Calculate whole-building maximum demand for DNO supply applications, main fuse sizing, and ADMD for multi-dwelling developments according to BS 7671
Calculate the maximum demand for an entire building or development for DNO supply applications and main fuse/cable sizing. This tool applies BS 7671 diversity factors at the whole-building level and includes ADMD calculations for multi-dwelling developments.
Single-phase: Amps = kW x 1000 / 230V. Three-phase: Amps = kW x 1000 / (400V x 1.732).
Maximum demand is the highest rate of electrical energy consumption that an installation or building is expected to draw at any point in time. It is a critical value required by BS 7671 Regulation 311.1 for the design of every electrical installation. Unlike the total connected load (the sum of all equipment ratings), maximum demand accounts for the statistical improbability that all electrical equipment will operate at full capacity simultaneously.
Accurate maximum demand assessment is essential for selecting appropriate protective devices, sizing supply cables and main switches, and determining the capacity of the electricity supply from the Distribution Network Operator (DNO). Every Electrical Installation Certificate (EIC) must include a justified maximum demand figure -- it should never simply be copied from the service cut-out fuse rating.
The sum total of all equipment ratings if every load operated at maximum capacity simultaneously. This is always higher than maximum demand.
The realistic peak load after applying diversity factors. This is the value used for sizing supplies, cables, and protective devices.
The ratio of maximum demand to connected load (always less than 1.0). Reflects that not all loads operate simultaneously.
ADMD (After Diversity Maximum Demand) is the key metric used by Distribution Network Operators (DNOs) when assessing supply requirements for multi-dwelling developments such as housing estates, apartment blocks, and mixed-use schemes.
The ADMD per dwelling decreases as the number of units increases. This is because with more dwellings, the statistical probability of all occupants using maximum power simultaneously becomes negligible. A single dwelling might have a peak demand of 15-20 kW, but the ADMD allocated per dwelling in a 100-unit development may be only 1.5-4 kVA depending on heating type.
ADMD values are guided by ENA Engineering Recommendation P2/7 and are subject to regional variation. Always confirm values with your local DNO.
When the calculated maximum demand exceeds the existing supply capacity, or for all new developments, an application must be made to the Distribution Network Operator (DNO). In London, this is typically UK Power Networks (UKPN). The application must include an accurate maximum demand assessment.
These diversity factors from the IET On-Site Guide Appendix A are applied at the whole-building level to determine maximum demand. They differ from circuit-level diversity applied to individual final circuits.
| Circuit Type | Diversity Allowance |
|---|---|
| Lighting | 66% of total current demand |
| Heating & Power | 100% up to 10A + 50% of remainder |
| Cooking Appliances | 10A + 30% of full load remainder + 5A if socket on cooker control |
| Instantaneous Water Heaters | 100% of largest + 100% of second + 25% of remainder |
| Thermostatic Water Heaters | 100% of full load (no diversity) |
| Floor Warming / Storage Heaters | 100% of full load (no diversity) |
| Socket Outlets | 100% of largest circuit + 40% of every other circuit |
| EV Charge Points | 100% -- no diversity (BS 7671 requirement) |
| Heat Pumps | 100% -- continuous load, no diversity |
Note: Diversity should only be applied by competent persons with relevant knowledge and experience. These factors are guidance values from the IET On-Site Guide -- actual usage patterns should be considered.
A typical domestic maximum demand calculation for a 3-bedroom semi with gas central heating, electric cooker, and electric shower.
| Circuit | Connected Load | Diversity Rule | Demand (A) |
|---|---|---|---|
| Lighting (3 circuits) | ~15A total | 66% | 9.9A |
| Ring final 1 (kitchen) | 32A | 100% of largest | 32.0A |
| Ring final 2 (downstairs) | 32A | 40% of others | 12.8A |
| Ring final 3 (upstairs) | 32A | 40% of others | 12.8A |
| Cooker (12kW) | 52.2A | 10A + 30% + 5A (socket) | 27.7A |
| Shower (9.5kW) | 41.3A | 100% (instantaneous heater) | 41.3A |
| Immersion heater (3kW) | 13A | 100% (thermostatic) | 13.0A |
| Total Maximum Demand | 149.5A → ~80A service fuse | ||
Wait — 149.5A on an 80A fuse? This is the theoretical maximum if every circuit hits peak simultaneously. In practice, the shower and cooker rarely run at full load together, and socket diversity further reduces actual peak. An 80A supply is considered adequate for this installation. If an EV charger (32A) is added, the demand exceeds 80A and a supply upgrade or load management device is required.
Box 311.1 on the Electrical Installation Certificate requires the assessed maximum demand of the installation. This should be the calculated demand after applying diversity factors — not the main fuse rating. Many electricians incorrectly enter “80A” or “100A” (the service fuse). The correct entry is the diversified maximum demand you calculated (e.g., “65A”). NICEIC and NAPIT inspectors check this field during audits.
Calculate the maximum demand including the new load (EV charger, heat pump, etc.). If it exceeds your main fuse rating (check the fuse at the meter — typically 60A, 80A, or 100A), you need either a supply upgrade from the DNO (typically £500-2,000 for fuse upgrade, more if new cable needed) or a load management device that curtails lower-priority loads (e.g., pausing the EV charger when the shower is running).
No. BS 7671 states that EV charge points should be taken at 100% of their rated current with no diversity applied for individual installations. A 7.4kW (32A) EVCP adds 32A to your maximum demand calculation. For multi-dwelling developments, the DNO may apply ADMD diversity across multiple charge points using smart charging profiles, but the individual installer must assess at full load.
Maximum demand is the peak load for a single installation. ADMD (After Diversity Maximum Demand) is the average peak demand per dwelling in a multi-dwelling development, accounting for the statistical diversity between households. ADMD decreases as the number of dwellings increases (e.g., 2.0 kVA per dwelling for 100 gas-heated flats vs 8-15 kVA for a single house).
Not usually, provided the new circuit doesn't push the total maximum demand above the existing service fuse rating. However, if the property already has a high-power shower, EV charger, and you're adding a 15kW range cooker, recalculate the total demand. If it exceeds the main fuse, you need a DNO application for a fuse upgrade before energising the new circuit.
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