Maximum Demand & Diversity for a House
Size the main switch and supply conductors realistically — because everything is never on at once.
IET On-Site Guide (diversity allowances)BS 7671
If you added up the rating of every circuit in a house you'd get a frightening number and specify a supply nobody needs. Diversity is the recognition that loads don't all run at full whack simultaneously. Applying the standard allowances gives a realistic maximum demand to size the main switch, tails and supply.
Connected load vs maximum demand
Connected load is the sum of everything you could switch on. Maximum demand is what the installation will realistically draw at its busiest. Diversity is the set of allowances that converts one into the other. It's applied per load type, then summed.
Typical domestic diversity allowances
| Load | Allowance (single domestic) |
|---|---|
| Lighting | 66% of total connected load |
| Cooker | First 10 A + 30% of remainder + 5 A if the unit has a socket |
| Ring / radial socket circuits | 100% of the largest, + 40% of the rest |
| Instantaneous water heaters | 100% of the largest two, + 25% of the rest |
| Thermostatic water/space heating | 100% (no diversity — it's on a thermostat, not user choice) |
| EV charge point | 100% (no diversity — assume full load) |
IET On-Site Guide allowances for an individual household installation. Blocks of flats and commercial premises use different figures.
EV charging changed the sums
A 7.4 kW charger adds a solid 32 A with no diversity. On older properties this alone can push maximum demand past the supply capacity — which is why load management or a supply upgrade is often needed.
Worked example
A typical three-bed house at 230 V:
| Load | Connected | After diversity |
|---|---|---|
| Lighting (12 × 100 W ≈ 1200 W ≈ 5.2 A) | 5.2 A | 3.4 A (66%) |
| Cooker (10 kW ≈ 43.5 A, with socket) | 43.5 A | 10 + (33.5×0.3) + 5 = 25 A |
| Two socket ring circuits (32 A each) | 64 A | 32 + (32×0.4) = 44.8 A |
| Immersion (3 kW thermostatic ≈ 13 A) | 13 A | 13 A (100%) |
| EV charger (7.4 kW ≈ 32 A) | 32 A | 32 A (100%) |
| Maximum demand (sum) | ≈ 118 A |
Illustrative. ~118 A comfortably fits a 100 A supply only if load management limits the EV charger — otherwise a supply upgrade or a load-limiting device is required.
Always sanity-check against the cut-out
Most UK domestic supplies are fused at 60–100 A. If your maximum demand approaches or exceeds the DNO fuse, you either apply load management or arrange an upgrade — you can't just fit a bigger main switch.
Reference material for qualified electrical work. Figures follow BS 7671 and the IET On-Site Guide unless stated; always confirm against the current edition and the standard that applies to your installation.