TradeCalculator.co.uk
Maximum Demand Calculator — BS 7671 Diversity
Calculate the maximum electrical demand for a domestic installation using BS 7671 diversity factors. Determine the correct main fuse or incomer size.
Number of lighting circuits (typically 6A MCB each)
32A ring finals for socket outlets
20A radial circuits
Total cooker rating in kilowatts
Adds 5A to the cooker diversity calculation per IET On-Site Guide Table A2 (only when the cooker switch incorporates a socket-outlet)
Shower rating (8.5kW, 9.5kW, 10.5kW typical)
Thermostatic water heaters take no diversity (100%) per Table A2. Typically 3kW. Fixed space/panel/storage heating is not covered by this tool — add it separately at 100% up to 10A + 50% of the excess (Table A2 ‘heating and power’ row).
3.6kW, 7kW, or 22kW typical
UK nominal supply voltage (default 230V)
Typical wattage per lighting circuit (default 1200W)
IET On-Site Guide Table A2 lighting diversity (default 0.66)
Safety notice
Electrical work in dwellings can be notifiable under Part P of the Building Regulations. Treat these figures as planning guidance only: circuits must be designed, installed and certified to BS 7671 by a competent person, normally a registered electrician.
Next up: related pages
How We Calculate This
This calculator applies the diversity allowances from the IET On-Site Guide (BS 7671:2018+A2:2022), Appendix A, Table A2, to estimate the maximum demand of a domestic electrical installation. (Assumed current demands come from Table A1.) Maximum demand is always less than the total connected load because not all circuits operate at full capacity simultaneously.
Diversity allowances used (Table A2)
- Lighting: 66% of total connected lighting load
- Socket-outlets (rings and radials combined): 100% of the current demand of the largest circuit + 40% of the current demand of every other circuit (32A per ring final, 20A per radial)
- Cooker: First 10A at 100%, remainder at 30%, plus 5A if the control unit incorporates a socket-outlet
- Electric shower / instantaneous water heater: 100% — no diversity (single high-demand load)
- Immersion heater (thermostatic water heater): 100% — no diversity
- EV charger: 100% — continuous load, no diversity (BS 7671 Section 722)
Note: socket-outlet circuits are a single category — only the largest of all your ring and radial circuits is taken at 100%; every other one (ring or radial) is taken at 40%. Fixed space/panel/storage heating is not a field here; add it separately at 100% up to 10A plus 50% of the excess (Table A2 ‘heating and power’ row).
The calculation
Maximum Demand = Sum of all diversified circuit loads
The total is compared against standard UK main fuse ratings (60A, 80A, 100A) to recommend the appropriate incomer size. All calculations use 230V as the UK nominal supply voltage per BS 7671. Diversity is an estimating aid — final supply sizing and any DNO notification must be confirmed by the installation designer.
Frequently Asked Questions
Maximum demand is the highest expected electrical load at any one time. It is lower than the total connected load because not all appliances run simultaneously. The IET On-Site Guide (BS 7671:2018+A2:2022) Appendix A, Table A2 provides diversity allowances to calculate a realistic maximum demand. This determines the size of the main fuse, incoming supply, and meter. Use our Electrical Load Calculator to list all appliances and calculate connected load, then apply Diversity Calculator factors.
Diversity allowances account for the fact that not all circuits operate at full load simultaneously. They are published in the IET On-Site Guide (BS 7671:2018+A2:2022), Appendix A, Table A2 — not in BS 7671 itself, and not in the older 'Table 1B' numbering. For example, lighting is assessed at 66% of connected load; for cookers, the first 10A is at 100% with the remainder at 30%; and socket-outlet circuits (rings and radials together) are taken as 100% of the largest circuit plus 40% of every other circuit.
Possibly. A 7kW EV charger draws about 30A continuously at 230V (7000 ÷ 230 = 30.4A); the common 7.4kW unit draws the full 32A. EV chargers take no diversity — they are sized at 100% of the charger’s maximum demand per BS 7671 Section 722. If your existing maximum demand plus the EV charger exceeds your main fuse rating (typically 60A or 80A for older properties), you will need a supply upgrade from your DNO (Distribution Network Operator), or a load-curtailment device. This calculator helps you check. For detailed EV cable sizing, see our EV Charger Cable Sizing Calculator.
Most modern UK houses have an 80A or 100A main fuse (service cut-out). Older properties may have a 60A fuse. If your calculated maximum demand approaches or exceeds the main fuse rating, contact your DNO (e.g., UK Power Networks, Western Power Distribution) to discuss a supply upgrade.
Electric showers are high-demand appliances — typically 8.5kW to 10.5kW (37A to 46A at 230V). No diversity is applied to showers because they draw full load when in use. A 9.5kW shower alone uses about 41A, which is a significant portion of a 60A or 80A supply. Use our Circuit Breaker Sizing Calculator to determine the correct MCB rating for a shower circuit.
Related Calculators
Was this calculator helpful?
Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.