Maximum demand

Maximum demand is the highest current an installation (or a part of it) is realistically expected to draw at any one time. It's the number everything upstream is sized from — the consumer mains, the main switch, the main protective device, and the supply itself.

The trap is to add up every connected load and size for that. That total — the connected load — almost never happens, because not everything runs flat out at the same moment. Maximum demand is the connected load brought down to what genuinely happens together.

Diversity is the allowance that loads don't all peak at once. A house full of power points, an oven, a hot water service and air conditioning has a big connected total, but in practice only some of it runs together — so the assessed demand is a fraction of the sum. Different load types get different allowances, because some (like a fixed heater) run near continuously while others (like power points) are only lightly used at any moment.

Apply too little diversity and you oversize everything and waste money. Apply too much and you undersize the mains and protection, and the installation nuisance-trips or runs hot under normal use. Getting the balance right is the whole point of a demand assessment.

There are a few recognised ways to arrive at a figure: assessment by calculation (applying diversity allowances to each load group), assessment from measured or metered figures on an existing installation, or assessment by an experienced judgement of the installation type. On a three-phase supply the demand is also assessed per phase, because an unbalanced board can overload one phase while the others sit idle.

The actual allowances, methods and limits are set by the wiring rules and are revised over time — this is licensed design work. Treat any figure as something to verify, not to copy.

Most 'the board trips but nothing looks overloaded' calls trace back to demand: extra loads added over the years, a sub-board grown beyond what its supply was sized for, or single-phase loads piled onto one phase. A main switch or sub-main that trips under normal use, or a switchboard that runs warm, is the installation telling you the real demand has outgrown the supply.

When you add significant load — a new appliance circuit, EV charger, air conditioning, or a sub-board — the demand has to be reassessed, not assumed. That's how you avoid overloading mains that were never sized for it.