Diversity

Diversity in this context is all about how one establishes the maximum demand for an electrical installation or an electrical circuit. The guidelines shown here are some of those as presented in the On Site Guide. Note however that these are just that - guidelines. They are based on statistical probabilities and typical usage. As a circuit or installation designer you may choose to make different assessments of current demand based on your knowledge of the actual installation.

Note also that the figures presented here are for domestic properties only.

Allowances at point of use

The following guidelines are used when estimating the load for a particular final circuit, and shows how appliances and power points etc fed from the circuit should be accounted for:

Point of use / appliance	Current demand to be assumed
Other Socket outlets (i.e. not 2A or 13A)	Rated current of the socket
2A Sockets	at least 0.5A
Lighting outlet	Actual rating of lamp or 100W (whichever is greater) per lamp holder. For circuits (also) supplying discharge lighting (i.e. linear fluorescent tube lights, sodium / mercury vapour lamps, or any lighting technology that requires the use of a current limiting ballast) then use VA load rating if known, since this will allows for any ballast losses and the power factor. If the VA rating of the lighting units is unknown, then use the rated wattage of the actual tubes/lamps used multiplied by 1.8 to arrive at an estimate of the VA rating for the lamp and ballast. (you can omit this step if you know all the fittings you are using contain modern HF ballasts or power factor correction). e.g. 10 x 58W Single tube linear fluorescent lamps, gives a total real power consumption of 580W. Multiply by 1.8 to give 1044 VA, and hence 4.5A
Shaver socket, bell transformer, electric clock point, or any appliance under 5 VA	May be neglected for the purposes of assessment.
Cooking appliance	10A of total load plus 30% or remainder. Add additional 5A if cooker point socket is fitted.
All other fixed equipment	Use nominal current as specified on the appliance ratings plate.

Examples

A lighting circuit with 6 traditional 1 lamp fittings, plus a 500W Halogen flood lamp, 
would be treated as 6 x 100 + 500 = 1100W or 4.8A

A 2.6kW double oven, plus 6.8kW hob, and a cooker point with a fitted socket. Total load is 9.4kW or 41A peak
load. Hence the diverse load is 10A + 30% of 31A + 5A for the socket = 24.3A

Allowances for Diversity

The following table is typically used for when assessing the load of a group of circuits fed from a single distribution board or CU. Note that the total current value used is the total current after diversity has been applied - not just the nominal rating of the MCB or Fuse

Purpose of circuit	Diversity applicable (domestic)	Notes
Lighting	66% of total current
Heating and Power	100% of total demand up to 10A, + 50% of the remainder.	Where space heating is connected to a general purpose power circuit.
Cooking appliances	100% of total demand up to 10A, + 30% of the remainder.	Add a further 5A if a socket is fitted to the cooker point.
Water heaters (instant)	100% of first appliance, 100% of second, and 25% of any remaining appliances.
Water heaters (storage)	No diversity allowable
Under floor heating	No diversity allowable
Storage Heaters	No diversity allowable
Standard Circuits	100% of current demand of largest circuit. + 40% of demand for every other circuit.	These are standard final circuits as described in appendix 8 of the OSG

Example

A domestic consumer unit with the following circuits:

Circuit No.	MCB Nominal Rating	Circuit Function	Design current (Ib)	Diversity Allowance	Demand after diversity
1.	32A	Cooker (12kW)	23A	100%	23A
2.	32A	Ring 1 (kitchen)	32A	100%	32A
3.	32A	Ring 2 (downstairs)	32A	40%	13A
4.	32A	Ring 3 (upstairs)	32A	40%	13A
5.	16A	Immersion heater (3kW)	13A	100%	13A
6.	6A	Lights (1kVA)	4.3A	66%	3A
7.	6A	Lights (600VA)	2.6A	66%	2A
Total			148A		99A

You will note that while the final load including diversity is within the nominal capacity of a domestic 100A supply, the maximum load including individual circuit diversity alone is significantly in excess of it. This is an increasingly common situation that rarely seems to cause any problem in practice. It also worth noting that there are still many many properties with only 60A or 80A supplies rather than 100A.

Part of the cause of this apparent problem is the assumptions upon which the calculations are based. These are now somewhat outdated and don't reflect modern usage for many properties. The guidance in the OSG for example has remained unchanged for many years, and yet in that time we have seen:

• an "explosion" of use of small low load devices in the home;
• a large growth in the numbers of socket outlets deemed adequate
• The introduction of the 17th edition of the wiring regs which has tended to further increase the number of circuits used due to the increased reliance on RCDs and the need to maintain discrimination under fault conditions;
• a general fall in the numbers of houses that rely on electricity for extensive day time heating as oil or gas fired central heating has become almost ubiquitous.

Couple this to the reality that the incomer fuse will typically support quite significant overload for short durations, and the supply impedance in the majority of properties is high enough that there will be a small amount of load shedding[1] at high loads, and it becomes apparent why they are not fusing on a regular basis.

[1] Most properties will have a measurable supply impedance of a few fractions of an ohm. This will mean as the load climbs toward the maximum, the available voltage will sag. This has a small natural limiting effect of reducing current drawn by the majority of domestic appliances. e.g. a property with a 240V supply, and 0.2 ohm impedance, will drop 20V at full load, reducing the current demand by nearly 10A.

See Also

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