Maximum Demand & Diversity: Sizing for the Load That Actually Happens
Cable Sizing · 7 July 2026 · For contractors and project engineers
Add up the rating of every circuit in a building and size the supply to match, and you'll specify an incomer two or three times bigger than you need — because not everything runs flat-out at the same moment. Diversity is how you size for the realistic peak instead. Over-apply it, though, and you undersize the supply and trip the main. It's a balance, and modern loads have quietly shifted it.
Connected Load vs Maximum Demand
Connected load is the arithmetic sum of everything that could draw power. Maximum demand is the largest load the installation actually draws at any one time. The gap between them exists because loads don't all peak together — the lights aren't at full output while every socket is loaded while the whole kitchen is cooking. The diversity factor is the allowance that bridges connected load down to maximum demand.
Where the Allowances Come From
The IET On-Site Guide and BS 7671 give diversity allowances by installation type and load category — lighting, socket outlets, cooking appliances, space and water heating, motors and so on. Crucially, each category diversifies differently: lighting is fairly predictable, sockets get a large allowance, and some loads get almost none. You apply the allowance category by category, then sum, rather than slapping one factor across the whole board.
A Quick Example
Take a small commercial unit with, say, 40 A of lighting, 60 A of general sockets and a 30 A fixed heater. Summed flat, that's 130 A. Apply sensible category diversity — most of the lighting, a portion of the socket load after the first, the full heater — and the realistic maximum demand might come out nearer 80–90 A. That's the difference between a 100 A supply and needing a 150 A one.
The Traps — and Why Modern Loads Bite
- Wrong context. Domestic diversity allowances are generous; applying them to a commercial or industrial installation undersizes it.
- Diversifying continuous loads. EV chargers, heat pumps, server rooms and process loads often run at or near full output for long periods — they have far less diversity than traditional loads, and treating them like ordinary sockets is a growing cause of undersized supplies.
- No headroom. Sizing to today's exact demand with nothing left for the load the client adds next year.
Why It Matters
Maximum demand sizes the incomer, the main tails, the distribution transformer or standby generator, and the DNO connection. Overestimate it and the client pays for capacity and copper they'll never use; underestimate it and the main protection trips, or the DNO connection is too small to extend later.
What This Means for Your Design
Maximum-demand and load-schedule work is repetitive and easy to get subtly wrong by hand — a perfect candidate for a purpose-built tool. We build bespoke load-schedule and demand calculators that apply your diversity rules consistently and roll up to a documented maximum demand, and we produce the downstream cable calculations once the demand is set.