Cable Size Calculator

To calculate cable size according to AS/NZS 3008.1.1:2025, you need to: 1) Determine your load current, 2) Identify the installation method, 3) Apply derating factors for temperature, grouping, and burial depth, 4) Select a cable with adequate current carrying capacity, 5) Verify voltage drop is within the 5% limit, and 6) Check fault current withstand capability. Our calculator above does steps 3 to 6 for you, with every AS/NZS 3008.1.1:2025 derating factor.

For a 32 Amp single-phase load on copper two-core thermoplastic (V-75) cable at 40°C, AS/NZS 3008.1.1:2025 Table 3.15 gives 4mm² unenclosed (34 A) or 6mm² enclosed in conduit (39 A). Voltage drop then sets the size on longer runs: at 230 V and 32 A, 4mm² reaches 5% at about 32m, 6mm² at about 48m and 10mm² at about 80m. Grouping with other circuits or a hotter ambient needs a larger size. Use our calculator for your specific conditions.

For a 20 Amp single-phase circuit, 2.5mm² copper two-core cable carries 23 A enclosed in conduit (AS/NZS 3008.1.1:2025 Table 3.15) and keeps voltage drop within 5% of 230 V up to about 32m; 4mm² goes to about 51m. If the cable is bunched with other circuits or in high ambient temperatures, derating may require upsizing to 4mm² regardless of length.

AS/NZS 3008 (Electrical installations - Selection of cables) is the Australian/New Zealand Standard that specifies requirements for selecting cables. It covers current carrying capacity tables for different installation methods, voltage drop calculation from cable resistance and reactance (with tabulated mV/A.m values), derating factors for various conditions, and short-circuit performance. Part 1.1 (current edition AS/NZS 3008.1.1:2025) covers cables for voltages up to and including 0.6/1 kV a.c. and 1500 V d.c. under typical Australian installation conditions.

Derating factors reduce a cable's rated current capacity based on installation conditions that affect heat dissipation. Key factors in AS/NZS 3008.1.1:2025 include: ambient air and soil temperature (Tables 3.44 and 3.45) - higher temps mean more derating, cable grouping (Tables 3.33 to 3.43) - multiple cables together generate more heat, depth of laying (Tables 3.46 and 3.47) - deeper burial reduces heat dissipation, and soil thermal resistivity (Table 3.48) - affects buried cable ratings. All applicable factors are multiplied together.

AS/NZS 3000:2018 Clause 3.6.2 limits the voltage drop from the point of supply to any point in the installation to 5% of the nominal voltage. That is one total for consumer mains, submains and final subcircuits together, so the designer decides how to share it. It may be 7% where the installation has its own substation on the premises, and extra-low voltage installations are limited to 10% (Clause 7.5.7). Your distributor's service and installation rules may set further limits, such as on consumer mains, so check them.

Copper has about 1.6x the conductivity of aluminium, so copper cables are smaller for the same current capacity. Copper is preferred for most installations due to better conductivity, easier termination, and no galvanic corrosion issues. Aluminium is mainly used for large feeders (95mm² and above) where cost savings are significant. Aluminium requires special terminations and is not recommended for sizes below 16mm².

Installation method significantly affects current ratings because it determines heat dissipation capability. Cables in free air have the highest ratings. Cables in conduit have reduced ratings due to restricted air flow. Buried cables ratings depend on soil conditions and depth. Cables on trays have good ratings with proper spacing. AS/NZS 3008.1.1:2025 provides separate current rating tables for each installation method.