Maximum Demand Calculation Using AS/NZS 3000 Tables C1 and C2

Maximum demand is the greatest average electrical load drawn by an installation over a defined period. It determines the main switch rating, the supply cable size, and the connection capacity the supply authority must provide. In Australia, maximum demand is calculated using diversity factors from AS/NZS 3000:2018 Tables C1 (domestic) and C2 (commercial). This guide walks through both tables, explains diversity and demand factors, and includes worked examples for a residential dwelling and a commercial office. The Maximum Demand Calculator implements both tables automatically.

What is maximum demand and why electricians calculate it

The total connected load of an installation is the sum of the ratings of every piece of equipment. A typical Australian home might have 30 to 50 kW of connected load across lighting, power points, oven, cooktop, hot water, air conditioning, and an EV charger. But not all of these run at full power at the same time.

Maximum demand accounts for this diversity. It is the highest load the installation is likely to draw simultaneously, taking into account the usage patterns of each load type. The main switch, the supply cable from the meter to the switchboard, and the supply authority connection must all be rated for the maximum demand, not the total connected load.

Diversity factor vs demand factor

A demand factor is the ratio of the maximum demand of a load to its total connected rating. Table C1 applies this idea directly, for example fixed space heating and airconditioning in load group (d) is assessed at 75 percent of connected load rather than at its full rating.

A diversity factor accounts for the fact that multiple circuits do not all peak at the same time. AS/NZS 3000 Tables C1 and C2 combine both concepts into a single set of factors per circuit category, so you do not need to calculate them separately.

Table C1: demand factors for domestic circuits

Table C1 applies to single-domestic dwellings (one home) and multi-domestic dwellings (apartments, units). Each circuit category has its own demand factor:

• Lighting, load group (a)(i): 3 A for 1 to 20 points, plus 2 A for each additional 20 points or part thereof, per phase. Lighting track counts as two points per metre. Outdoor lighting exceeding a total of 1000 W is a separate load group at 75 percent of connected load.
• Socket outlets not exceeding 10 A, load group (b)(i): 10 A for 1 to 20 points, plus 5 A for each additional 20 points or part thereof, per phase. Permanently connected equipment not exceeding 10 A counts as an additional point. If the installation includes one or more 15 A socket outlets the base loading increases by 10 A, and if it includes 20 A socket outlets it increases by 15 A (15 A total if both are present).
• Ranges, cooking appliances, laundry equipment and socket outlets rated above 10 A for their connection, load group (c): 50 percent of connected load for a single domestic installation or individual living unit. For blocks of living units it is 15 A per phase for 2 to 5 units, or 2.8 A per living unit for 6 or more.
• Water heating: storage water heaters, load group (f), at full load current. Instantaneous water heaters, load group (e), meaning units with element ratings greater than 100 W per litre, at 33.3 percent of connected load for a single domestic installation.
• Fixed space heating or airconditioning equipment and saunas, load group (d): 75 percent of connected load. Note 11 to Table C1 says that where the installation has both an airconditioning system for hot weather and a heating system for cool weather, only the system with the greater load is taken into account, so the smaller system is not counted at all.
• Motors, load group (l): Table C1 refers you to Paragraph C2.4.1 and Table C2 Column 2, which is full load of the highest rated motor plus 50 percent of the full load of the remainder. Lifts are a separate load group with their own allocation.
• EV charger: continuous load at 100 percent.

Table C2: demand factors for non-domestic circuits

Table C2 applies to commercial, retail, industrial, and hospitality installations:

• Lighting, load group (a): 75 percent of connected load for residential institutions, hotels, boarding houses, hospitals, accommodation houses and motels (Column 2), or full connected load for factories, shops, stores, offices, business premises, schools and churches (Column 3). Connected load comes from the lamp ratings in Note 2 to Table C2, not from floor area. There is no watts per square metre method in AS/NZS 3000.
• Socket outlets not exceeding 10 A, load group (b)(i): 1000 W for the first outlet plus 750 W for each additional outlet (Column 3), or 1000 W plus 400 W for each additional outlet (Column 2). The allocation is per outlet, not per circuit. Where the building or part of it has permanently installed heating or cooling equipment, load group (b)(ii) applies instead: 1000 W for the first outlet plus 100 W for each additional outlet. Socket outlets exceeding 10 A fall under load group (b)(iii): full current rating of the highest rated outlet plus 75 percent of the remainder (Column 3), or plus 50 percent (Column 2).
• Appliances for cooking, heating and cooling, including instantaneous water heaters, load group (c)(i): full connected load of the highest rated appliance plus 75 percent of the full load of the remainder (Column 3), or plus 50 percent (Column 2). Heating elements associated with thermal storage heaters, including water heaters, space heaters, swimming pools, spas and saunas, are load group (g) at full load current.
• Motors other than lifts and fuel dispensing units, load group (d): full load of the highest rated motor, plus 75 percent of the full load of the second highest rated motor, plus 50 percent of the full load of the remainder (Column 3). For Column 2 premises it is full load of the highest rated motor plus 50 percent of the full load of the remainder.

