Maximum Demand Calculator — BS 7671 Diversity

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.