How do you calculate maximum demand under AS/NZS 3000?

AS/NZS 3000 Appendix C sets out the calculation method. List every final subcircuit by load type (lighting, power outlets, ranges, water heating, fixed appliances, motors, air conditioning). Apply the demand assessment formula or percentage given in Table C1 for the relevant installation type (single domestic, multiple domestic, or non-domestic). Sum the per-phase amps across all categories. The largest phase result is the maximum demand for the installation. The result drives the consumer mains size, the main switch rating, and the switchboard busbar rating.

What is the difference between connected load and maximum demand?

Connected load is the total nameplate rating of every load in the installation if everything ran at full output at the same time. Maximum demand is the realistic peak load, calculated by applying diversity factors from AS/NZS 3000 Table C1 to the connected load. Connected load might be 400 amps, but the after-diversity maximum demand for a small office could be 120 to 180 amps. You size cables and switchboards to maximum demand, not connected load.

Should maximum demand be calculated in amps or kVA?

AS/NZS 3000 Appendix C calculates per-phase current in amps, which is what you need to size cables (AS/NZS 3008.1.1) and the main switch. Convert to kVA when you talk to the network distributor about supply capacity, because connection agreements are written in kVA. The conversion for a three-phase 400 V supply is kVA = (amps multiplied by 400 multiplied by square root of 3) divided by 1000, which simplifies to kVA = amps multiplied by 0.693. A 200 A three-phase maximum demand is roughly 138 kVA.

What diversity factor applies to general power outlets in a commercial building?

Under AS/NZS 3000 Appendix C Table C1, non-domestic socket outlets are assessed at 10 amps per outlet for the first 20 outlets per phase, then 5 amps per outlet for the next 20, then a reducing percentage beyond that. For a small office with 30 outlets per phase, the calculation is (20 multiplied by 10) plus (10 multiplied by 5) equals 250 amps connected, but Table C1 allows the engineer to apply a more realistic load profile based on the actual equipment. Most engineers cross-check Table C1 against measured kW/m2 figures from AS/NZS 3008 or AIRAH guidance for the building type.

AS/NZS 3000 Maximum Demand Calculation Guide

Section 1

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 a calculation table (Table C1) that lists every common load type and the assessment rule that applies to it. Diversity factors are baked into Table C1 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, because AS/NZS 3000 is referenced in the National Construction Code (NCC 2025) as the deemed-to-satisfy electrical design pathway.

This memo walks through Section C of AS/NZS 3000, shows a worked example for a small commercial building, and covers the common over-sizing traps that drive electrical capital cost above what the project actually needs.

Section 2

Code Requirements and Calculation Methods

AS/NZS 3000:2018, Appendix C is the legal calculation method for maximum demand in Australia. Section C2 covers single-phase domestic. Section C3 covers multiple-phase domestic and non-domestic. Section C4 covers commercial and industrial. The engineer must assess each load category separately, apply the per-category rule from Table C1, 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 C1 lists nine load categories: lighting, socket outlets, permanently connected appliances, ranges and cookers, water heaters, motors, air conditioning, electric vehicle charging (added in Amendment 2:2020), and floor heating. 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, Table C1)
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 2025 Volume One, Section J6 and J5)

Section 3

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 C1 per category (per phase)

Load category	Connected (A)	Table C1 rule	MD (A)
Lighting	5.2	100% of connected	5.2
Socket outlets (20 per phase)	200	10 A x first 20	15.0
Hot water (3.6 kW, 1ph on phase A)	15.7	100% on assigned phase	15.7
Kitchenette (2.4 kW, 1ph on phase B)	10.4	100% on assigned phase	10.4
Air conditioning (VRF compressor)	22.0	100% of FLA, 3-phase	22.0
Indoor fan units	0.5	100% of FLA	0.5
Mech ventilation fan	2.5	100% of FLA	2.5
Lift motor	14.0	100% of largest motor	14.0
Per-phase maximum demand (worst phase, A)			~85 A

Phase A carries the hot water unit. Phase B carries the kitchenette. All three-phase loads apply equally across A, B, and C. Phase A total is the sum of three-phase loads (59.2 A) plus 15.0 A socket outlets plus 15.7 A hot water, equals roughly 90 A. Phase B total is 59.2 + 15.0 + 10.4, equals roughly 85 A. Phase C total is 59.2 + 15.0, equals roughly 74 A. Maximum demand is 90 A per phase, set by phase A.

Step 3: Convert to kVA for the network distributor

kVA = (90 A x 400 V x square root of 3) / 1000 = 62 kVA. The Connection Application to the network distributor states 62 kVA, three-phase, 400 V.

Step 4: Size the consumer mains and main switch

Design current is 90 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).

Section 4

Key Design Decisions

1

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.

Trade-off: Per-phase calculation often shows the maximum demand is 10% to 30% higher than a naive three-phase average suggests. Worth the extra work to avoid undersizing.

2

Use Table C1 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 C1 gives a category-specific rule. Lighting is typically 100% of connected. Socket outlets get 10 A per outlet for the first 20, then less. Motors get 100% of the largest plus a percentage 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.

Trade-off: A generic factor saves 30 minutes on the calc. A correct Appendix C calc takes 1 to 2 hours and produces a result you can defend in writing.

3

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.

Trade-off: Using FLA from manufacturer data typically reduces the air conditioning contribution to maximum demand by 15% to 25% versus a kW-based estimate. On a 35 kW VRF system, that is 4 to 6 amps per phase saved.

4

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.

Trade-off: A 250 A busbar costs roughly $2,000 to $4,000 more than a 100 A busbar at construction. A switchboard replacement at year five costs $25,000 to $60,000 plus tenant disruption.

Section 5

Who Needs to Know What

Section 6

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

AS/NZS 3000 Maximum Demand Calculation Guide for Commercial Buildings