Question 1

What is electrical diversity and why is it important?

Accepted Answer

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.

Question 2

How does BS 7671 Table 1B calculate cooking diversity?

Accepted Answer

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.

Question 3

Does diversity apply to ring circuit socket outlets?

Accepted Answer

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.

Question 4

Should I apply diversity to electric showers and EV chargers?

Accepted Answer

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.

Question 5

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

Accepted Answer

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

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BS 7671 Table 1B Diversity Factors

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