Maximum Demand and Diversity Calculation Explained — BS 7671 On-Site Guide

What Is Maximum Demand?

Every electrical installation has a limit to how much current it can safely supply. Maximum demand is the term used to describe the greatest total load that an installation is expected to draw at any one time. Understanding how to calculate it is a fundamental skill for any electrician, whether you are wiring a new build or adding circuits during a renovation.

If you simply add up the current ratings of every circuit in an installation, you will almost certainly arrive at a figure that exceeds the capacity of the incoming supply. A typical domestic property with a 100 amp cut-out fuse might have circuits that total 140 amps or more on paper. Does that mean the supply is inadequate? Not necessarily — and that is where diversity comes in.

Why Diversity Matters

In practice, not every circuit in a dwelling will be operating at full load simultaneously. Your cooker, shower, ring finals, lighting, and panel heaters will not all draw their maximum rated current 24 hours a day, every day of the year. Diversity is the method of accounting for this real-world usage pattern, giving you a far more realistic estimate of the actual demand on the supply.

Not everything will be on 24 hours of the day, every day of the year, using its full load current — so we apply diversity, and we’ll see what that does to these values.

Without applying diversity, you may conclude that a perfectly adequate supply is insufficient, leading to unnecessary and costly upgrades. With diversity applied correctly, you can demonstrate that a proposed installation is well within the capacity of the existing supply.

The On-Site Guide Method — Appendix A

BS 7671 is the national standard for electrical installations in the United Kingdom, and it is accompanied by a series of Guidance Notes published by the IET. The On-Site Guide (sometimes referred to alongside Guidance Note 3) includes a straightforward method for estimating maximum demand with diversity applied, found in Appendix A.

This method is particularly useful for domestic installations and is one of the simplest approaches available. You can do it on your phone’s calculator while on site — no complex software or spreadsheets required.

The Two-Step Calculation

The method works as follows:

1. Take 100% of the largest circuit — identify the highest-rated circuit in the installation and use its full current rating.
2. Take 40% of the remainder — add up the ratings of all other circuits, then multiply the total by 0.4.

Finally, add the two figures together. The result is your estimated maximum demand with diversity applied.

A Worked Example

Consider a one-bedroom house renovation. The property has a 100 amp cut-out fuse and 25 mm² tails. The existing circuits are being retained (with satisfactory test results for insulation resistance and other parameters), and two additional 16 amp circuits are being added for electric panel heaters to replace the old Economy 7 storage heaters.

Old circuit arrangement — without diversity:

Adding up all existing circuit ratings gives a total of 140 amps. That already exceeds the 100 amp supply on paper.

Old circuit arrangement — with diversity applied:

• Largest circuit: 32 amps (100% = 32 A)
• Remainder of circuits: 108 amps (40% = 108 × 0.4 = 43.2 A)
• Maximum demand: 32 + 43.2 = 75.2 amps

New circuit arrangement (with two extra 16 A circuits) — without diversity:

The total rises to 172 amps — well beyond the 100 amp supply.

New circuit arrangement — with diversity applied:

• Largest circuit: 32 amps (100% = 32 A)
• Remainder of circuits: 140 amps (40% = 140 × 0.4 = 56 A)
• Maximum demand: 32 + 56 = 88 amps

We’ve added two circuits, and the maximum demand including diversity is 88 amps — so we can continue with our proposed plans.

With diversity applied, the maximum demand of 88 amps sits comfortably within the 100 amp supply. The renovation can proceed without needing a supply upgrade from the DNO.

When to Use This Method

This calculation is appropriate for domestic dwellings where the circuit arrangements are in accordance with Appendix H of the On-Site Guide — that is, typical domestic layouts with ring final circuits, dedicated cooker circuits, lighting circuits, and so on.

You should perform a maximum demand calculation whenever you are:

• Adding circuits to an existing consumer unit
• Replacing a consumer unit and incorporating existing and new circuits
• Completing an EIC or EICR — documenting the maximum demand is part of the certification process
• Assessing whether the existing supply is adequate for a proposed alteration

For more complex installations — commercial or industrial premises, or installations with large motor loads — different methods and more detailed calculations may be required, as outlined in the IET Guidance Notes and BS 7671 itself.

Key Points to Remember

• Always start with the largest circuit at 100%, not just any circuit
• Do not include the largest circuit again when calculating the 40% remainder — it has already been accounted for
• The 40% factor is a general domestic diversity allowance; other installation types may use different percentages
• If your calculated maximum demand exceeds the supply rating, you must either redesign the installation or arrange a supply upgrade with the DNO
• Document your maximum demand calculation as part of your electrical certification paperwork

Connecting This to Health and Safety

Understanding maximum demand is not just a technical exercise — it has direct implications for electrical safety. An overloaded supply can lead to overheating of cables and connections, nuisance tripping of protective devices, or in the worst case, electrical fires.

Under the Health and Safety at Work Act 1974 and the Electricity at Work Regulations 1989, duty holders have a legal obligation to ensure that electrical systems are designed, installed, and maintained to prevent danger. Correctly calculating maximum demand and applying diversity is one of the ways electricians fulfil that obligation.

For those preparing for the ECS HS&E test, the electrotechnical topic area covers safe working practices, testing procedures, and an understanding of electrical hazards — all of which connect to the competence demonstrated by performing calculations like these. A solid grasp of BS 7671 principles also supports your broader knowledge of electrical safety on site.

Other Methods of Calculating Diversity

The On-Site Guide Appendix A method is just one approach. Other methods include:

• Table-based diversity from BS 7671 Guidance Note 1 — which provides specific diversity percentages for different circuit types (lighting, cooking appliances, water heating, etc.)
• Detailed assessment — measuring actual current demand using clamp meters over a period of time
• Software-based calculations — using design tools that model load profiles for larger or more complex installations

Each method has its place depending on the type and scale of the installation. The Appendix A method is valued for its simplicity and practicality, particularly for domestic work where a quick on-site assessment is needed.

How Sparky Safety Can Help

Whether you are revising for your ECS HS&E test or brushing up on your BS 7671 knowledge, the Sparky Safety app has you covered. Our 10 built-in BS 7671 calculators help you work through common electrical calculations, and our study materials cover the electrotechnical principles that underpin real-world tasks like maximum demand assessment.

With 300+ ECS HS&E practice questions spanning all 11 topic areas — including the electrotechnical section where maximum demand concepts are tested — you can build the confidence you need to pass your test first time. Our reference guides, topic-by-topic study guides, and realistic mock tests are designed to help you understand the reasoning behind each answer, not just memorise facts.

Download the Sparky Safety app today and take your revision — and your on-site competence — to the next level.