AS/NZS 3000 Maximum Demand Calculation Guide for Commercial Buildings
What You Need to Know
Maximum demand is the realistic peak current that an installation will draw, expressed in amps per phase. It is not the sum of every nameplate rating in the building. Maximum demand drives three sizing decisions that flow through the entire electrical design: the consumer mains cable, the main switch and switchboard busbar rating, and the supply capacity you negotiate with the network distributor.
AS/NZS 3000:2018 (the Wiring Rules) sets out the method in Appendix C. The standard provides calculation tables: Table C1 for domestic and Table C2 for non-domestic. They list every common load type and its assessment rule. Diversity factors are baked into each table for each category, so the engineer does not pick a generic "use 70%" multiplier. The factor is specific to lighting, socket outlets, ranges, water heaters, motors, and air conditioning, and it varies by installation type: single domestic, multiple domestic, or non-domestic.
Get the calculation right and the switchboard, mains, and supply are right-sized for the building. Get it wrong and you either pay too much for an oversized supply that never gets used, or you trip the main switch on a hot day with the chillers running. The calculation method is the same across NSW, VIC, QLD, SA, WA, TAS, NT, and ACT. State and territory electrical safety laws make AS/NZS 3000 mandatory for electrical work.
This memo walks through Appendix C of AS/NZS 3000 and shows a worked example for a small commercial building. It also covers the common over-sizing traps that drive electrical capital cost above what the project actually needs.
Code Requirements and Calculation Methods
- AS/NZS 3000:2018, Appendix C is the standard calculation method for maximum demand in Australia. It is informative guidance: Clause 2.2.2 also allows assessment, measurement over 30 minutes, or limitation. Table C1 covers single and multiple domestic installations. Table C2 covers non-domestic installations, with one column for hotels and motels and another for offices, shops and factories. The engineer must assess each load category separately, apply the per-category rule from the table, then sum the per-phase result. The largest phase total is the maximum demand. (AS/NZS 3000:2018, Clause 2.2.2 and Appendix C)
- Table C2 (non-domestic) lists the load groups. They cover lighting, socket outlets, cooking and cooling appliances, motors, lifts, fuel dispensing, water heaters, welding, X-ray and other equipment. Amendment 1 added electric vehicle charging to both Table C1 and Table C2. Each category has a specific assessment formula. Some are "X amps per outlet for the first N outlets, then Y amps for the next N." Some are "100% of the largest plus X% of the rest." There is no single diversity factor that applies across the whole installation. (AS/NZS 3000:2018, Tables C1 and C2)
- Three-phase balancing matters because the standard requires the engineer to calculate maximum demand per phase, not as a three-phase total. If one phase is loaded with 200 A and the other two phases are at 80 A each, the maximum demand is 200 A. The supply, mains, and switchgear must be rated for the worst phase. Single-phase loads should be distributed across the three phases at the design stage to minimise imbalance. (AS/NZS 3000:2018, Clause 2.2.4)
- Cable sizing under AS/NZS 3008.1.1 uses the maximum demand result as the design current. The cable must carry the maximum demand at the installed temperature and grouping, with voltage drop within the limit set by AS/NZS 3000 Clause 3.6 (typically 5% from the point of supply to the furthest load). The maximum demand calculation flows directly into the cable schedule. (AS/NZS 3008.1.1:2017, Section 3)
- Network distributor approval is required for any new connection or supply upgrade. Ausgrid (NSW), Endeavour Energy (NSW), Essential Energy (NSW regional), Energex (QLD), AusNet Services (VIC), and others all require a Connection Application that states the maximum demand in kVA and the proposed supply size. Distributors will challenge the calculation if the requested capacity looks oversized for the building type. (Service and Installation Rules of the relevant network)
- NCC 2025 Section J does not change the maximum demand calculation method, but it does cap lighting power density (W/m2 by space type) and sets minimum efficiency requirements for HVAC equipment. These caps reduce the connected load that feeds into the Appendix C calculation, which often reduces the maximum demand by 10% to 25% on a well-designed Section J compliant building. (NCC 2022 and NCC 2025 Volume One, Parts J7 and J6)
Worked Example: Small Commercial Building
Project: Two-storey commercial fitout, 600 m2 total floor area, mixed use (office on level 1, retail on ground). Three-phase 400 V supply. NCC 2025 Class 5 (office) and Class 6 (retail).
Step 1: List the loads by category
- Lighting: 600 m2 at 6 W/m2 average (Section J compliant) equals 3,600 W. Three-phase, so 1,200 W per phase, or 5.2 A per phase at 230 V.
- Socket outlets: 60 single 10 A GPOs total, distributed roughly 20 per phase.
- Permanently connected appliances: hot water unit 3.6 kW (single phase), kitchenette equipment 2.4 kW (single phase).
- Air conditioning: VRF system, 35 kW total cooling capacity, three-phase. Compressor full load amps from manufacturer data: 22 A per phase. Indoor units: 0.5 A per phase total.
