Max demand is the starting point for every installation design. Get it wrong and you undersize the supply or waste money on oversized equipment. This guide walks through diversity factors, worked examples for domestic and commercial installations, and the common mistakes that trip electricians up.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
Maximum demand is the largest current an installation draws at any one time, and BS 7671 Reg 311.1 requires it to be assessed — with diversity allowed — before sizing the supply, main cables and protective devices. Add the full connected load of every circuit, then apply a diversity factor to each load category (lighting, sockets, cooker, water heating, motors) from the IET On-Site Guide, because not everything runs together. A domestic cooker, for example, is commonly taken as the first 10 A plus 30% of the remaining rated current, plus 5 A if the control unit has a socket.
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Max demand (sometimes written as maximum demand) is the greatest electrical load, measured in amperes or kilowatts, that an installation is expected to draw from the supply at any one time. It is not the total of every circuit added together — it is a realistic assessment of the peak simultaneous load, taking into account that most circuits will not be at full capacity at the same moment.
BS 7671:2018+A4:2026 requires the designer of an electrical installation to assess the maximum demand as part of the general characteristics assessment under Part 3, specifically Regulation 311.1 and the guidance in Appendix A. This assessment directly affects the sizing of the incoming supply cable, the main protective device, the distribution board, and every downstream cable and device.
The concept is straightforward: a typical domestic property might have a 10 kW electric shower, a 13 kW cooker, a 3 kW kettle, a 3 kW immersion heater, a 7 kW EV charger, and dozens of socket outlets. If you added up the rating of every circuit and appliance, you might reach 200 A or more. But in practice, you will never run the shower, cooker, kettle, immersion heater, and EV charger all at full load simultaneously while also loading every socket circuit to its maximum. Max demand accounts for this by applying diversity factors.
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Getting the max demand calculation right is critical for two reasons: safety and cost.
For domestic installations, the max demand assessment is often a quick calculation using the standard diversity factors. For larger commercial and industrial installations, it requires a detailed load analysis — and may need to be agreed with the DNO based on historical load data or projected usage patterns.
Diversity factors are percentage reductions applied to the rated load of each circuit type to reflect the realistic likelihood of simultaneous use. BS 7671 Regulation 311.1 requires the maximum demand to be assessed; the numeric allowances are set out in the IET On-Site Guide Appendix A, Table A2 (Allowances for Diversity). Table A1 in the same appendix gives typical current demands per point of utilisation. The key circuit types and their diversity allowances for a typical domestic installation are:
| Circuit Type | Diversity Allowance |
|---|---|
| Lighting | 66% of total connected load |
| Socket outlets (general use) | 100% of largest circuit + 40% of remaining |
| Cooking appliances | 10 A + 30% of remainder + 5 A for socket in cooker control |
| Instantaneous water heaters (showers) | 100% of largest + 100% of second largest (if any) |
| Immersion heater | 100% of full load (no diversity) |
| Space heating (fixed) | 100% of total demand up to 10 A + 50% of any demand in excess of 10 A (individual household — OSG Table A2) |
| EV charger | 100% (dedicated circuit); where a load-curtailment or smart-charging system automatically reduces or disconnects the charger, that reduction may be taken into account when determining max demand (Reg 722.311.201) |
These factors apply to domestic and small commercial installations. For larger installations, diversity is typically assessed on a case-by-case basis using load profiles, power monitoring data, or industry-specific guidance. The IET Guidance Note 1 (Selection and Erection) provides further commentary on applying diversity in practice.
Consider a typical three-bedroom house with the following circuits:
Total connected load: 194 A
Total max demand: 161 A — down from 194 A connected load
This exceeds a standard 100 A single-phase supply. The designer would need to consider load management (such as an EV charger with load balancing), a three-phase supply, or negotiating a higher supply capacity with the DNO.
Work out maximum demand with diversity per the IET On-Site Guide. Free calculator plus worked examples for domestic and commercial installations.
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For a small commercial unit (office or retail), the diversity assessment is different. Socket outlet loading in commercial premises is typically higher and more sustained than in domestic properties, and specific loads such as air conditioning, commercial kitchen equipment, or server rooms need individual assessment.
Total max demand: approximately 137 A — likely requiring a three-phase supply or careful load balancing on a 100 A single-phase supply.
For commercial installations, it is common to use a spreadsheet or calculator tool to document the max demand assessment. This forms part of the design records required by BS 7671 and should be retained with the Electrical Installation Certificate.
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Start free trialThe following table shows typical max demand values for common installation types. These are approximate figures based on standard UK installations and should be verified with a proper calculation for each specific project.
| Installation Type | Typical Max Demand | Supply |
|---|---|---|
| 1-bed flat (gas heating) | 30 to 50 A | Single-phase 60 A |
| 3-bed house (gas heating, electric shower) | 60 to 80 A | Single-phase 100 A |
| 3-bed house (electric heating, shower, EV charger) | 120 to 160 A | Three-phase or load management |
| Small retail unit | 60 to 100 A | Single-phase 100 A |
| Medium office (20+ desks) | 100 to 200 A | Three-phase |
| Restaurant with commercial kitchen | 150 to 300 A | Three-phase |
| Small industrial workshop | 100 to 400 A | Three-phase |
These are guidelines only. Every installation must be assessed individually. The presence of high-load items such as electric vehicle chargers, heat pumps, or commercial kitchen equipment can significantly increase the max demand beyond the typical ranges shown.
Elec-Mate includes a purpose-built max demand calculator as part of its suite of 50+ electrical calculators. The calculator is designed for UK electricians working to BS 7671 and does the following:
Select the circuit type from a dropdown (lighting, sockets, cooker, shower, immersion, space heating, EV charger, etc.) and enter the rated current. The calculator applies the correct diversity factor from IET On-Site Guide Appendix A (Table A2) automatically.
The result shows each circuit type, the connected load, the diversity factor applied, and the diversified load — so you can see exactly how the final max demand figure is derived. This transparency makes it easy to verify and to include in design documentation.
Max demand is just one of over 70+ calculators on Elec-Mate. Others include cable sizing, voltage drop, earth fault loop impedance (Zs), PFC, adiabatic equation, conduit fill, trunking fill, power factor, and three-phase power.
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