AS/NZS 3000 Wire Sizing Guide: Australian & New Zealand Wiring Rules

Overview of AS/NZS 3000

AS/NZS 3000 is published jointly by Standards Australia and Standards New Zealand. It covers installations operating at voltages up to 1000 V AC or 1500 V DC. The standard is legally referenced in Australian state and territory electrical safety legislation, making compliance mandatory. Licensed electricians in Australia and New Zealand must demonstrate competence in its application. The current edition (2018, Amendment 2:2021) aligns closely with IEC 60364 but includes region-specific requirements for harsh climates, underground installations, and solar PV systems.

Cable Sizing with AS/NZS 3008.1

AS/NZS 3008.1 provides current-carrying capacity tables for copper and aluminium conductors. The process is: (1) Determine design current from maximum demand (AS/NZS 3000 Section 2). (2) Apply derating factors for ambient temperature, grouping, soil thermal resistivity (buried cables), and depth of burial. (3) Select a cable with derated ampacity ≥ design current. (4) Check voltage drop. The standard sizes follow IEC: 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300 mm².

Current-Carrying Capacity Tables

For single-phase copper PVC cables installed in air (clipped to surface), AS/NZS 3008.1 Table 3 gives: 1.5 mm² = 17.5 A, 2.5 mm² = 24 A, 4 mm² = 32 A, 6 mm² = 41 A, 10 mm² = 57 A, 16 mm² = 76 A, 25 mm² = 97 A. These values assume an ambient temperature of 40 °C (not 30 °C as in IEC), reflecting the hotter Australian climate. For XLPE cables, ampacities are higher: 2.5 mm² = 33 A, 4 mm² = 44 A, 6 mm² = 56 A.

Voltage Drop: 5% Maximum

AS/NZS 3000 Clause 3.6.2 limits voltage drop to 5% between the point of supply and the furthest point of the installation. Unlike IEC/BS which split 3% lighting / 5% power, AS/NZS uses a single 5% limit for all circuit types. The formula uses cable impedance values from AS/NZS 3008.1 Table 35 (mV/A/m). For 2.5 mm² single-phase PVC cable: 18.1 mV/A/m. Voltage drop: ΔU = mV/A/m × I × L / 1000. At 230 V, 5% = 11.5 V maximum. For 20 A over 25 m: ΔU = 18.1 × 20 × 25 / 1000 = 9.05 V = 3.9%.

Maximum Demand Calculation

AS/NZS 3000 Section 2 provides methods for calculating maximum demand, which determines the design current for sub-mains and the main switchboard. For residential installations, diversity factors are applied: first 10 A of lighting at 100%, remainder at 50%; power points use a lookup table based on the number of outlets. Air conditioning and fixed appliance loads are added at full rating. The total maximum demand determines the main switch and sub-main cable size. This differs from NEC Article 220, which uses different demand factors.

RCD and Safety Switch Requirements

AS/NZS 3000 requires 30 mA RCD protection (called safety switches in Australia) for all final sub-circuits of a domestic installation, including lighting, socket outlets, and fixed appliances. This goes further than BS 7671, which exempts some circuits. In New Zealand, the requirements are slightly different, with RCDs required for socket outlets and circuits likely to supply outdoor equipment. All RCDs must be tested every 6 months by the user (push-button test) and verified during periodic inspection.

Practical Example: 20 A Circuit in Australian Conditions

A 20 A general-purpose circuit at 230 V in Sydney (40 °C ambient), 30 m run, cables clipped to surface: Step 1 — Design current = 20 A. Step 2 — At 40 °C ambient (reference temperature for AS/NZS), no temperature derating needed. 2.5 mm² = 24 A ≥ 20 A. Step 3 — Voltage drop: ΔU = 18.1 × 20 × 30 / 1000 = 10.86 V = 4.72%. Under 5% — acceptable. Step 4 — Protection: 20 A MCB, 2.5 mm² cable. If the run were 35 m: ΔU = 12.67 V = 5.5%, exceeding 5%, so upsize to 4 mm² (11 mV/A/m → 7.7 V = 3.3%). Final selection: 2.5 mm² for 30 m, 4 mm² for 35 m.

FAQ