Circuit diversity estimator
Total connected load vs a realistic maximum demand. Enter the connected load per category — standard domestic diversity allowances do the rest.
Connected load by category
66% diversity applied — lights are rarely all on at once.
First 2.3kW at 100%, 40% of the remainder.
First 2.3kW at 100%, 30% of the remainder.
No diversity — usually the only one of its kind.
Showers, storage heaters, etc. — no diversity applied.
Estimated maximum demand
— diversity trims about 36% off the raw total.
Simplified estimate — not a BS 7671 diversity assessment. Standard domestic allowances only. Larger, mixed or commercial installations need the full assessment from a qualified electrician.
Add up the rating of every circuit in a property and you get a number that's almost never what actually gets drawn — nobody runs the cooker, the immersion heater and every socket flat out at the same time. Diversity is the allowance for that: a set of category-based factors applied to connected load to get to a realistic maximum demand. It's what stops a supply, main switch or distribution board being sized for a load that will never really happen.
Where these allowances come from
The estimator above uses the standard rule-of-thumb allowances for a typical domestic installation, in line with the diversity guidance in the IET On-Site Guide: lighting at 66% of connected load; socket and general power circuits at 100% of the first 2.3kW (10A) plus 40% of whatever's left; cooking appliances at 100% of the first 2.3kW plus 30% of the remainder; and water heating and other fixed equipment (immersion heaters, showers, EV chargers treated individually) at 100% — no diversity applied, because each is usually the only one of its kind and can run at full load on its own.
A worked example
Say a house has 2kW of lighting, 6kW of socket circuits, 8kW of cooking load and a 3kW immersion heater — 19kW connected in total. Lighting assesses at 66% (1.32kW). Sockets: 2.3kW at 100% plus 3.7kW at 40% (1.48kW) — 3.78kW. Cooking: 2.3kW at 100% plus 5.7kW at 30% (1.71kW) — 4.01kW. The immersion heater stays at its full 3kW. Add those together and the assessed maximum demand comes to roughly 12.1kW — about 53A at 230V, against 19kW (83A) connected. That gap is the entire point of diversity.
Why this matters on site
Maximum demand, not total connected load, is what you size a main switch, tails, or a supply upgrade discussion against. Get it wrong in either direction and you either oversize everything for a load that will never happen, or undersize a supply that then trips or overheats under genuine simultaneous use. It's also the number a DNO wants when a property's demand is changing — an extension, an EV charger, an upgrade from gas to electric heating.
What this isn't
This is a simplified, general-purpose estimate for typical domestic properties — not the full diversity assessment in BS 7671 Appendix A, which has finer categories, different allowances for larger or commercial installations, and covers cases this tool doesn't (multiple water heaters, three-phase supplies, mixed instantaneous and stored water heating, and more). Use it to sanity-check a figure or scope a conversation, then run the full assessment — or get a qualified electrician to — before anything gets specified or installed.
Seen enough? Claim your patch.
TradesInvo captures every enquiry, replies first, and runs it to a booked, quoted, paid job. Early access is five founder slots per postcode — check yours.