AS/NZS 3000 Maximum Demand Calculation — The Quick Method
AS/NZS 3000 Appendix C provides a simplified maximum demand method. Here's how to apply it correctly, including the diversity factors most people misuse.
February 26, 2026
Overview
AS/NZS 3000:2018 Appendix C provides the standard method for calculating maximum demand in installations. This determines the size of the main switch, consumer mains, and supply authority connection.
Load Categories
AS/NZS 3000 Table C1 divides loads into categories with specific demand rules:
Category A — Lighting and Power
- First 10A at 100% (no diversity)
- Remainder at 50%
- Applies to: general lighting, general power outlets (GPOs)
Category B — Cooking Appliances
Based on the number and rating of appliances:
- Single appliance up to 14A: at 100%
- Two appliances: sum at 75%
- Three or more: sum at 50% + 5A for each additional unit after the second
Category C — Heating and Cooling
- Largest appliance at 100%
- All others at 50%
- Note: only the larger of heating OR cooling is counted (they don't operate simultaneously)
Category D — Motor Loads
- Largest motor at 100%
- Second largest at 80%
- All remaining at 60%
- Plus starting current allowance per Clause C3.4
Category E — Other Loads
- Water heaters: at full rating
- Pool/spa heaters: at full rating
- EV chargers: at full rating (continuous load)
Worked Example
A residential installation:
| Load | Rating | Category | Demand Calculation | Demand (A) |
|---|---|---|---|---|
| Lighting (20 points) | 20A | A | 10A @ 100% + 10A @ 50% | 15.0A |
| GPOs (15 outlets) | 15A | A | 10A @ 100% + 5A @ 50% | 12.5A |
| Oven + cooktop | 32A + 20A = 52A | B | 52A @ 75% (2 appliances) | 39.0A |
| Air conditioner | 15A | C | 100% (largest) | 15.0A |
| Fan heater | 10A | C | 50% (not largest) | 5.0A |
| Hot water | 13A | E | 100% | 13.0A |
| EV charger | 32A | E | 100% (continuous) | 32.0A |
Total maximum demand: 131.5A
With the addition of starting allowance for the air conditioner compressor motor: add 33% of motor rated current = 5.0A.
Final maximum demand: 136.5A
This determines: main switch ≥ 160A, consumer mains sized for 136.5A with voltage drop check, and supply authority notified of 137A maximum demand.
Common Mistakes
- Counting both heating AND cooling at demand: only the larger applies (they're mutually exclusive)
- Ignoring continuous load rules for EV chargers: these must be at 100%, no diversity
- Applying diversity to water heaters: hot water systems are counted at full rating
- Forgetting motor starting allowance: required for the largest motor only
Verification
Always cross-check the calculated maximum demand against the supply authority's minimum supply threshold. Most utilities require notification or application for supplies exceeding 100A single-phase or 63A per phase three-phase.
Calculate your demand: Use the Maximum Demand Calculator for automatic diversity factor application.
Frequently Asked Questions
What standards govern cable sizing calculations?
The primary standards are AS/NZS 3008.1.1:2017 (Australia/NZ), BS 7671:2018 (UK), IEC 60364-5-52 (International), and NEC Article 310 (USA). Each has different assumptions for ambient temperature, installation methods, and derating factors.
Why do different standards give different cable ratings?
Standards differ in reference ambient temperature (AS/NZS uses 40°C, BS 7671 uses 30°C), test conditions, grouping factor calculations, and installation method classifications. A 50mm² XLPE cable can vary by 15% between standards.
Related Articles
- Max Demand - Interactive calculator with standards compliance
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