Calculate maximum demand using the IET On-Site Guide diversity factors (Table 1B). Supports domestic and commercial load assessments including cookers, showers, EV chargers, and heating loads.
Calculate maximum demand with IET On-Site Guide diversity allowances
MD = Sum of diversified loads per IET On-Site Guide. I = (MD × 1000) / V
Maximum demand is the highest rate of electrical energy consumption that an installation is expected to draw from the supply at any given moment, taking into account the likelihood that not all connected equipment will be operating simultaneously at full load. It is expressed in amperes (A) or kilowatts (kW) and is the single most important figure for determining the size of the electricity supply, the main protective device, and the meter tails.
Understanding maximum demand is essential for every electrician, whether working on a new domestic installation, an alteration to an existing property, or a full commercial fit-out. If the maximum demand is underestimated, the supply will be inadequate — the main fuse or breaker will trip under peak load conditions, equipment will not function correctly, and the installation may need an expensive supply upgrade from the distribution network operator (DNO). If the maximum demand is overestimated, the client pays for a larger supply than necessary, with higher connection charges and standing costs.
The key concept that makes maximum demand calculations practical is diversity. In any real installation, it is statistically unlikely that every appliance will be switched on and running at full power at exactly the same time. A domestic kitchen may have a cooker rated at 12 kW, a kettle at 3 kW, a washing machine at 2.4 kW, a dishwasher at 2.2 kW, and several other appliances — but they are rarely all drawing maximum current simultaneously. Diversity factors quantify this statistical reduction and are published in the IET On-Site Guide, Table 1B.
Getting maximum demand right is particularly important in the current era, as installations increasingly include EV chargers (7.4 kW for a typical single-phase home charger), heat pumps (3 to 12 kW), and battery storage systems. These new loads can significantly increase the maximum demand beyond what the existing supply was designed to handle, and electricians must assess the impact before adding them. Use the cable sizing calculator to size meter tails once maximum demand is confirmed, and the voltage drop calculator to verify long submain runs.
The IET On-Site Guide — the companion publication to BS 7671:2018+A4:2026 — addresses maximum demand estimation. It provides guidance on assessing the maximum demand of an installation and includes Table 1A (standard circuit arrangements for domestic premises) and Table 1B (allowances for diversity). Diversity guidance is not contained in a BS 7671 appendix.
Table 1A lists the standard circuit arrangements for a typical domestic dwelling, including the number and type of circuits for lighting, socket outlets, cooker, immersion heater, shower, and other fixed appliances. It serves as a reference for what circuits are expected in a standard domestic installation.
Table 1B is the critical table for maximum demand calculations. It lists the diversity allowances that may be applied to different types of final circuits. The allowances are expressed differently depending on the circuit type:
It is important to understand that the Table 1B factors are guidance, not absolute rules. They represent typical usage patterns for domestic and small commercial installations. For unusual installations, high-demand premises, or applications where reliability is critical, a more conservative approach may be appropriate.
Consider a typical three-bedroom semi-detached house with the following circuits and loads. We will calculate the maximum demand step by step using the Table 1B diversity factors.
Total maximum demand: 5.7 + 32.0 + 12.8 + 12.8 + 27.7 + 41.3 + 13.0 + 32.0 = 177.3 A
Wait — this exceeds the standard 100 A supply. But this is before applying the overall assessment. In practice, the shower and cooker are unlikely to run at full load simultaneously with the EV charger and all ring finals loaded. A realistic assessment, using engineering judgement alongside Table 1B, would place this installation at approximately 80 to 90 A — within the capacity of a 100 A supply, though marginal. If the maximum demand is genuinely expected to exceed 100 A (for example, if the EV charger and shower are frequently used at the same time), a supply upgrade or load management system would be required.
This example illustrates why maximum demand assessment requires professional judgement as well as table look-ups. The Table 1B factors are a starting point, but the electrician must consider the actual usage patterns of the occupants. A household with two electric vehicles, an electric shower, and an induction hob will have very different peak demands from a retired couple with a gas cooker and no EV.
Commercial maximum demand assessments follow the same fundamental principles as domestic ones, but with additional complexity. Three-phase supplies are standard for commercial premises, and loads must be balanced as evenly as possible across the three phases. The maximum demand of the installation is determined by the most heavily loaded phase.
For commercial premises, the diversity factors from Table 1B still apply where the circuit types match, but many commercial loads — such as three-phase motors, air conditioning units, commercial kitchen equipment, and server rooms — require specific assessment. Motor loads should include the starting current requirements, as some motor types draw five to eight times their full load current during start-up, which can affect the sizing of the main protective device and the supply capacity.
