Maximum demand and diversity calculator
Estimate the maximum demand of a domestic installation by applying the standard On-Site Guide diversity allowances, then size the supply and main switch.
Household diversity per the IET On-Site Guide. A design guide. Confirm against the installation.
Assessing maximum demand without over-sizing
Add up the nameplate rating of everything in a house and you get a figure far higher than the installation will ever draw, because not everything runs at once. Diversity turns connected load into a realistic maximum demand. It is the difference between a sensible 80 A supply and an unnecessary three phase service.
Household allowances used here
- Lighting: 66% of the connected lighting load.
- Socket and ring circuits: 100% of the largest circuit, plus 40% of the rest.
- Cooker: the first 10 A in full, then 30% of the remainder (add 5 A if the control unit has a socket).
- Instantaneous showers and EV chargers: no diversity, taken at full load.
These come from the IET On-Site Guide tables for household installations. Showers and EV points get no diversity because they are high, sustained loads that often coincide with everything else on a winter evening.
From demand to supply
With the maximum demand you can size the main switch and confirm the DNO supply fuse is adequate. Most UK homes sit under a 100 A single phase service. Where an EV charger pushes a property over, a load-management device that curtails the charger at peak is usually cheaper than a supply upgrade. Record the actual assessment on the certificate.
FAQs
How do you calculate maximum demand?
Add up each circuit's current after applying diversity. For a typical home: lighting 66%, socket circuits 100% of the largest plus 40% of the rest, cooker 10 A plus 30% of the remainder, with showers and EV chargers at full load.
Does an EV charger have diversity?
No. A dedicated EV charge point is assessed at 100% of its rated current, commonly 32 A for a 7 kW unit, unless an approved load-management device is fitted.
Why not just add up every appliance?
Connected load assumes everything runs at once, which never happens, so it overstates demand. Diversity scales each circuit to a realistic simultaneous figure and prevents over-sizing the supply and switchgear.