Design calculations

Maximum demand & diversity (domestic)

List every circuit or fixed appliance in the dwelling, pick its category, and the calculator applies the correct On-Site Guide Table A1 diversity rule to each group — then totals the assessed maximum demand and compares it against the service fuse and cut-out rating. Enter loads in amps or kilowatts; kW is converted at the working voltage.

Calculator inputs & results

Working voltage

Socket in cooker unit

Adds 5 A to the cooking groupService fuse / cut-out

Assessed demand by diversity group
Group	Connected	After diversity
Lighting66% of total demand.	7.00 A	4.62 A
Cooking appliancesFirst 10 A + 30% of the balance (+5 A if the cooker unit has a socket).	43.48 A	20.04 A
Instantaneous water heaters / showers100% of largest + 100% of second largest + 25% of the rest.	41.30 A	41.30 A
Thermostatic water heaters (immersion)No diversity allowable.	13.04 A	13.04 A
Socket-outlet circuits & stationary equipment100% of largest circuit + 40% of every other circuit.	64.00 A	44.80 A

Results

Total connected load168.83A

Assessed maximum demand123.81A

Equivalent power at working voltage28.48kW

Over service fuse by23.81A

Assessed demand exceeds the cut-out rating — reduce load, apply load management, or ask the DNO about a supply upgrade before proceeding.

Formula

Lighting = 66% of total
Heating / motors / sockets = largest + 40% of rest
Instantaneous water = largest + 2nd largest + 25% of rest
Cooking = 10 A + 30% of balance (+5 A socket)
Immersion / floor warming / EV = 100%
I (A) = kW × 1000 ÷ V

Reference

BS 7671 Regulation 311.1 requires maximum demand to be assessed with diversity taken into account, but gives no factors. The percentages used here are the typical domestic figures from Appendix A, Table A1 of the IET On-Site Guide (further detail in Guidance Note 1, Appendix 8). They are guidance only — the designer remains responsible for judging the actual pattern of use, and diversity should not be applied to an EV charge point unless load curtailment is provided under Section 722.

Disclaimer — no liability

These calculators are provided for guidance and educational use only. All results must be verified by a competent person against BS 7671:2018+A3:2024/A4:2026, manufacturer data and the specific site conditions before any design, installation or certification decision is made.

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