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Cable Size Calculator AS/NZS 3008

Cable size calculator for current rating, voltage drop, loop impedance, earth cable and short circuit, based on Australia and New Zealand standard AS/NZS 3008.

Underground wiring enclosure

See Also

Standard parameters

  • Standard: Select the relevant part in AS/NZS 3008: 2017.
    • Part 1: Australian conditions. Air 40°C. Soil 25°C.
    • Part 2: New Zealand conditions. Air 30°C. Soil 15°C.
  • The differences between the parts are:
    • Rating tables, Table 4 to 15.
    • Air temperature derating, Table 27(1).
    • Soil temperature derating, Table 27(2).

Load parameters

  • Phase: Select the phase arrangement. 3 Phase AC, 1 Phase AC, or DC.
  • Voltage (V): Select the voltage. If the voltage is not available in the select list, choose Other and specify any value.
  • Rating: Specify the the load rating in kW, kVA, A, or hp. Enter the per‑phase current for 3‑phase systems. For example, enter only 100 A in the following scenario.

    Phase 1 100 A
    Phase 2 100 A
    Phase 3 100 A
    Enter only 100 A
  • PF: Specify the load power factor when the load rating is specified in kW or hp. It is also required for A and kVA ratings, when the Load Power Factor option is selected as Specified under Advanced Voltage Drop options.

Voltage drop parameters

  • Max. volt drop (%): The maximum allowable voltage drop at the load.
  • Distance (m): The cable length in metres from the source to the load. The return length is automatically included by the calculator for single-phase and three-phase installations.
  • Advanced options: Select to show the following two options.
  • Conductor temperature: Select Calculate or Maximum.
    Calculate (default) The calculated operating temperature is used to select the cable resistance for voltage drop calculations. Read more here.
    Maximum The maximum rated temperature of the insulation is used to select the cable resistance for voltage drop calculations. This is the most conservative option.
  • Load power factor: Worst case or Specified.
    Worst case (default) The worst-case power factor is used to calculate the voltage drop. This is the most conservative option. Read more here.
    Specified The specified power factor of the load is used to calculate the voltage drop. Read more here.

Active cable parameters

  • Insulation: Select the insulation for the cables. For multi-core cables, this applies to the live, neutral and earth conductors. For single-core cables, this applies to the live and neutral conductors. The earth-cable select is selected under Earth Cable Insulation.

    Popular
    PVC V-90 Standard 75°C Most popular insulation.
    Max. temperature: 75°C.
    For high temperature, see PVC V-90 Operate at 90°C .
    XLPE X-90 Standard 90°C Most popular XLPE.
    Max. temperature: 90°C
    Special PVC
    PVC V-75 Traditional 75°C Older type PVC cables.
    Traditionally labelled V-75.
    Max. temperature: 75°C.
    PVC V-90 Operate at 90°C Standard V-90 PVC.
    Max. temperature: 75°C.
    Can operate at 90°C if there is no mechanical pressure on the insulation.
    PVC V-90HT Operate at 105°C High temperature PVC.
    Max. temperature: 75°C.
    Can operate at 105°C for 500 hours per year, if there is no mechanical pressure on the insulation.
    Special XLPE
    X-110 High Temp 110°C High temperature XLPE.
    Max. temperature: 110°C.
    X-HF-90 Fire rated 90°C Fire rated XLPE.
    Max. temperature: 90°C.
    X-HF-110 Fire and High Temp Fire rated XLPE.
    High temperature.
    Max. temperature: 110°C.
  • Cable type: Select the cable type as shown below.

    1 Phase AC or DC
    Multi-core 2C+E Multi-core cable. 1 live core, 1 neutral core and 1 earth core.
    Single-cores 2x1C+E Single core cables: 1 live cable, 1 neutral cable and 1 earth cable.
    3 Phase AC
    Multi-core 3C+E Multi-core cable. 3 phase cores, and 1 earth core.
    Multi-core 4C+E Multi-core cable. 3 phase cores, 1 neutral core and 1 earth core.
    Single-cores 3x1C+E Single-core cables: 3 phase cables and 1 earth cable.
    Single-cores 4x1C+E Single-core cables: 3 phase cables, 1 neutral cable and 1 earth cable.
  • Flexible: Select if a flexible cable is used.
  • Conductor: Select Copper or Aluminium.
  • Active size (mm2): Select a conductor size or select Auto. The Auto option will automatically select the smallest cable that meets the following criteria.

