Calculate diversity factors for domestic and commercial electrical installations using IET On-Site Guide Table 1A allowances. Instantly determine whether the DNO supply is adequate.
Calculate electrical demand after applying IET On-Site Guide diversity allowances
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Diversity is a fundamental concept in electrical installation design. It recognises a simple practical fact: not every electrical appliance in a building operates at the same time. In a typical domestic property, you would never simultaneously run the oven at full power, boil the kettle, heat water with the immersion heater, charge an electric vehicle, run the shower, and switch on every light and socket outlet. The probability of all loads drawing their full rated current simultaneously is effectively zero.
Because of this, the IET Wiring Regulations and the associated On-Site Guide allow electricians to apply diversity allowances when calculating the maximum demand of an installation. Without diversity, a modern 3-bed house with electric cooking, a shower, an immersion heater, and an EV charger could have a total connected load of 30 kW or more. This would require a supply far larger than the standard 100 A single-phase supply provided by the Distribution Network Operator (DNO). With diversity correctly applied, the assessed maximum demand typically falls to 12-18 kW, which is well within the capability of the standard supply.
Getting diversity right is critical. Apply too much diversity and the supply could be overloaded, causing the DNO fuse to blow or the main switch to trip. Apply too little diversity and the installation may require an unnecessarily expensive supply upgrade, larger main tails, and an oversized distribution board. The IET On-Site Guide Table 1A provides standardised diversity allowances based on decades of measured usage patterns in UK installations. Use the cable sizing calculator to size main tails once your diversified demand is confirmed, and the EV charger load calculator to assess how a new charge point affects your total demand.
The IET On-Site Guide, which accompanies BS 7671:2018+A4:2026, contains Table 1A setting out the recommended diversity allowances for individual domestic and similar installations. These are the values that every UK electrician should use when assessing maximum demand:
It is essential to note that showers, immersion heaters, storage heaters, and EV charge points are always taken at their full rated current. These are high-power, long-duration loads that can realistically operate at full power for extended periods, so no diversity is allowed. Lighting and socket outlets, by contrast, are statistically unlikely to all be at full load simultaneously, which is why diversity can be applied.
Let us work through a complete diversity calculation for a typical 3-bedroom semi-detached house. This is the most common type of domestic installation an electrician will encounter.
Lighting: 66% allowance
7.83 A total. First 2 kW (8.7 A) at 100% but load is less, so: 7.83 A x 0.66 = 5.17 A
Socket Outlets: First 10 A at 100%, then 50%
10 A + (64 - 10) x 0.5 = 10 + 27 = 37 A
Cooker: 10 A + 30% of remainder + 5 A socket
10 + (52.2 - 10) x 0.3 + 5 = 10 + 12.66 + 5 = 27.66 A
Electric Shower: 100% (no diversity)
41.3 A
Immersion Heater: 100% (no diversity)
13 A
EV Charger: 100% (no diversity)
32 A
Total Diversified Maximum Demand
156.13 A
Without diversity: 210.33 A total connected load
At 156 A, this installation exceeds the standard 100 A supply. The EV charger and shower combined push the demand beyond a single-phase 100 A supply. Options: upgrade to three-phase, install a load management system for the EV charger, or apply for a DNO supply upgrade.
Diversity directly determines two critical aspects of every electrical installation: the size of the incoming supply and the size of the main tails. If the diversified maximum demand exceeds the rating of the DNO supply cut-out (typically 60 A or 100 A for domestic single-phase), the electrician must either apply for a supply upgrade or redesign the installation to reduce the maximum demand.
The main tails — the cables connecting the meter to the consumer unit — must be sized to carry the assessed maximum demand. For a 100 A supply, 25 mm² meter tails are standard. If diversity calculations show the maximum demand is only 60 A, 16 mm² tails may be sufficient, saving material cost. Conversely, if the assessed demand is 120 A, the standard tails are inadequate and must be upgraded.
In modern installations, the addition of electric vehicle charge points (typically 7.4 kW / 32 A) and heat pumps has significantly increased maximum demand in domestic properties. Many existing 60 A or 80 A supplies are no longer adequate when these loads are added. The diversity calculation is the tool that quantifies this and determines whether an upgrade is needed. Elec-Mate performs this calculation automatically, saving time and eliminating arithmetic errors.
Purpose-built for UK electricians assessing maximum demand to IET On-Site Guide standards.
Enter your connected loads by type — lighting, sockets, cookers, showers, EV chargers — and get the diversified maximum demand calculated instantly to…
Pre-built templates for typical domestic installations (1-bed flat to 5-bed house) and small commercial units.
All diversity percentages from the IET On-Site Guide Table 1A are built into the calculator. No need to look up tables or remember which loads get 66%…
Automatically compares your diversified maximum demand against the DNO supply rating (60 A, 80 A, or 100 A) and warns if an upgrade is needed.
Visual pie chart showing how each load type contributes to the total maximum demand. Instantly see which circuits are driving the demand.
All diversity calculations follow the current 18th Edition wiring regulations including Amendment 4.
Enter your connected loads by type — lighting, sockets, cookers, showers, EV chargers — and Elec-Mate applies IET On-Site Guide Table 1A diversity…
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A diversity factor is a percentage allowance applied to the connected load of an electrical installation, reflecting the fact that not all loads will be operating simultaneously at their maximum rating. For example, in a domestic property, the oven, kettle, immersion heater, and all socket outlets are unlikely to all be drawing full current at the same time. Diversity allows the electrician to reduce the calculated maximum demand so that cables, protective devices, and the incoming supply are not oversized.
The IET publishes diversity allowances in the On-Site Guide, Table 1A (for individual domestic and small commercial installations) and Table 1B (for blocks of flats and other grouped installations). These tables give the recommended diversity percentages for each type of load: lighting, heating, cooking appliances, socket outlets, water heaters, showers, and electric vehicle charge points. The values are based on decades of recorded usage patterns in UK properties.
No. Certain loads must always be taken at their full rated current with no diversity applied. These include electric showers (100%), immersion heaters (100%), storage heating (100%), and electric vehicle charge points (100% for the first, with possible diversity for additional units in multi-charger installations). The IET On-Site Guide Table 1A specifies exactly which circuits can have diversity applied and which must be taken at full load.
Without diversity, the total connected load of a typical 3-bed house might exceed 30 kW, requiring a 125 A supply and very large main tails. With diversity correctly applied, the maximum demand typically comes down to 12-18 kW (around 50-80 A), which matches the standard 100 A DNO supply cut-out. Diversity therefore determines whether the existing supply is adequate or whether an upgrade is needed, and it directly affects the size of the main tails, meter tails, and distribution board main switch.
They are closely related but not identical. Diversity factor is the ratio of actual maximum demand to the total connected load, always less than 1. Demand factor is sometimes used interchangeably in UK practice, particularly in the IET On-Site Guide. In some international standards, demand factor refers to the ratio for a single load, while diversity factor refers to the combined effect across a group of loads. In UK electrical work to BS 7671, the IET On-Site Guide Table 1A terminology is used.
Yes. When completing the maximum demand section of an Electrical Installation Certificate (EIC) or Electrical Installation Condition Report (EICR), you should apply diversity in accordance with IET On-Site Guide Table 1A. The assessed maximum demand figure is entered on the certificate and is used to verify that the incoming supply is adequate for the installation. Elec-Mate calculates this automatically when you enter the circuit details.
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