrical loads.
A diversity factor defined by the following formula:
Diversity Factor = Total Connected Load / Actual Maximum Load
We have recognized this issue so have developed an electrical Diversity Calculator to assist with the calculation of diversity which consists of 3 different methods for calculating and applying diversity to electrical loads which include the OSG guide values, rule of thumb method, and a custom diversity factor with an additional usage factor which could be applied to the OSG method.
All information and calculations are intended only for guidance because it is impossible to specify the appropriate allowances for diversity for every type of installation and such allowances call for special knowledge of the installation and how it is being or likely to be used.