Maximum Demand Calculator
Calculate maximum demand with diversity factors for load assessment
| Circuit Type | Rating | Qty | Connected Load | Diversity | Diversified Load | |
|---|---|---|---|---|---|---|
W | 1.20 kVA | 66% | 0.79 kVA | |||
A | 13.80 kVA | 70% | 9.66 kVA | |||
| Board Total | 15.00 kVA | 10.45 kVA | ||||
This indicative calculation applies BS 7671 maximum-demand principles with IET On-Site Guide Appendix A diversity guidance. BS 7671:2018+A3:2024 remains valid until 15 October 2026 during the Amendment 4 transition period.
For commercial installations, load-management systems, generation/storage, or complex loads, consult the designer and DNO regarding supply requirements.
| Type of Final Circuit | Households | Small Shops | Hotels |
|---|---|---|---|
| Lighting | 66% of total demand | 90% of total demand | 75% of total demand |
| Discharge Lighting | 66% total x 1.8 (PF & gear losses) | 90% total x 1.8 (PF & gear losses) | 75% total x 1.8 (PF & gear losses) |
| Heating & Power | 100% up to 10A + 50% balance | 100%X + 75%(Y+Z) | 100%X + 80%Y + 60%Z |
| Cooker/Hob | 10A + 30% balance + 5A if socket | 100%X + 80%Y + 60%Z | 100%X + 80%Y + 60%Z |
| Motor (not lifts) | 100%X + 80%Y + 60%Z | 100%X + 80%Y + 60%Z | 100%X + 50%(Y+Z) |
| Instantaneous Water Heater | 100%X + 100%Y + 25%Z | 100%X + 100%Y + 25%Z | 100%X + 100%Y + 25%Z |
| Thermostatic Water Heater | 100% (no diversity) | 100% (no diversity) | 100% (no diversity) |
| Floor Warming | 100% (no diversity) | 100% (no diversity) | 100% (no diversity) |
| Thermal Storage Heating | 100% (no diversity) | 100% (no diversity) | 100% (no diversity) |
| Standard Circuit | 100%X + 40%(Y+Z) | 100%X + 50%(Y+Z) | 100%X + 50%(Y+Z) |
| Socket Outlets & Stationary Equip. | 100%X + 40%(Y+Z) | 100%X + 75%(Y+Z) | 100%X + 75%Y + 40%Z |
| Ring Circuit (30/32A) | 100% first + 40% remainder | 100% first + 75% remainder | 100% first + 75% second + 40% remainder |
| Electric Shower | 100% (no diversity) | 100% (no diversity) | 100% (no diversity) |
| EV Charger | 100% (no diversity) | 100% (no diversity) | 100% (no diversity) |
Note: Board connected load is shown separately from its diversified contribution to the installation maximum demand. Distribution-board and protective-device selection still requires competent design judgement.
Maximum demand calculation determines appropriate supply capacity. It accounts for equipment not operating simultaneously at full load, applying diversity factors for realistic assessment. The diversity factors vary based on premises type (household, commercial, or hospitality).
X, Y, Z Methodology
For multiple loads of the same type, the On-Site Guide expresses some diversity allowances using the X/Y/Z principle:
- X = Full load current of the largest appliance
- Y = Full load current of the second largest
- Z = Full load current of remaining appliances
Lighting Circuits
Households: 66%, Small shops: 90%, Hotels: 75% of total demand. Discharge lighting multiplied by 1.8 for PF and gear losses.
Ring Circuits & Sockets
Uses X/Y/Z methodology. Households: 100%X + 40%(Y+Z). Commercial: 100%X + 75%(Y+Z). Hotels vary by load type.
Cookers
Households: 10A + 30% remainder + 5A if socket fitted. Commercial/Hotels: 100%X + 80%Y + 60%Z methodology.
Water Heaters
Thermostatic: 100% (no diversity allowed). Instantaneous: 100%X + 100%Y + 25%Z for all premises types.
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