What is electrical diversity and why is it important?

Diversity recognises that not all circuits in an installation will carry their maximum current simultaneously. BS 7671 Table 1B provides allowances: 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.

How does BS 7671 Table 1B calculate cooking diversity?

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 has a socket outlet, add 5A. This reflects that all hob rings and oven rarely operate at maximum simultaneously.

Does diversity apply to ring circuit socket outlets?

Per IET On-Site Guide Table A1 (dwellings): 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.

Should I apply diversity to electric showers and EV chargers?

Electric showers and EV chargers are typically taken at 100% of their rated current — no diversity is applied because they are single high-demand loads that run continuously during use. An 8.5kW shower = 37A, a 7.4kW EV charger = 32A. Both are added in full to the maximum demand calculation. If there are multiple showers, some engineers apply diversity to the second unit.

What supply size do I need for a typical UK house?

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.

Electrical Diversity Calculator — BS 7671 Table 1B

Calculate diversified maximum demand for a domestic installation using BS 7671 Table 1B diversity factors. Essential for DNO supply applications and consumer unit sizing.

Lighting Total Current

Total of all lighting circuits

Portable Heating Current

Largest portable heater

Water Heating Current

Immersion heater / instant water heater

Cooking Appliance Current

Total cooker/hob/oven current

Socket Outlets Assessed Current

Total ring/radial socket circuit current

Electric Shower Current

Shower kW ÷ 230V

EV Charger Current

Dedicated EV charger load

How We Calculate This

This calculator applies BS 7671 Table 1B diversity factors to your circuit loads to determine the diversified maximum demand for a domestic installation.

BS 7671 Table 1B Diversity Factors

Lighting: 66% of total current demand
Portable heating: 100% of largest appliance (all others are assumed off)
Water heating: 100% of largest (no diversity for single immersion)
Cooking: First 10A at 100%, remainder at 30%, plus 5A if socket on cooker unit
Socket outlets (IET OSG Table A1): 100% of largest circuit + 40% of remainder (approximated as first 32A + 40%)
Electric shower: 100% of rated current
EV charger: 100% of rated current

The total diversified load should not exceed the main supply fuse rating (typically 60A or 100A for single-phase domestic).