Domestic Diversity Calculations: How to Use Appendix H

Calculating maximum demand for a domestic installation can produce some surprisingly large figures. Add every circuit together without diversity and an ordinary house can appear to need far more current than its 100 A supply could provide.

 

Applying the traditional circuit-by-circuit allowances in Appendix A of the IET On-Site Guide can reduce the result, but it may still leave you with a figure that looks unrealistically high. For certain individual household installations, Appendix H provides another approach: 100% of the largest circuit plus 40% of everything else.

 

That does not make Appendix H an automatic shortcut. Diversity is a design judgement, and the method can only be used when the installation meets the relevant conditions. Here is how to recognise those conditions, complete the calculation, and avoid the common mistakes.

What Maximum Demand and Diversity Mean

BS 7671 Regulation 311.1 requires the maximum demand of an installation to be assessed. Diversity may be taken into account when determining that demand.

 

Maximum demand is the greatest current the installation is reasonably expected to draw under normal service conditions. It is not necessarily the sum of every connected appliance, every circuit-breaker rating, or every theoretical load.

 

Diversity recognises that loads do not normally operate together at their full ratings. In a typical home:

• Heating appliances cycle through thermostatic controls.
• Lighting is switched off in unoccupied rooms.
• Cooking appliances do not use every element continuously.
• Socket circuits rarely carry their maximum possible current at the same time.
• Demand rises around breakfast and evening periods, then falls considerably overnight.

 

Key point: Diversity is not permission to ignore loads. It is an engineering assessment of how the installation is reasonably expected to be used.

 

The On-Site Guide offers useful methods, but it also makes clear that one formula cannot represent every installation. The designer must consider the connected loads, duty cycles, patterns of use and characteristics of the supply.

 

Why Adding Every Circuit Gives an Unrealistic Result

Suppose the design currents of all circuits in a typical dwelling add up to 170.5 A. If the property has a 100 A supply protective device, recording 170.5 A as its expected maximum demand would suggest the supply is substantially undersized.

 

The problem is that this total assumes every circuit reaches its design current simultaneously. It might assume full loading of several socket circuits while the cooker, lighting and every other connected load are also operating at their maximum.

 

That combination is possible in theory but unlikely to represent the installation’s normal peak demand.

 

Assessment	Maximum Demand
Total before diversity	170.5 A
Supply protective-device rating	100 A
Difference above supply rating	70.5 A

 

Remember: Add the circuit design currents, not simply every protective-device rating. A circuit protected by a 32 A device does not automatically have a 32 A design current, although that figure may be appropriate for a socket circuit capable of being loaded to the device rating.

 

The Appendix A Approach

Appendix A of the On-Site Guide provides recommended diversity allowances for different types of final circuit. Instead of applying one percentage to the whole installation, it treats circuit categories separately.

 

Typical examples include:

Circuit Type	Typical Diversity Treatment
Socket-outlet circuits	100% of the largest circuit plus a percentage of the remaining circuits
Lighting	A percentage of the connected lighting load
Cooking appliances	10 A plus 30% of the remainder, with an additional allowance where applicable
Water heaters	Treatment depends on capacity, control and expected use
Space heating	Often little or no diversity unless load control or operating conditions justify it

 

Applying the Appendix A recommendations to our example reduces the maximum demand from 170.5 A to 117.4 A.

 

Stage	Assessed Demand
Before diversity	170.5 A
After Appendix A allowances	117.4 A
Supply protective-device rating	100 A

 

The result is much lower, but it still exceeds the 100 A supply rating. That does not automatically prove the installation is unsafe, nor does it justify changing the answer until it fits. It means the designer should examine whether another recognised assessment method better represents the installation.

 

For more guidance on navigating these supporting tables, see The On-Site Guide: How to Use the Standard Final Circuit Tables.

 

When Appendix H Can Be Used

Appendix H of the On-Site Guide covers standard circuit arrangements for domestic and similar premises. Under the conditions identified in Appendix A, a different whole-installation diversity method may be used for an individual household installation whose circuits comply with the relevant Appendix H arrangements.

 

The method is:

 

Maximum demand = 100% of the largest circuit + 40% of all remaining circuits

 

Before using it, confirm that:

Check	Question to Ask
Type of premises	Is this an individual household installation?
Circuit arrangements	Do the circuits meet the applicable Appendix H arrangements?
Socket circuits	Do they comply with the relevant requirements of Table H2.1?
Known loads	Are there unusual or substantial loads requiring separate consideration?
Expected use	Does the resulting figure credibly represent the occupants’ demand?
Current edition	Are you using the tables and notes from the correct edition of the On-Site Guide?

 

Table and page numbers can change between editions, so navigate by the appendix and table title rather than relying only on a page number copied from older training material.

 

Important: Appendix H is conditional. Do not apply the 100% plus 40% formula to every installation simply because it produces a lower answer.

 

Appendix H Worked Example

Our example installation has a total design current of 170.5 A. Its largest circuit is a 32 A kitchen socket circuit.

 

The calculation has three steps.

