Add up the nameplate rating of everything in a house and the total runs far past the size of the consumer mains. The mains are not undersized. The loads never all run at once, and maximum demand is the estimate of what the installation actually draws at its busiest.
What maximum demand decides
Maximum demand sets the size of the consumer mains, the rating of the main switch, the submain to each distribution board, and the capacity a designer asks the network for. It is the first number on a job and the one that is most expensive to get wrong in either direction.
Too low and the mains overheat or the main switch trips under normal use. Too high and the installation pays for conductor, switchgear and a network connection it never uses.
Four methods, one clause
Cl 2.2.2 of AS/NZS 3000:2018 sets out the ways to arrive at the figure. They differ in evidence, not in principle.
Calculation from the tables. Appendix C gives a demand per group. This is the default for a new installation with no history.
Assessment. A competent person judges the demand from the way the installation will be used. The clause allows it where the tables do not fit the building.
Measurement. An existing installation is metered over a representative period. This is the strongest evidence, and it only exists after the fact.
Limitation. The demand is held down by a device, such as a load controller or a smaller main switch. The limit becomes the demand.
The calculator on this site applies the first method.
Groups, not a load list
Table C1 covers domestic installations and Table C2 covers other occupancies. Neither works from a list of appliances. Each works from groups, and each group has its own rule.
Lighting is counted in points. Socket outlets are counted in points, and the rule changes with the count. Ranges, fixed heating and water heating are entered in kilowatts. Some groups take a share of the connected load. Some take a fixed figure regardless of how many items are installed.
That is where the diversity lives. The standard does not apply one blanket percentage. It applies a different rule per group, because a lighting circuit and a water cylinder behave differently across a day.
Table C4 covers the diversity that applies upstream, where several installations share one set of mains.
A worked house
Take a single domestic installation on a 230 V single-phase supply. The load schedule is 20 lighting points in group A(i), 24 socket outlets in group B(i), a 9 kW range in group C, 6 kW of fixed space heating in group D, and a 3 kW storage water heater in group F.
The tool applies Table C1 group by group.
- A(i) lighting: 3 A
- B(i) socket outlets and permanently connected equipment: 15 A
- C ranges, cooking appliances and laundry equipment: 19.57 A
- D fixed space heating, air-conditioning and saunas: 19.57 A
- F storage water heaters: 13.04 A
The maximum demand is 70.18 A, and the per-phase figure is the same because the supply is single phase.
The connected load is far larger. The range and the heating together are 15 kW of plate rating, and the tool carries 39.14 A forward for the pair. The 70.18 A result is what the rules give, not what the nameplates add up to.
The per-phase rule
On a three-phase supply the number that matters is the current in the worst phase, not the total. The tool divides the demand across the phases and reports a per-phase figure. That is the figure a network application asks for, and the figure the main switch has to carry.
Balance is an assumption, not a fact. A three-phase board with every large single-phase load on one phase does not behave like the calculation. Spreading the large loads is part of the design, not a detail for the installer.
Regulations sit beside the standard
The Electricity (Safety) Regulations 2010 place limits that a calculated demand has to respect. Reg 57 and reg 59 are the two that apply to the size of a domestic installation and its connection. They are stated in the regulations and they are not outputs of any calculator. Read them by number and check the current reprint.
Nothing on this page states who may carry out electrical work or certify it.
Three habits worth keeping
Record the schedule, not the answer. A demand figure without its group-by-group input cannot be reviewed or reused when the client adds a load.
Re-run the demand when the brief changes. An electric vehicle charger, a second heat pump or a pool all land in a group and move the mains.
Say which method you used. A calculated demand and a measured demand carry different weight with a network, and the difference matters in a dispute.
This page is a design aid. Verify every value against the current edition of the standard.