When assessing a commercial installation, it is essential to obtain a detailed load schedule from the client or the building services engineer. This schedule should list every item of fixed equipment, its power rating, the number of phases it uses, and an estimate of its duty cycle (how often and how long it operates). From this, you can build up the maximum demand per phase and verify that the three-phase supply is adequately sized.
Common commercial loads and their typical diversity treatment include: office socket outlets (100% of largest circuit plus 40% of subsequent), commercial lighting (90% of total — higher than domestic because commercial lighting runs for longer periods), air conditioning (100% of largest unit plus 80% of subsequent units due to simultaneous operation), and commercial kitchen equipment (which may need the full cooker diversity formula applied to each appliance, or a combined assessment based on the kitchen consultant's load schedule).
The Elec-Mate calculator supports both single-phase and three-phase installations. For three-phase, you assign each circuit to a phase (L1, L2, or L3), and the calculator shows the load on each phase, the degree of imbalance, and the total maximum demand.
While diversity factors are valuable for typical domestic and commercial installations, there are scenarios where applying diversity would be unsafe or inappropriate. Recognising these situations is an important part of professional competence.
Data centres and server rooms typically run at high utilisation continuously, and the supply must be sized for the full connected load plus cooling. Applying domestic diversity factors to a data centre would result in an inadequate supply and potential downtime.
Industrial process installations where all equipment operates simultaneously as part of a production line must be assessed at full load. Shutting down part of the line due to supply overload could have serious safety and financial consequences.
Emergency and life safety systems — fire alarm systems, emergency lighting, sprinkler pumps, smoke ventilation — must be available at full capacity at all times and should never have diversity applied to their supply calculations.
Standby generator sizing requires careful consideration. If a generator must supply the entire installation during a mains failure, it must be sized for the maximum demand that will occur when all loads reconnect simultaneously (this is often higher than normal running demand due to motor starting currents and battery charging surges). Load shedding and staged reconnection can mitigate this.
The maximum demand figure ultimately determines the DNO supply requirements. A standard domestic single-phase supply in the UK is typically provided with a 60 A or 100 A cutout fuse. If the calculated maximum demand exceeds this, the options are: apply load management (such as a smart EV charger that reduces power when other loads are high), request a supply upgrade from the DNO (which may involve a new service cable, cutout, and meter), or consider a three-phase supply. The Elec-Mate calculator flags when the maximum demand approaches or exceeds standard supply ratings.
Create a complete list of every final circuit in the installation, noting the type of circuit (lighting, socket outlet, cooker, shower, immersion heater, EV charger, etc.) and the total connected load in watts or the design current in amps. For existing installations, take readings from the distribution board schedule. For new designs, use the equipment ratings from the specification.
For each circuit type, find the applicable diversity factor from the IET On-Site Guide, Table 1B. Note that the factors differ depending on the circuit type: lighting uses a percentage method, cooking appliances use the "first 10 A plus 30% of remainder" formula, and some loads like immersion heaters and showers are taken at full load with no diversity (unless there are multiple units).
Calculate the diversified demand for each circuit by applying the Table 1B factor to the connected load. For ring final circuits, Table 1B allows the current demand to be taken as the design current of the circuit. For lighting, the factor depends on the number of points and the type of premises. Work through each circuit systematically.
Add together all the diversified demand values from each circuit to obtain the total maximum demand for the installation. Express this in both amps and kilowatts. For three-phase installations, balance the loads as evenly as possible across the three phases and calculate the maximum demand per phase — the highest phase determines the supply requirement.
Use the total maximum demand to select the appropriate main switch or isolator rating, meter tail cable size, and cutout fuse rating. Verify that the DNO supply capacity is sufficient. For a standard domestic single-phase supply, the cutout fuse is typically 60 A or 100 A. If the calculated maximum demand exceeds the available supply capacity, you may need to apply load management, request a supply upgrade from the DNO, or consider a three-phase supply.
Purpose-built for UK electricians working to BS 7671. Faster and more reliable than manual calculations with Table 1B.
All BS 7671 Table 1B diversity factors built in. Select the circuit type and load, and the correct factor is applied automatically.
Pre-configured for typical domestic circuits: ring finals, lighting, cooker, shower, immersion heater, EV charger.
Supports three-phase installations with automatic phase balancing. Add commercial loads including motors, air conditioning, lifts, and catering equipment.
Includes EV charger loads with guidance on when to apply diversity. Supports single charger domestic and multi-charger commercial scenarios.
Compares your calculated maximum demand against common UK supply ratings (60 A, 80 A, 100 A single-phase; three-phase options).
All calculations follow the diversity methodology in the IET On-Site Guide (Table 1B), the companion to BS 7671:2018+A4:2026. Verified against the current edition of the IET Wiring…
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