Maximum Demand in NZ: How AS/NZS 3000:2018 Table C1 Works

"Maximum demand" sounds like it should mean exactly what it says: add up every load in the installation and size the mains for that total. It doesn't work that way, and it's not supposed to. If mains and switchboards were sized for every load running flat out simultaneously, houses would need commercial-grade supply for no real benefit, since a range, a heat pump and an instantaneous water heater are never all pulling full load at the same instant in a typical home.

That's what maximum demand calculations actually solve: a realistic design load, not a worst-case sum. In New Zealand, that calculation is done under AS/NZS 3000:2018, using Table C1 for domestic installations.

Why diversity is the whole point

The standard applies a "diversity" factor to most load categories, a way of accounting for the fact that not everything in a house runs at once. Lighting, socket outlets, cooking appliances, fixed heating and hot water each get their own treatment, because each behaves differently: lighting load scales with the number of points, socket outlets have their own allowance, cooking appliances get a specific formula because ranges rarely all run flat out together, and so on. Table C1 isn't one blanket percentage, it's a separate rule for each category.

Single dwellings: each load is its own item

For a single dwelling, every load group is genuinely independent: lighting, outdoor lighting, socket outlets, ranges and cooking, fixed heating and air conditioning, both types of water heater (instantaneous and storage get treated differently), spa and pool heaters, and EV charging. None of these derive from each other, each has its own formula and its own contribution to the final figure. Add them up correctly and you get the installation's maximum demand in amps, the number that actually determines mains and switchboard sizing.

Blocks of living units: a different shape entirely

Blocks of units work differently, and it trips people up if they try to apply the single-dwelling logic to a multi-unit job. The per-unit loads, lighting, sockets, ranges, heating, and water heating, aren't calculated independently per unit. Instead they're driven by one shared multiplier based on the number of living units and how they're split across phases, applied against a single lookup. On top of that core figure, communal loads (communal lighting, communal sockets, shared dryers or water heaters, communal spa or pool heating, communal EV charging) and lifts or motors are added as their own separate items, with lifts and motors getting their own group treatment: the largest motor contributes at a higher factor, the next largest at a reduced factor, and the rest at a lower factor again, reflecting that they don't all start simultaneously either.

Why getting this right actually matters

Undersize the mains and you've got a genuine safety and compliance problem, potentially expensive to fix once the installation's in the ground or in the wall. Oversize it and the client's paying for cable, switchgear and supply capacity they'll never use. Table C1 exists to land on the real number, not a guess in either direction, which is exactly why it's worth doing the calculation properly rather than eyeballing it from experience on a job that doesn't quite match the last one.

We built CertMate's free Maximum Demand calculator around exactly this structure: add each load as its own item to a running schedule, single dwelling or block of units, and get your total in amps with a branded PDF you can hand straight to a client or building consent authority. No login required.