 

Step	Calculation	Result
1	Identify the largest circuit	32 A
2	Subtract it from the total: 170.5 − 32	138.5 A
3	Find 40% of the remainder: 138.5 × 0.4	55.4 A

 

Now add the largest circuit back at 100%:

 

Maximum demand = 32 A + 55.4 A

 

Maximum demand = 87.4 A

 

Assessment Method	Result
No diversity	170.5 A
Appendix A circuit-by-circuit method	117.4 A
Appendix H method	87.4 A
Supply protective-device rating	100 A

 

The Appendix H assessment produces a maximum demand below the 100 A supply rating and may better reflect the expected peak demand of this qualifying household installation.

 

Key point: The figure has not been chosen because it is below 100 A. It is acceptable only if the Appendix H conditions apply and the designer considers the result realistic.

 

Keep the calculation with the designer’s records and certification documents. A clear calculation provides far better evidence than entering a figure on a certificate without recording how it was obtained.

 

Choosing the Largest Circuit

Only one circuit is included at 100% under this method. All other applicable circuits are included in the 40% remainder, even if another circuit has the same rating.

 

For example, if a dwelling has three 32 A socket circuits, one 32 A circuit is selected as the largest. The other two remain within the group assessed at 40%.

 

Choose the largest circuit by design current, not automatically by the number printed on the circuit-breaker. Where several circuits have the same design current, it is sensible to select the one expected to experience the most sustained loading.

 

A kitchen socket circuit is often a reasonable choice because appliances such as kettles, toasters, dishwashers and washing machines may operate around the same period. The decision must still reflect the actual installation.

 

When the Appendix H Method May Not Be Suitable

Modern homes can contain loads that were once uncommon. A simple whole-installation allowance may not properly represent an installation with substantial or sustained demand.

 

Loads requiring closer consideration can include:

Load	Why It Needs Attention
EV charge point	Can operate at high current continuously for several hours
Electric shower	Large instantaneous load with little opportunity for circuit-level diversity
Heat pump	Demand depends on operating mode, auxiliary heating and controls
Electric space heating	Several heaters may operate together during cold conditions
Battery storage system	Import and export modes can alter installation demand
Instantaneous water heating	High current may coincide with other household loads
Home workshop equipment	Use may differ substantially from an ordinary dwelling

 

Load limiting, energy management and manufacturer-defined operating controls may affect the assessment, but their operation must be properly understood and documented.

 

Smart-meter information can also provide useful evidence for an existing occupied property. Recorded peak demand does not replace design judgement, but it can help the designer compare a calculated figure with real usage.

 

Important: A low historical reading does not guarantee that future demand will remain low. Consider changes of occupant, new equipment and loads that were not operating during the monitoring period.

 

Maximum Demand and Circuit Protection

Maximum demand and overload protection are related, but they are not the same calculation.

 

Regulation 433.1.1 uses the familiar relationship:

 

Ib ≤ In ≤ Iz

 

Symbol	Meaning
Ib	Design current of the circuit
In	Rated current or current setting of the protective device
Iz	Current-carrying capacity of the conductor

 

This relationship is applied to an individual circuit. The diversified maximum demand calculation assesses the installation as a whole and helps determine whether the supply and distribution equipment are suitable.

 

You should not use diversity to reduce an individual circuit’s design current where its load is expected to operate fully. A 7.4 kW EV charge point, for example, may draw approximately 32 A for hours. Its circuit still needs to be designed for that load even though the installation’s overall maximum demand assessment may consider load management or coincident demand.

 

For a fuller explanation of the circuit relationship, read Circuit Design and Overload Protection: Revision Notes for the 18th Edition.

 

Percentage Calculations Made Simple

To find 40% of a number, convert the percentage to a decimal and multiply:

 

40% = 40 ÷ 100 = 0.4

 

Calculation	Answer
240 × 0.4	96
60 × 0.4	24
138.5 × 0.4	55.4

 

The same method works for other percentages:

Percentage	Decimal
30%	0.3
40%	0.4
50%	0.5
66%	0.66

 

Exam tip: Write the formula and each intermediate value before using your calculator. It reduces the chance of applying 40% to the full total instead of to the remainder.

 

If rearranging formulae or handling percentages slows you down, see Transposition of Electrical Formulae: The Maths You Need for the IET Exam.

 

Common Diversity Calculation Mistakes

Mistake	Why It Causes a Problem
Adding protective-device ratings without checking Ib	Device ratings do not always represent the actual circuit loads
Applying 40% to the complete total	The largest circuit must first be separated and included at 100%
Including every 32 A circuit at 100%	The Appendix H method uses 100% of one largest circuit
Using Appendix H without checking its conditions	The alternative method only applies to qualifying installations
Reducing the answer until it fits the supply	Diversity must be justified, not selected to produce a convenient result
Ignoring continuous or unusual loads	EV charging, heating and similar loads may materially change demand
Confusing installation diversity with circuit protection	Diversity does not remove the need to satisfy Ib ≤ In ≤ Iz
Using page numbers from an older guide	Tables, notes and page numbers may move between editions

 

Remember: A credible maximum-demand figure should be supported by the installation details, the selected method and the designer’s judgement.

 

Practice Maximum Demand Calculations

Maximum demand questions test more than arithmetic. You need to recognise which loads can reasonably be diversified, distinguish design current from protective-device rating, and find the correct supporting guidance quickly.

 

The 18th Edition Prep app includes practice questions covering BS 7671 calculations, circuit design and maximum-demand principles. Use the topic-specific quizzes to strengthen individual weak areas, then complete timed mock exams to practise finding regulations and completing calculations under exam conditions.

 

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