Electrical Diversity Calculator — IET On-Site Guide (BS 7671)

Diversity recognises that not all circuits in an installation will carry their maximum current simultaneously. BS 7671 does not itself contain a diversity table — Regulation 311.1 simply requires the designer to assess the maximum demand and permits diversity to be taken into account. The recognised allowances come from the IET On-Site Guide (Appendix A): for example, only 66% of the lighting load is assumed to be on at any time. Applying diversity gives a realistic maximum demand figure, which is essential for sizing the incoming supply, main switch, and distribution equipment. Without diversity, you would massively oversize the supply.

Per the IET On-Site Guide (Appendix A), for cooking appliances the first 10A is taken at 100% and the remainder at 30%. For example, a 13kW cooker drawing 56.5A: first 10A = 10A, remainder 46.5A × 30% = 13.95A, total = 23.95A. If the cooker control unit incorporates a socket outlet, add a further 5A. This reflects that all hob rings and the oven rarely operate at maximum simultaneously.

Per the IET On-Site Guide (Appendix A, Table A2 'Allowances for diversity', individual household): 100% of the largest socket circuit + 40% of remaining circuits. This calculator approximates by taking 100% of the first 32A (largest ring) + 40% of the remainder. For a single 32A ring with modest load, no diversity reduction applies.

An electric shower is an instantaneous water heater. The On-Site Guide (Appendix A) rule is 100% of the largest + 100% of the second largest + 25% of the remainder — so for a single shower it works out at the full 100% (an 8.5kW shower = ~37A). No diversity is withheld on the first two because of the small number of units, not because they 'run continuously'. With three or more showers the third and subsequent units do get 25% diversity. EV chargers: this calculator takes a charger at 100% as a conservative default. Since BS 7671 Amendment 4:2026 (published 15 April 2026, mandatory from 15 October 2026) the load-assessment guidance permits around 60% diversity on a single domestic 7kW charger for maximum-demand and cable-sizing purposes (commercial rapid chargers stay at 100%). Taking 100% over-orders slightly, so it remains a safe figure.

Most UK dwellings have a 60A or 100A single-phase supply. A typical 3-bed house with gas heating, electric shower, cooker, and standard socket loads will have a diversified maximum demand of 40-60A. Adding an EV charger (32A) may push this over 80A, requiring a supply upgrade. For all-electric properties with heat pumps and EV charging, a three-phase supply (3 × 100A) may be needed. Contact your DNO for supply upgrades. Use our Maximum Demand Calculator for a full circuit-by-circuit assessment.