Free online cable sizing calculator for Australian & New Zealand electrical installations
Enter your circuit parameters below to calculate the recommended cable size based on AS/NZS 3008 current carrying capacity and voltage drop requirements.
Cable sizing is a critical aspect of electrical installation design. Selecting the correct cable size ensures safe operation, prevents overheating, minimizes voltage drop, and complies with Australian and New Zealand electrical standards. This guide explains the methodology used in AS/NZS 3008 for selecting power cables.
The current carrying capacity (or ampacity) of a cable is the maximum continuous current it can safely carry without exceeding its temperature rating. This is determined by:
Key Principle: The cable's derated current carrying capacity must be greater than or equal to the design current (Ib) of the circuit. Mathematically: Iz × derating factors ≥ Ib
Derating factors reduce the cable's rated capacity based on real-world installation conditions. Multiple derating factors are multiplied together. The key factors from AS/NZS 3008 are:
Standard current ratings assume 40°C ambient temperature for cables in air, or 25°C for buried cables. Higher temperatures require derating:
When multiple cables are grouped together, heat dissipation is reduced. Derating factors depend on the number of circuits and arrangement:
For buried cables, installation depth affects heat dissipation. Standard ratings assume 0.5m depth:
Voltage drop occurs due to the resistance and reactance of conductors. AS/NZS 3000 limits voltage drop to ensure equipment operates correctly:
The voltage drop formula is:
Single-phase: Vd = (mV/A/m × Ib × L × 2) / 1000
Three-phase: Vd = (mV/A/m × Ib × L × √3) / 1000
Where: mV/A/m is from AS/NZS 3008 tables, Ib is design current (A), L is cable length (m)
To select the correct cable size, follow these steps:
Standard Sizes (mm²)
1, 1.5, 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500
Voltage Drop Limits
• Consumer mains: 2%
• Subcircuits: 5%
Insulation Types
• PVC (V-75): 75°C
• PVC (V-90): 90°C
• XLPE (X-90): 90°C
Current carrying capacity for copper and aluminium cables with thermoplastic (PVC) insulation at 40°C ambient temperature. Based on AS/NZS 3008.1.1 Table 4 (enclosed in conduit).
| Size (mm²) | Current (A) | mV/A/m |
|---|---|---|
| 1 | 14 | 44 |
| 1.5 | 17.5 | 29 |
| 2.5 | 24 | 18 |
| 4 | 32 | 11 |
| 6 | 41 | 7.3 |
| 10 | 57 | 4.4 |
| 16 | 76 | 2.8 |
| 25 | 101 | 1.8 |
| 35 | 125 | 1.3 |
| 50 | 151 | 0.93 |
| 70 | 192 | 0.63 |
| 95 | 232 | 0.46 |
| 120 | 269 | 0.36 |
| 150 | 300 | 0.29 |
| 185 | 341 | 0.25 |
| 240 | 400 | 0.195 |
| Size (mm²) | Current (A) | mV/A/m |
|---|---|---|
| 16 | 57 | 4.6 |
| 25 | 75 | 2.9 |
| 35 | 92 | 2.1 |
| 50 | 112 | 1.55 |
| 70 | 140 | 1.1 |
| 95 | 170 | 0.78 |
| 120 | 200 | 0.61 |
| 150 | 230 | 0.50 |
| 185 | 265 | 0.41 |
| 240 | 315 | 0.32 |
| 300 | 360 | 0.265 |
| Ambient Temp | PVC 75°C | PVC 90°C | XLPE 90°C |
|---|---|---|---|
| 35°C | 1.04 | 1.04 | 1.04 |
| 40°C | 1.00 | 1.00 | 1.00 |
| 45°C | 0.91 | 0.96 | 0.96 |
| 50°C | 0.82 | 0.93 | 0.93 |
| 55°C | 0.71 | 0.89 | 0.89 |
| 60°C | 0.58 | 0.84 | 0.84 |
Professional Features
Our professional cable sizing software goes beyond basic calculations to give you accurate, compliant results every time.
Temperature derating (Table 27)Grouping factors (Table 22)Burial depth (Table 28)Soil resistivity (Table 29)Accurate mV/A/m valuesSingle & three-phaseCompliance indicatorsAny cable lengthZs calculationsProtection coordinationMCB/RCD verificationEarth fault currentProfessional formatFull calculation breakdownCertification readyCustom brandingCopper & AluminiumPVC, XLPE, EPRAll installation methodsManufacturer dataSave calculationsOrganize by projectCable schedulesRevision history
Frequently Asked Questions
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.
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