Maximum Demand Calculator AS3000

What is Maximum Demand in Electrical Terms?

Maximum demand is the maximum current that is expected to flow through an electrical installation, or a specific part of it, at any given time. If you added up the wattage of your cooktop, oven, air conditioning systems, hot water system, pool pump, kettle, and all the power points in the house, you would get a massive number. In a typical single-phase house, that sum could easily exceed 150 Amps.

If we sized incoming service cables and consumer mains to handle that absolute sum, copper bills would skyrocket and every street transformer would be over-engineered. In the real world, you do not run every appliance at maximum capacity simultaneously. You do not weld in the garage while baking a roast, boiling three kettles, and running three ducted air conditioners.

AS/NZS 3000 Appendix C outlines standard diversity factors that account for this statistical usage. By applying these factors, we arrive at a realistic maximum demand. This figure determines the size of the mains cables, the rating of the switchboard busbars, and the capacity of the main protective devices.

How Does the Maximum Demand Calculator Work?

Our calculator automates the manual math laid out in AS/NZS 3000 Appendix C. It separates connected loads into distinct groups, applies the respective diversity factors, and sums them up to get the total load in Volt-Amps (VA) and Amps:

• Lighting: Calculated based on the floor area of the dwelling. We assume a standard density of 10 VA per square metre. The calculator takes the first 1,000 VA at 100%, and the remaining lighting load at 50%.
• General Power Points (GPOs): Based on the count of points. The first outlet is counted at 1,000 VA (100% demand). The next 19 points are calculated at 125 VA each. Any points after the first 20 are calculated at 62.5 VA each. This reflects that adding more power points increases convenience, not total concurrent load.
• Cooking Appliances: Covers stoves, cooktops, and ovens. The first 8,000W of cooking appliances is calculated at 60% load. Any wattage above 8,000W is calculated at 30%.
• Water Heating: Controlled loads (off-peak tariff hot water) are excluded because they run when other domestic demand is low. Continuous-rate hot water elements are added at 100% of their rating.
• Heating and Cooling: Air conditioning units and space heaters are loaded at 100%. However, because you do not run heating and cooling simultaneously, the calculator only includes the larger of the two loads.
• Fixed Motors: The largest motor (like a pool pump) is taken at 100%, while any additional motors are added at 50%.
• EV Chargers and Dedicated Loads: Sized at 100% without diversity, as these represent sustained high-power draws.

Once the sum is calculated in Volt-Amps, the calculator divides the total by the nominal supply voltage (230V for single-phase, or 400V × 1.732 for three-phase) to output the demand in Amps.

AS/NZS 3000 Table C1 Diversity Factors Explained

Table C1 in Appendix C of the Wiring Rules is the source of truth for residential maximum demand calculations in Australia and New Zealand. These factors are based on decades of load monitoring by distribution networks. They ensure that under hot summer conditions or freezing winter nights, consumer mains will not overheat.

For example, power points contribute 1000 VA for the first point and only 125 VA for the next 19 points because households have a limit on how many heavy appliances (vacuum cleaners, heaters, irons) are plugged in and running at the same time. The additional outlets are usually just powering low-draw devices like phone chargers, TVs, and lamps.

Similarly, cooking appliances have a heavy diversity factor because hotplates cycle on and off via thermostats, and ovens rarely run all elements continuously at full heat.

Worked Example — Maximum Demand for a 3-Bedroom House

Let's run through a typical domestic maximum demand calculation for a 3-bedroom brick veneer house in Western Sydney.

Common Mistakes with Maximum Demand Calculations

• Adding Heating and Cooling Combined: Electricians sometimes add the ducted air conditioning load and the electric space heaters together. This violates the interlock rule, as you will not run both at the same time. Always take only the larger of the two.
• Over-Sizing General Outlets: Adding up the nominal rating of every power point (e.g. 10A per GPO) rather than applying the Table C1 diversity factors. Doing this results in massive, unnecessary demand figures.
• Including Off-Peak Hot Water: Adding off-peak hot water load straight into the main demand sum. If the hot water system is wired to a controlled load tariff, it runs off-peak and is excluded from maximum demand calculations by most supply networks.
• Ignoring Future EV Charger Loads: Sizing switchboards with zero headroom. A standard single-phase EV charger draws 32A continuously. Sizing consumer mains without accounting for this can lead to expensive upgrades when the homeowner purchases an electric vehicle.

When Do You Need a Maximum Demand Calculation?

Maximum demand calculations are required in several regulatory and design scenarios:

• New Dwelling Construction: Supply authorities (like Ausgrid or Energex) require a certified maximum demand figure before approving a new connection and sizing the service line.
• Switchboard Upgrades: Upgrading an old fuse box to a modern RCBO board, or upgrading from single-phase to three-phase supply.
• Adding Heavy Fixed Loads: Installing a new ducted air conditioner, a pool heater, an induction cooktop, or a fast EV charger. You must prove the existing mains can handle the extra load.
• Subdivisions and Granny Flats: Adding a secondary dwelling requires calculating the total site demand to see if the street connection is adequate.

Supply Authority Requirements Across Australia

While AS/NZS 3000 provides the base rules, individual supply authorities (electricity distributors) have specific submission processes:

• Ausgrid (NSW): The Service Connection Application requires the maximum demand in Amps. They may inspect installations that appear marginal.
• Energex (QLD): Accepts AS/NZS 3000 maximum demand calculations but also provides their own domestic load assessment forms.
• Essential Energy (NSW): Requires the calculated maximum demand on the Connection Application form, especially for rural sites with long service runs.
• Western Power (WA): Electricians must submit calculations through their online Portal before booking a meter installation.

Always check your local distributor guidelines. Once you have calculated your maximum demand, you can proceed to size the switchboard supply cables.