AS/NZS 3008.1.1:2017 — Cable Selection Standard Overview

AS/NZS 3008.1.1:2017 — Electrical installations — Selection of cables — Part 1.1: Cables for alternating current voltages up to and including 0.6/1 kV — is the joint Australian and New Zealand standard for selecting low-voltage cables in electrical installations. Published by Standards Australia and Standards New Zealand, it provides the methods, tables, and procedures for determining:

• Current-carrying capacity of cables for various installation methods
• Derating factors for ambient temperature, grouping, soil conditions, and other installation variables
• Voltage drop impedance values (mV/A/m) for cable sizing verification
• Short circuit withstand capacity of cables
• Earth fault loop impedance for protective device coordination

The current edition is AS/NZS 3008.1.1:2017, with the 2025 edition published in late 2024. The standard covers copper and aluminium conductors with PVC (V-75, V-90), XLPE (X-90), and elastomeric insulation types, in single-core and multicore configurations.

AS/NZS 3008 is referenced directly by AS/NZS 3000:2018 (Wiring Rules), which is the mandatory installation standard in both countries. Any cable installed under the Wiring Rules must be sized in accordance with AS/NZS 3008.

The standard is organised around a comprehensive set of tables that form the basis of all cable sizing calculations. Understanding which table to use and how to read it is essential.

AS/NZS 3008 Table 3 defines 29 installation method configurations, each of which determines the cable's heat dissipation characteristics and therefore its current-carrying capacity. The main categories are:

• Enclosed in wiring enclosure (conduit, trunking): Columns 1–8. Restricted heat dissipation due to enclosure. Typical for commercial and industrial wiring.
• Clipped direct to a surface: Column 9. Better heat dissipation than conduit. Common for exposed cable runs in industrial settings.
• Cable tray (perforated and unperforated): Columns 10–15. Good heat dissipation, widely used for cable distribution in commercial buildings and industrial plants. Perforated tray allows better ventilation than solid tray.
• Suspended from catenary wire: Column 16. Free-air conditions with good heat dissipation.
• Free air (spaced from surface): Columns 17–19. Maximum heat dissipation. Provides the highest current ratings but is rarely practical for everyday installations.
• Buried direct in ground: Columns 20–23. Heat dissipation depends on soil type and moisture. Requires burial depth, soil thermal resistivity, and ground temperature derating factors.
• In underground ducts/conduit: Columns 24–29. Most restricted below-ground installation. Commonly used for road crossings and areas where future cable replacement may be required.

The choice of installation method has a dramatic impact on cable size. For example, a 16 mm² V-90 multicore copper cable is rated at 76 A in free air (Column 17) but only 57 A enclosed in conduit (Column 6) — a 25% reduction.

Current ratings in AS/NZS 3008 tables are based on specific reference conditions. When actual installation conditions differ, derating factors must be applied:

The 40°C reference ambient temperature is notably higher than BS 7671 and NEC (both 30°C). This means that at 30°C, AS/NZS 3008 tabulated ratings already have built-in margin and actually need an increase factor (k > 1.0). Conversely, in hot climates exceeding 40°C — common in inland Australia — significant derating is required.

AS/NZS 3008 and AS/NZS 3000 work together as a pair:

• AS/NZS 3000:2018 (Wiring Rules) is the overarching installation standard. It defines requirements for the design, construction, and verification of electrical installations, including maximum demand calculation (Part 2), protection requirements, earthing, and voltage drop limits.
• AS/NZS 3008 is specifically referenced by AS/NZS 3000 for cable selection. Clause 3.5 of AS/NZS 3000 requires cables to be selected in accordance with AS/NZS 3008.

The typical workflow is:

1. Calculate maximum demand per AS/NZS 3000, Part 2 (diversity factors, load grouping)
2. Determine the design current for each circuit
3. Select cables per AS/NZS 3008 (current rating, derating, voltage drop)
4. Verify voltage drop complies with AS/NZS 3000, Clause 3.6.2 (5% power, 3% lighting)
5. Verify earth fault loop impedance per AS/NZS 3000, Clause 5.8

In practice, both standards work together: maximum demand calculations use AS/NZS 3000 diversity tables, while cable sizing uses AS/NZS 3008 current rating and voltage drop tables.

The AS/NZS 3008.1.1:2025 edition was published in late 2024, replacing the 2017 edition. Key changes include:

• New cable types: Updated current rating tables include modern cable constructions such as low-smoke zero-halogen (LSZH) and fire-rated cables that have become prevalent since the 2017 edition.
• Updated derating factors: Revised grouping factors in Table 25 reflecting recent research on mutual heating effects, particularly for cables on trays and in enclosed trenches.
• Harmonisation with IEC 60364-5-52: Closer alignment of installation method definitions and table structures with the international standard, making it easier for engineers working across both AS/NZS and IEC jurisdictions.
• Enhanced buried cable provisions: Updated soil thermal resistivity data and refined depth-of-burial derating factors based on field measurement data from Australian conditions.
• Solar PV cable provisions: New guidance for selecting DC cables for photovoltaic installations, reflecting the growth of rooftop and utility-scale solar systems.

The transition period allows use of either the 2017 or 2025 edition until the 2025 edition is formally cited in the next update to AS/NZS 3000. Professional tools should support both editions to allow comparison during the transition period.

Professional cable sizing work requires access to the complete AS/NZS 3008.1.1:2017 table set:

• All 29 installation methods from Table 3
• Current rating tables 13, 14, and 15 for all conductor materials and insulation types
• Full derating factor tables 22–27 (ambient temperature, ground temperature, grouping, depth, soil resistivity)
• Voltage drop impedance tables 30–42 with temperature correction
• Cross-referencing between installation method and correct current rating column

Calculation results should include full standard references — table numbers, column numbers, and applicable clause references — so each step can be verified against the physical standard.