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OSG-style diversity for a single domestic installation — engineering judgement still applies.
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This calculator estimates the maximum demand of a domestic installation using On-Site Guide style diversity allowances. UK electricians use it when assessing whether a consumer unit, main switch, tails or supply can support an addition — an EV charger, a shower, an extension — without simply adding up every breaker rating and getting a uselessly pessimistic answer.
Diversity recognises that loads do not all run at once. The tool applies the customary allowances for a single household: 66% of the total lighting load, the first 10A of the cooker plus 30% of the remainder (plus 5A if the cooker unit has a socket), 100% of the largest ring final plus 40% of each additional ring, and shower, immersion and other fixed loads at full value. The result is a realistic assessed demand in amps.
Demand = 0.66 × lighting + [10 + 0.3 × (cooker − 10)] + shower + immersion + [32 + 0.4 × 32 × (rings − 1)] + other
Lighting is taken at 66% of its total connected current. The cooker allowance is its full load up to 10A, plus 30% of everything above 10A, with 5A added if the control unit incorporates a socket-outlet. Ring final circuits are counted as 32A for the first ring plus 40% of 32A for each additional ring. Showers, immersion heaters and other fixed loads are added at 100% because a thermostat or user can run them at full load for sustained periods. The sum is the assessed maximum demand in amps.
Enter the total connected lighting current in amps. The tool applies the 66% diversity allowance automatically.
Enter the cooker's full-load current and say whether its control unit has a socket. Add shower and immersion/water-heating currents — these are taken at 100% with no diversity.
Enter the number of ring final circuits (first at 32A, each additional at 40% of 32A) and any other fixed loads at their full current.
The result breaks out the diversified lighting, cooker and ring figures and totals the estimated maximum demand, which you can compare against the main switch, tails and supply fuse capacity.
The allowances follow the familiar On-Site Guide approach for a single domestic installation supplied at 230V. They are deliberately conservative assessments of what a household plausibly draws at once, not guarantees — a house with two electric showers and an EV charger needs more thought than a table can give.
Diversity is an exercise of engineering judgement, and BS 7671 leaves the assessment to the designer. Loads that run for long periods under thermostatic or user control — showers, immersion heaters, EV charging — get no diversity here, and that is intentional. If the assessed demand approaches the supply fuse rating (commonly 60A, 80A or 100A in the UK), talk to the distributor before adding load.
These allowances are for individual households only. Blocks of flats, commercial premises and installations with unusual load profiles need a proper assessment rather than domestic diversity tables. Guidance only — engineering judgement still applies.
OSG-style diversity for a single domestic installation — engineering judgement still applies.
Diversity is the recognition that connected loads do not all operate simultaneously, so an installation's realistic maximum demand is less than the sum of every circuit rating. Applying assessed percentages to each load type gives a demand figure used to size the supply, tails and switchgear sensibly.
For a single household cooking appliance: take the first 10A of its full-load current in full, add 30% of the remainder, and add 5A if the cooker control unit includes a socket-outlet. A 45A cooker therefore assesses at 10 + 0.3 × 35 = 20.5A, or 25.5A with a socket in the unit.
An electric shower draws its full rated current for the whole time it runs, and its use is not shortened by diversity of habit in any dependable way. The same applies to immersion heaters under thermostatic control, so both are assessed at 100% of full load.
It gives you the starting point: assess the existing demand, then add the charger at its full current since it draws a sustained load. If the total approaches the cut-out fuse rating, load-curtailment (a charger that monitors the supply) or an application to the distributor for a larger supply are the usual routes. Treat the result as guidance, not a substitute for a proper assessment.
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