For mixed-use sites (e.g. a shop with a flat above), calculate each part separately using the appropriate table and add them.

Worked example: residential dwelling

Table C1 is worked in amperes per phase and counts points, not circuits, so start by counting outlets and lighting points. A single-phase 230 V dwelling has 24 lighting points, 30 socket outlet points not exceeding 10 A, one 15 A socket outlet, one 6 kW oven, one 3.6 kW storage hot water system, one 7.4 kW ducted air conditioner, and one 7.4 kW EV charger (32 A).

• Lighting, load group (a)(i): 24 points = 3 + 2 = 5 A
• Socket outlets not exceeding 10 A, load group (b)(i): 30 points = 10 + 5 = 15 A
• 15 A socket outlet, load group (b)(ii): 10 A
• Oven, load group (c): 6,000 / 230 = 26.1 A at 50 percent = 13.0 A
• Storage hot water, load group (f): 3,600 / 230 = 15.7 A at full load current
• Ducted air conditioning, load group (d): 7,400 / 230 = 32.2 A at 75 percent = 24.1 A
• EV charger, load group (j)(iv): fully connected load = 32 A

Total maximum demand = 5 + 15 + 10 + 13.0 + 15.7 + 24.1 + 32 = 114.8 A per phase. The next standard main switch rating is 125 A. Note that if the dwelling also had a fixed heating system, Note 11 to Table C1 means you count only the larger of the heating and airconditioning loads, not both.

Worked example: commercial office

An office is a Column 3 premises, so lighting is full connected load and socket outlets are counted per outlet, not per circuit. A three-phase 400/230 V office has 3,000 W of connected lighting load, 30 socket outlets not exceeding 10 A, and fixed appliances of a 3 kW kitchenette and a 2 kW server. Assume the lighting and the socket outlets are balanced, so 10 outlets and 1,000 W of lighting per phase, and that both appliances sit on the same phase.

• Lighting, load group (a): 1,000 W per phase at full connected load = 1,000 / 230 = 4.3 A
• Socket outlets, load group (b)(i): 1000 + (9 x 750) = 7,750 W per phase = 7,750 / 230 = 33.7 A
• Appliances, load group (c)(i): 3,000 + (0.75 x 2,000) = 4,500 W = 4,500 / 230 = 19.6 A

Total on the heaviest loaded phase = 4.3 + 33.7 + 19.6 = 57.6 A. The next standard main switch rating is 63 A three-phase. If the office has permanently installed heating or cooling equipment, load group (b)(ii) applies to the socket outlets instead: 1000 + (9 x 100) = 1,900 W per phase = 8.3 A, which brings the heaviest phase down to 32.2 A and a 40 A main switch. Which socket outlet load group applies makes a large difference, so check Note 4 to Table C2 before choosing.

Demand factor vs actual load and safety margin

The calculated maximum demand is an estimate, not a measurement. The demand factors in Tables C1 and C2 are based on statistical analysis of typical usage patterns. In most cases the actual simultaneous load is lower than the calculated maximum demand, providing a built-in safety margin.

How to apply diversity when multiple loads share a feeder

When multiple dwellings share a common feeder, for example in an apartment building, you do not apply an extra diversity factor of your own. AS/NZS 3000 handles it inside Table C1: Columns 3, 4 and 5 give separate allocations for blocks of 2 to 5, 6 to 20, and 21 or more living units, and those allocations already account for the fact that not every dwelling peaks at once. Use the column matching the number of living units on the feeder rather than summing individual maximum demands and discounting the total.

Supply authority requirements and coordination

For installations under 100 A single-phase, most Australian supply authorities do not require formal maximum demand approval. For larger connections, three-phase, or sites with high inrush loads, the supply authority may require a maximum demand assessment, and may impose conditions such as off-peak water heating, demand limiters, or controlled EV charging.

Common mistakes

• Adding every load at 100 percent. This ignores diversity and dramatically oversizes the main switch and supply cable.
• Using Table C1 for a commercial site. Table C1 is domestic only. Commercial sites use Table C2.
• Forgetting the EV charger. EV chargers are continuous loads (100 percent demand factor) and can significantly increase maximum demand.
• Not checking with the supply authority. Larger installations may require formal approval and load management conditions.
• Ignoring footnotes in the tables. Tables C1 and C2 have footnotes that modify the demand factors for specific scenarios.

Where the calculator fits in

The ElecCalc Maximum Demand Calculator implements both Table C1 (residential) and Table C2 (commercial). Enter your circuits and loads, and the calculator applies the demand factors, sums the demands, and recommends a main switch rating. For the supply cable, use the Cable Sizing Calculator with the calculated maximum demand as the design current.