Cable Size Calculator

To calculate cable size according to AS/NZS 3008, 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 handles steps 4-5 automatically.

For a 32 Amp load with copper conductors and PVC insulation, you need at least 4mm² for short runs (under 15m) or 6mm² for typical installations up to 30m. For longer runs, 10mm² may be required to keep voltage drop under 5%. The exact size depends on installation method, ambient temperature, and any grouping with other cables. Use our calculator for your specific conditions.

For a 20 Amp circuit, 2.5mm² copper is typically adequate for short runs in conduit. For runs over 20 meters, consider using 4mm² to minimize voltage drop. 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 methods using mV/A/m values, derating factors for various conditions, and cable construction requirements. Part 1.1 covers cables for AC voltages up to and including 0.6/1kV.

Derating factors reduce a cable's rated current capacity based on installation conditions that affect heat dissipation. Key factors include: ambient temperature (Table 27) - higher temps mean more derating, cable grouping (Table 22) - multiple cables together generate more heat, burial depth (Table 28) - deeper burial reduces heat dissipation, and soil thermal resistivity (Table 29) - affects buried cable ratings. All applicable factors are multiplied together.

According to AS/NZS 3000 (Wiring Rules), the maximum voltage drop from the point of supply to any point in the installation should not exceed 5% for final subcircuits under normal conditions. For consumer mains (from supply to main switchboard), the typical limit is 2%. These limits ensure equipment operates correctly and efficiently.

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 provides separate current rating tables for each installation method.