    • Current rating.
    • Voltage drop.
    • Fault current rating.
    • Loop impedance.
    • Protection device thermal trip current.

    This applies to the live and neutral conductors.

  • Use parallel cables: Generally, only one cable is required for multi-core applications. Or one cable group is required for single-core applications.

    This option enables you to use parallel cables in high-current installations, where more than one multi-core cable or more than one single-core cable group is required. This means more than one live core per phase.

    For single-core applications. The whole cable group is counted as one. A cable group includes three phase cables, the neutral cable, and the earth cable. It can be installed in a flat or trefoil formation.

  • No of cables or cable groups: Specify the number of parallel cables or parallel cable groups.

    Multi-core cables Number of cables
    Single-core cable groups Number of cable groups

Earth cable parameters

  • Earth core size (mm2): Select a core size or select Auto. The Auto option will automatically select an earth core size from AS 3000-2018, Table 5.1, "Minimum Copper Earthing Conductor Size". The fault rating and the earth loop impedance of the selected earth core are then checked.

    If the earth core size is selected manually, the selected core is checked against AS 3000-2018, Table 5.1. And the fault rating and the earth loop impedance of the selected earth core are checked.

    Note that the earth core sizes from AS 3000-2018, Table 5.1 will match the earth core size in most commercially available multi-core cables.

  • No of earth cores or cables: This option is visible when parallel cables are used.

    Multi-core cables Matches the number of parallel cables.
    Cannot be changed.
    Single-core cable groups Limited to one earth cable.
    Cannot be changed.
    Please let me know if you need to use more than one.
  • Insulation: Applicable to single-core cables. Earth cable insulation. It can be different than the active cable insulation. The options are the same as the active cable insulation options.
  • Flexible: Applicable to single-core cables. Select if a flexible earth cable is used.
  • Conductor: Applicable to single-core cables. Select Copper or Aluminium. Note that you cannot select Aluminium for a copper active cable. Please let me know if you need to.
  • Earth size: Applicable to single-core cables. Select a cable size or select Auto. The Auto option will automatically select an earth cable size from AS 3000-2018, Table 5.1, "Minimum Copper Earthing Conductor Size".

Installation parameters

  • Installation method: Select the installation method. Consider the worst-case section of the cable installation. That is, the installation section with the lowest current rating.
  • Formation: Trefoil or laid flat.
  • Show derating options: Check this box if you want to specify derating options. If not, the following default derating options are used.

    Australian conditions New Zealand conditions
    Number of cables or cable groups 1 1
    Air temperature 40 °C 30 °C
    Soil temperature 25 °C 15 °C
    Cable depth 0.5 m 0.5 m
    Soil resistivity 1.2 °C.m/W 1.2 °C.m/W
    Circuit derating None None
  • Cable support: The cable support options depend on the installation method.
  • Number of cables, cable groups, or enclosures: This is required when multiple cables are installed close to each other, where the heat radiated from adjacent cables affects the current rating.

    It includes parallel multi-core cables and parallel single-core cable groups. For parallel multi-core cables, each cable is considered in the derating. For parallel single-core cable groups, each cable group is considered in the derating -not the individual cables for each phase or the neutral.

    Here are a few examples.

    Multi-core cables Number of cables
    Single-core cables Number of cable groups
    Multi-core cables
    One cable per enclosure
    Number of enclosures
    Single-core cable
    One cable group per enclosure
    Number of enclosures
    Single-core cables
    One cable per enclosure
    Number of cable groups
  • Space between cables, cable groups or enclosures: Select the spacing between multi-core cables, single-core cable groups or enclosures (underground multi- and single-core). The table below shows the spacing that can be selected for different installations and cable types.