- Mechanical ventilation: 1.5 kW three-phase fan. Full load amps 2.5 A per phase.
- Lift: hydraulic passenger lift, 7.5 kW three-phase. Full load amps 14 A per phase. Starting current is handled separately.
Step 2: Apply Table C2, Column 3 (offices and shops), per category (per phase)
| Load category | Connected (A) | Table C2 rule | MD (A) |
|---|---|---|---|
| Lighting | 5.2 | Full connected load | 5.2 |
| Socket outlets (20 per phase) | 200 | 1000 W + 100 W per extra outlet (building has permanent cooling) | 12.6 |
| Hot water (3.6 kW, 1ph on phase A) | 15.7 | Storage water heater, full load | 15.7 |
| Kitchenette (2.4 kW, 1ph on phase B) | 10.4 | Cooking and cooling appliances: 75% of all but the largest | 7.8 |
| Air conditioning (VRF compressor) | 22.0 | Largest cooling appliance, full load | 22.0 |
| Indoor fan units | 0.5 | 100% of FLA | 0.5 |
| Mech ventilation fan | 2.5 | Largest motor, full load | 2.5 |
| Lift motor | 14.0 | Largest lift, 125% of full load | 17.5 |
| Per-phase maximum demand (worst phase, A) | 76 A | ||
Phase A carries the hot water unit. Phase B carries the kitchenette. All three-phase loads, including socket outlets, apply equally across A, B, and C. Their sum is 60.3 A per phase. Phase A total is 60.3 + 15.7 hot water, equals 76.0 A. Phase B total is 60.3 + 7.8, equals 68.1 A. Phase C total is 60.3 A. Maximum demand is 76 A per phase, set by phase A.
Step 3: Convert to kVA for the network distributor
kVA = (76 A x 400 V x square root of 3) / 1000 = 53 kVA. The Connection Application to the network distributor states 53 kVA, three-phase, 400 V.
Step 4: Size the consumer mains and main switch
Design current is 76 A. Apply AS/NZS 3008.1.1 derating for installation method (in conduit, grouped). A 25 mm2 copper XLPE cable in conduit carries 96 A at 45 degrees Celsius ambient. Main switch rated 100 A, 4-pole. Switchboard busbar rated 250 A to allow for future expansion (see Decision 4 below).
Key Design Decisions
Calculate per Phase, Not Three-Phase Total
The most common error in junior calculations is summing all three phases and dividing by three. AS/NZS 3000 requires per-phase calculation because the supply, mains cable, and main switch are sized to the worst phase. A single-phase 3.6 kW hot water unit on phase A pulls 15.7 A on phase A only and zero on phases B and C. Distribute single-phase loads carefully at the design stage to keep the worst phase as close to the average as possible.
Use Table C2 Diversity, Not a Generic Multiplier
Some designers apply a flat 70% or 80% diversity factor to total connected load. This is not Appendix C compliant and produces wildly different results depending on the load mix. Table C2 gives a category-specific rule. In offices and shops, lighting is 100% of connected. Socket outlets get 1000 W for the first outlet plus 750 W for each extra outlet, or 100 W where permanent heating or cooling is installed. Motors get 100% of the largest, 75% of the second and 50% of the rest. The category-by-category method is more work but defensible if the supply size is challenged by the network distributor or the certifier.
Air Conditioning at Full Load Amps, Not Compressor kW
HVAC equipment is the single biggest source of over-sizing in commercial maximum demand calcs. The trap is using cooling kW divided by COP to estimate electrical input, then converting to amps. Manufacturer Full Load Amps (FLA) is the correct figure and it is always lower than the kW conversion suggests, because the compressor only draws full FLA in extreme conditions. Pull FLA from the equipment schedule on the mechanical drawings or the manufacturer cut sheet, not from a back-of-envelope calculation.
Switchboard Busbar Sizing for Future Growth
The main switch and consumer mains are sized to the calculated maximum demand. The switchboard busbar should be sized higher to allow for future tenant fitouts, equipment upgrades, or EV charging additions. A common rule is busbar at 1.5 to 2x the current maximum demand, capped at the next standard busbar rating (100, 250, 400, 630, 800, 1000, 1250 A). Adding capacity at construction is cheap. Replacing a switchboard later because the busbar maxed out is expensive and disruptive.
Who Needs to Know What
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References
- AS/NZS 3000:2018, Electrical Installations (Wiring Rules), including Amendment 1:2018 and Amendment 2:2020, Appendix C
- AS/NZS 3008.1.1:2017, Electrical Installations, Selection of Cables, Part 1.1: Cables for Alternating Voltages up to and Including 0.6/1 kV, Typical Australian Installation Conditions
- National Construction Code 2025, Building Code of Australia, Volume One, Section J
- Ausgrid, Network Standard NS 116, Service and Installation Rules
- Endeavour Energy, Service and Installation Rules of New South Wales
- Energex, Queensland Electricity Connection Manual