How Do You Calculate Electrical Maximum Demand?

Why You Cannot Just Add Up Every Circuit

If you added the full rated current of every circuit in a house, lighting, every socket ring, the cooker, the shower, the immersion heater, you would get a number nobody’s house actually draws, because not everything runs at once. Nobody boils a kettle on every socket circuit while the shower, the cooker and every light are on at the same time. Diversity is the standard’s recognised allowance for that reality: a set of published percentages that reduce each circuit’s contribution to a realistic combined figure. It exists to size the main fuse, the incoming cut-out fuse and the supply cable correctly: oversize it and you pay for capacity nobody uses, undersize it and the supply is overloaded the day everything genuinely does run together.

Where the Allowances Come From

The diversity allowances used in UK domestic practice are published in the IET On-Site Guide (BS 7671:2018+A2:2022), Appendix A, Table A2, not in BS 7671 itself. They are guidance rather than a rigid formula, which is exactly why applying them correctly to a real, mixed installation is a job for someone trained to read the table’s conditions and exceptions, not a rule of thumb.

• Lighting: 66% of the total connected lighting load
• Socket-outlet circuits (rings and radials together): 100% of the current demand of the largest circuit, plus 40% of the current demand of every other socket circuit (a ring final is assessed at 32A, a radial at 20A, unless the installed protective device says otherwise)
• Cooker: the first 10A of the design current at 100%, the remainder at 30%, plus 5A if the cooker control unit has a built-in socket-outlet
• Electric shower or instantaneous water heater: 100%, no diversity applied
• Immersion heater (thermostatically controlled): 100%, no diversity applied
• EV charger: 100%, no diversity applied, per BS 7671 Section 722
• Fixed space and water heating (other than the above): 100% of the first 10A plus 50% of the remainder

Notice the pattern: circuits that genuinely run continuously at their full rating, a shower, an immersion heater, an EV charger, get no diversity at all, because there is no reasonable assumption to make other than “it might be fully loaded”. Circuits with many points that are never all in use together, lighting, general sockets, get the largest reductions.

Worked Example: A Typical 3-Bed Semi (Illustrative Only)

This example exists to show the arithmetic, not to size a real supply. None of the connected loads below are a recommendation for any specific property: a registered electrician works from the loads actually installed.

• Lighting: 1,500W connected. Design current: 1,500 ÷ 230 = 6.52A. After 66% diversity: 6.52 × 0.66 = 4.30A
• Sockets: two 32A ring finals and one 20A radial. Largest circuit at 100% (32A), the other two at 40% of their combined demand: 32 + (0.40 × (32 + 20)) = 32 + 20.8 = 52.80A
• Cooker: 8kW. Design current: 8,000 ÷ 230 = 34.78A. First 10A at 100%, remainder at 30%: 10 + (0.30 × (34.78 − 10)) = 10 + 7.43 = 17.43A
• Electric shower: 9.5kW, no diversity. Design current: 9,500 ÷ 230 = 41.30A
• Immersion heater: 3kW, no diversity. Design current: 3,000 ÷ 230 = 13.04A

Total maximum demand: 4.30 + 52.80 + 17.43 + 41.30 + 13.04 = approximately 129A.

That already sits above a common 80A main fuse and close to a 100A one, on a fairly ordinary set of loads. Add an EV charger at a further 32A with no diversity applied and the total moves decisively past what many existing services can carry, which is exactly the kind of finding that has to go to the DNO (Distribution Network Operator), not be worked around on site. This is the practical reason maximum demand gets calculated properly rather than estimated: the answer decides whether your existing supply is adequate at all.

Diversity Allowances at a Glance

Why This Is Not a DIY Calculation

• The table has conditions and exceptions.Which circuits count as “the largest”, how mixed ring and radial circuits combine, and how unusual loads such as electric vehicle charging or heat pumps are treated, all need correct reading of the guide, not a shortcut.
• It feeds a legal design, not just a number. Maximum demand determines the main fuse rating, the meter tails, and whether the existing DNO supply is adequate. Getting it wrong risks an overloaded supply or an unnecessary, costly upgrade.
• New and growing loads change the picture. EV chargers, heat pumps and home batteries take no diversity and are increasingly what pushes an older 60A or 80A supply past its limit, which is why maximum demand is reassessed whenever a major new load is added.
• The result has to be certified. Once assessed, the design is documented as part of the electrical installation certificate for the work, not left as a private calculation.

Who Must Do This

A registered electrician assesses the connected load circuit by circuit, applies the correct diversity allowances, and confirms whether the existing main fuse and supply are adequate or need upgrading. Where the supply itself needs to change, the electrician liaises with your DNO(Distribution Network Operator, for example UK Power Networks or Western Power Distribution depending on your region), since the incoming cut-out fuse and service cable are the DNO’s equipment, not the installer’s. Any resulting new circuit or consumer unit work is notifiable under Part P of the Building Regulations in England and Wales and must follow one of the routes set out in Approved Document P: self-certified by a competent person scheme electrician, certified by a registered third-party certifier, or notified to building control before work starts.

Once you know your circuits, the diversity calculator walks through the same Table A2 allowances, and the electrical load calculator helps list every appliance and its connected load before diversity is applied.