    In air Multi-core cables
    Space between the edges of cables.
    Touching
    Spaced 1D The diameter of the largest cable.
    In air Single-core cables
    Space between the edges of cable groups.
    Touching
    Spaced 1D The diameter of the largest cable.
    Underground Multi-core cables
    Space between the centres of cables.
    Touching Buried direct

    Underground enclosure
    150 mm
    300 mm
    450 mm
    600 m
    Underground Single-core cables
    Space between the centres of cable groups.
    Touching Buried direct

    Underground enclosure
    150 mm
    300 mm
    450 mm
    600 m
  • No of tiers: Number of tiers.

    • No of horizontally tiered racks: The number of horizontal tiers (rows) of racks or ladders installed above each other. The minimum vertical space between racks is 300 mm. See AS/NZS 3008:2017 Tables 23 and 24.
    • No of horizontally tiered unperforated trays: The number of horizontal tiers (rows) of trays installed above each other. The minimum vertical space between trays is 300 mm. See AS/NZS 3008:2017 Tables 23 and 24.
    • No of horizontally tiered perforated trays: The number of horizontal tiers (rows) of trays installed above each other. The minimum vertical space between trays is 300 mm. See AS/NZS 3008:2017 Tables 23 and 24.
    • No of vertical back-to-back perforated trays: On or two vertical back-to-back trays. The minimum vertical space between trays is 230 mm. ee AS/NZS 3008:2017 Tables 23 and 24.
  • Air temperature (°C): For cables installed in air. The defaults (no derating) are.
    • Australian condictions: 40°C
    • New Zealand condictions: 30°C
  • Soil temperature (°C): For cables installed underground. The defaults (no derating) are:
    • Australian condictions: 25°C
    • New Zealand condictions: 15°C
  • Cable depth (m): The default (no derating) is 0.5 m.
  • Soil resistivity (°C.m/W): The default (no derating) is 1.2 °C.m/W.

Short circuit protection parameters

  • Check short circuit rating: Select to check the fault rating of the cable.
  • Check loop impedance: Select to check the earth loop impedance of the circuit. In other words, to check if the specified protection device will trip for an earth fault when the specified cable is used.

    The external source impedance is estimated, calculated or specified (measured). The method depends on the selected protection device. When it is calculated, it is calculated from the prospective earth fault current.

    Protection device Loop impedance medhod
    MCB Estimated, Calculated or Measured
    MCCB Calculated
    Generic device Calculated
  • Protection: Select protection device. MCB, MCCB or Generic.

    Protection device Description
    MCB Miniature circuit breakers. From 1 A to 125 A.
    MCCB Moulded case circuit breakers. From 16 A to 630 A.
    Generic device Generic phase and earth current fault protection.
    With definite time settings.

Short circuit protection parameters -MCB

  • MCB curve type: The MCB tripping curve: B, C or D.
  • MCB rating: Select an MCB rating or select Auto. The auto option will automatically select the recommended size from Tables C6 and C7 in AS/NZS 3000-2018. In Auto, the MCB size is selected for the load current. And then checked against the cable.
  • Source impedance: Specify the method to determine the source (external) loop impedance.
    • Estimate: Estimate according to AS/NZS 3000-2018, i.e. assume that 80% voltage is available at the cable source during an earth fault.
    • Calculate: Calculate from the prospective fault current.
    • Measured: Specify the measured impedance in Ohm.
  • Prospective earth fault current (kA): Specify the prospective earth fault current on the primary side of the circuit breaker. This parameter is required when the source impedance method is selected as "Calculate". It is used to calculate the external source loop impedance for earth faults.

