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 single load or circuit to its total connected rating. For example, a domestic lighting circuit rated at 1200 W might have a demand factor of 0.83, meaning the maximum expected load is 1000 W.

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 (placeholder values pending CPEng validation):

• Lighting circuits: first circuit at 1000 W, each additional circuit at 750 W.
• Power point circuits: first at 1000 W, each additional at 500 W.
• Cooking appliances: first at 100 percent, second first 1 kW at 100 percent then remainder at 80 percent, third and subsequent at 60 percent.
• Water heating: continuous load at 100 percent.
• Heating and cooling: larger load at 100 percent, the other at 50 percent.
• Motors: largest at 100 percent, others at 80 percent.
• 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: based on total watts and floor area, typically 75 percent diversity (placeholder).
• Socket outlet circuits: 0.5 kW per circuit at 50 percent diversity (placeholder).
• Fixed appliances: 100 percent.
• Motors: 85 percent average diversity (placeholder).

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

A single-phase 230 V residential dwelling has: 3 lighting circuits, 4 power point circuits, one 6 kW oven, one 3.6 kW hot water system, one 7.4 kW ducted air conditioner, and one 7.4 kW EV charger (32 A).

• Lighting: 1000 + 750 + 750 = 2,500 W
• Power points: 1000 + 500 + 500 + 500 = 2,500 W
• Oven: 6,000 W at 100 percent = 6,000 W
• Hot water: 3,600 W at 100 percent = 3,600 W
• Air conditioning: 7,400 W at 100 percent = 7,400 W
• EV charger: 7,400 W at 100 percent = 7,400 W

Total maximum demand = 29,400 W = 29.4 kW. At 230 V that is 29,400 / 230 = 127.8 A. The next standard main switch rating is 160 A.

Worked example: commercial office

A three-phase 400 V commercial office of 200 m squared has: lighting at 15 W per m squared (3,000 W), 10 socket outlet circuits, 5 kW of fixed appliances (kitchenette, server), and no motors.

• Lighting: 3,000 W at 75 percent = 2,250 W
• Socket outlets: 10 circuits at 500 W at 50 percent = 2,500 W
• Fixed appliances: 5,000 W at 100 percent = 5,000 W

Total maximum demand = 9,750 W = 9.75 kW. At 400 V three-phase that is 9,750 / (1.732 times 400) = 14.1 A per phase. A 40 A three-phase main switch is sufficient.

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 (e.g. in an apartment building), an additional diversity factor is applied to the sum of individual maximum demands. Two dwellings might each have a maximum demand of 30 kW, but the common feeder does not need to be sized for 60 kW because not all loads in both dwellings peak simultaneously. Common values are 0.8 for 2 dwellings, 0.6 for 5, and 0.5 for 10 or more.

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.