Maximum Demand Calculator
Calculate total maximum demand after diversity for domestic installations — IET On-Site Guide
Reference Info & Formulas
Lighting: 66% of total
Sockets: 100% largest + 40% others
Cooking: 10A + 30% remainder
Showers: 100% + 100% 2nd + 25% rest
Storage heating: 100% (no diversity)
EV charger: 100% (no diversity)
IET On-Site Guide Appendix A
60 A: Older properties
80 A: Standard modern
100 A: Large / new builds
Data: IET On-Site Guide — Appendix A (Domestic Diversity)
For guidance only. The responsibility for any electrical installation lies with the qualified person carrying out the work. Always verify calculations independently and apply professional judgement.
How This Calculator Works
Maximum demand is the total anticipated electrical load after applying standard diversity factors from the IET On-Site Guide.
For domestic installations, each circuit type has a defined diversity allowance that reduces the connected load to a realistic simultaneous maximum. The calculator applies these factors per circuit, then sums the diversified values to give the total maximum demand for the installation.
Maximum Demand = Σ (Circuit Load × Diversity Factor)- Σ
- = Sum of all diversified circuit loads
IET On-Site Guide Appendix A
Recording on the EIC
Domestic Diversity Factors
Standard diversity allowances for domestic premises
IET On-Site Guide Appendix A| Circuit Type | Diversity Allowance |
|---|---|
| Lighting | 66% of total connected load |
| Cooking appliances | 10A + 30% of remainder + 5A if socket fitted |
| Socket outlets (ring) | 100% of largest circuit + 40% of each remaining |
| Immersion heater | No diversity (100%) |
| Storage heaters | No diversity (100%) |
| EV charger | No diversity (100%) |
These factors apply to domestic installations only.
Practical Notes
EV Chargers and Heat Pumps
Cooker Diversity Saves Capacity
Frequently Asked Questions
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