“AS/NZS 3000 Maximum Demand Calculation: The Complete 2025 Free Guide for Australian Electricians”
When designing or assessing an electrical installation, one of the most important steps is determining the maximum demand on consumer mains, submains, and final subcircuits.
Under AS/NZS 3000:2018 (Wiring Rules), maximum demand represents the highest load an installation is expected to draw, considering the distribution of equipment, how it is used, and how conditions may vary over time. Calculating this accurately ensures that cables, protective devices, and supply arrangements are sized correctly for both safety and performance.
Why Maximum Demand Matters
If the maximum demand is underestimated, the installation could face:
- Overloaded cables
- Frequent nuisance tripping
- Unsafe operating conditions
If it is overestimated, the result could be:
- Oversized (and more expensive) cables
- Larger than necessary switchgear
- Increased installation costs
The Wiring Rules therefore provide recognised methods for accurately determining maximum demand, balancing safety with cost efficiency.
Four Methods to Determine Maximum Demand
AS/NZS 3000 recognises that no single method works for every situation, so it offers four main approaches:
- Calculation
The most common method, calculation, involves applying the diversity factors and formulas provided in the standard (with guidance in Appendix C for basic installations). While the standard gives a framework, alternative calculation methods may also be used if they take into account all relevant details of the installation. - Assessment
Used in cases where load patterns are complex or irregular, assessment is suitable for:
- Installations with fluctuating or intermittent loads.
- Large or complex electrical systems.
- Special occupancies where standard diversity factors may not apply.
- Installations with fluctuating or intermittent loads.
- Measurement
This involves measuring the highest demand sustained over 30 minutes using a maximum demand indicator or recorder. If the measured value exceeds the calculated figure, the measured value becomes the official maximum demand. - Limitation
In some cases, maximum demand is set by the rating of protective devices—such as the fixed current rating of a circuit-breaker or the adjustable setting on a protective device.
Appendix C of AS/NZS 3000, particularly Tables C1 and C2, offers guidance for calculating maximum demand for consumer mains and submains only. For final subcircuits, however, maximum demand is typically determined using either the assessment method or the limitation method.
“AS/NZS 3000 Maximum Demand Calculation: The Complete 2025 Free Guide for Australian Electricians”
Purpose
Our Ausinet Maximum demand calculator calculates the total electrical demand of a residential or commercial installation. It guides switchboard sizing and protection device selection.
Required Inputs
- Installation categories include:
- House, individual living unit or shed
- Hotels, boarding houses, hospitals, residential institutions
- Factories, shops, stores, offices, business premises
- Choose the load types provided in the installation categories. Such as:
- Lighting, general
- Lighting, outdoor > 1000W
- Socket-outlet, general use 10A
- Socket-outlet, general use 15A
- Socket-outlet, general use 20A
- Cooking range > 10A
- Cooking, other > 10A
- Laundry, appliance > 10A
- HVAC, air conditioner > 10A
- Space heating > 10A
- Sauna heating > 10A
- Water heater, instantaneous
- Water heater, storage
- Water heater for pool
- Water heater for spa
- Charging, electric vehicle
- Motor, lift
- Motor, general
- Other, appliances
- Enter the load for each item in amperes/points
Interpretation of Results
If Maximum Demand ≤ Supply Capacity:
The installation is compliant and safely sized.
If Maximum Demand > Supply Capacity:
Upgrade supply, redesign circuits.
Use the result to:
- Size the main switchboard
- Select protection devices
- Determine cable sizes
- Plan for future expansion (e.g. EV, solar, battery)