Max DemandCalculator

Maximum demand (MD) is the highest electrical load a premises is likely to draw from the supply at any one time, expressed in amps or kilowatts. It is used to size the incoming supply, the main switch, the consumer unit, and the service cable from the distribution network. If the MD exceeds the rated capacity of the supply (typically 100A single-phase for domestic premises), the DNO must be consulted about a supply upgrade. Oversizing wastes money; undersizing causes tripping, overheating, or voltage sag.

Diversity is the statistical likelihood that not all circuits will be at full load simultaneously. BS 7671 Appendix 1 (formerly Table A1 in the On-Site Guide) defines diversity factors for common domestic circuit types. For example, socket outlet rings: 100% of the first 2 sockets, then 40% of the remainder. Lighting: 66% of the total installed load. Cookers: 10A + 30% of the remaining rated load (after the first 10A) + 5A if a socket is incorporated. These percentages acknowledge that, in practice, not every socket, light, and appliance is on full blast simultaneously.

A standard 100A single-phase domestic supply (100A × 230V = 23kW) is sufficient for most UK homes without electric heating. With typical BS 7671 diversity applied, a house with a 9kW shower, a 12kW cooker, standard socket ring circuits, and lighting usually comes out at 60–80A after diversity — comfortably within 100A. However, adding a 7kW EV charger (32A dedicated circuit, no diversity allowed under PAS 1899), a heat pump (16A), and an electric shower simultaneously can push toward or past 100A, warranting a supply upgrade or load management device.

Commercial premises do not benefit from the same domestic diversity factors. For small commercial units, electricians typically calculate the connected load (sum of all circuit ratings) and apply a demand factor based on the type of business — typically 60–80% for most commercial uses. For larger premises, a full load schedule is drawn up, showing circuit types, installed watts, and demand factors. The IEE Guidance Note 1 (Selection and Erection) and Electricians' Guide to the Building Regulations both provide worked commercial examples.

You need to contact the Distribution Network Operator (DNO — e.g. UK Power Networks, Western Power Distribution, SP Energy Networks) when the calculated maximum demand exceeds the existing cut-out fuse rating (usually 100A for domestic). The DNO will arrange for the supply fuse and sometimes the meter tails to be upgraded — typically to 150A or 200A for single-phase, or conversion to three-phase. Lead times vary from a few weeks to several months. The DNO upgrade is separate from the consumer unit work done by your electrician.

A 7kW (32A) EV charger draws its full rated current for 4–8 hours overnight — it cannot be reduced below its design current in the same way a heating circuit can. PAS 1899 (the EV charger installation standard) requires that EV chargers be assessed at 100% of their rated current for maximum demand purposes, with no diversity. This is why a single 7kW EV charger can push a previously comfortable home close to its 100A supply limit. Smart charge scheduling (time-locking to off-peak hours when other loads are low) or a load-balancing gateway device are common mitigations.

Maximum demand is the peak instantaneous draw from the supply, used to size protective devices and supplies. A load calculation (or energy audit) considers both peak demand and duration — used for tariff selection, battery storage sizing, and predicting energy costs. A property can have a low maximum demand but high energy consumption (e.g. underfloor heating running 16 hours/day at 4kW) or a high peak demand but low consumption (e.g. a 9kW shower running 10 minutes/day). Both calculations are needed for different design decisions.