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
Shower rating (8.5kW, 9.5kW, 10.5kW typical)
Typically 3kW for standard immersion
3.6kW, 7kW, or 22kW typical
How We Calculate This
This calculator applies the diversity factors from BS 7671 18th Edition Table 1B to estimate the maximum demand of a domestic electrical installation. Maximum demand is always less than the total connected load because not all circuits operate at full capacity simultaneously.
Diversity factors used
- Lighting: 66% of total connected lighting load
- Ring final circuits: First ring at 100% (32A), additional at 40%
- Radial circuits: First at 100% (20A), additional at 40%
- Cooker: First 10A at 100%, remainder at 30%, plus 5A if socket outlet fitted
- Shower: 100% — no diversity (single high-demand load)
- Immersion heater: 100% — no diversity
- EV charger: 100% — continuous load, no diversity
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.
Frequently Asked Questions
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Last updated: February 2026
All calculations are estimates. Verify with your supplier.