Short circuit protection parameters -MCCB

  • Make: Currenly only Schneider is supported. Please let me know if you need other manufacturers.
  • Model: The number in the Schneider model name represents the current rating of the circuit breaker. For example, NSX250 is 250 A.
  • CB fault rating: The fault current rating of the circuit breaker. This affects the let-though energy.
  • Trip unit: Select between Thermal Magnetic (TM-D) and Micrologic 2.3.
    • Thermal Magnetic
      • Trip unit rating: Select the current rating or select Auto. The following are automatically selected in Auto: trip unit rating, thermal trip setting, and magnetic trip setting. The trip unit rating, and the thermal trip setting, are selected to be ≥ the load current.
      • Thermal trip: The thermal trip setting can be selected, if the trip unit rating was selected manually.
      • Magnetic trip: The magnetic trip setting can be selected, if the trip unit rating was selected manually.
    • Micrologic 2.3
      • Trip unit rating: Select the current rating or select Auto. The following are automatically selected in Auto: trip unit rating, thermal trip setting, short-time trip setting, and instantaneous trip setting. The trip unit rating, and the thermal trip setting, are selected to be ≥ the load current. The short-time trip setting is selected as 6x the thermal trip setting.
      • Thermal trip: The thermal trip setting can be selected, if the trip unit rating was selected manually.
      • Short-time trip: The short-time trip setting can be selected, if the trip unit rating was selected manually.
      • Instantaneous trip: The instantaneous trip setting can be selected, if the trip unit rating was selected manually.
    • Prospective phase fault: The prospective phase fault current on the primary side of the circuit breaker. Specify the 3-phase fault current for 3-phase AC supplies. Or specify the phase-to-neutral fault current for 1-phase AC and DC supplies. When Check short circuit rating is selected, this parameter is used to calculate the phase fault let-through-energy (I2t) of the circuit breaker, and compares it against the fault energy rating of the cable.
    • Prospective earth fault (kA): The prospective earth fault current on the primary side of the circuit breaker. Specify the phase-to-earth fault current for 3-phase AC, 1 Phase AC and DC supplies. When Check loop impedance is selected, this parameter is also used to calculate the external source loop impedance for earth faults. And when Check short circuit rating is selected, it is used to calculate the earth fault let-through-energy (I2t) of the circuit breaker, and compares it against the fault energy rating of the cable.

Short circuit protection parameters -Generic

  • Current limiting (yes/no): Specify if the circuit breaker or fuse can limit the fault energy. For example current limiting fuses and MCCBs.
    • Current limiting
      • Prospective phase fault current (kA): The prospective fault current on the primary side of the circuit breaker. It is used to calculate the phase fault current at the load.
      • Phase fault let through energy (A2s): It is checked against the fault energy rating of the live and neutral cores of the cable. The let-through energy (I2t) is available on current-limiting curves from the device manufacturer. It is a function of the prospective fault current.
      • Prospective earth fault current (kA): The prospective phase-to-earth fault current. It is used to calculate the earth fault current at the load. It is also used to calculate the external earth loop impedance.
      • Earth fault trip current (A): The earth fault trip setting of the protection device. It is used to check the earth loop impedance of the circuit.
      • Earth fault let through energy (A2s): It is checked against the fault energy rating of the live and earth cores of the cable The let-through energy (I2t) is available on current-limiting curves from the device manufacturer. It is a function of the prospective fault current.
    • Not current limiting
      • Prospective phase fault current (kA): The prospective phase fault current on the primary side of the circuit breaker. It is used to calculate the phase fault current at the load. It is also used to check the phase fault current rating of the live and neutral cores of the cable.
      • Phase fault trip time (ms): The phase fault clearing time of the protection device. It is used to check the fault current rating of the live and neutral cores of the cable.
      • Prospective earth fault current (kA): The prospective phase-to-earth fault current on the primary side of the circuit breaker. It is used the calculate the earth fault current at the load. It is also used to calculate the external earth loop impedance. And it is used to check the earth fault current rating of the live and earth cores of the cable.
      • Earth fault trip current (A): The earth fault trip setting of the protection device. It is used to check the earth loop impedance of the circuit.
      • Earth fault trip time (ms): The earth fault clearing time of the protection device. It is used to check the earth fault current rating of the live and earth cores of the cable.

Cable group definition

A cable group refers to a set of single-core cables in a circuit. For example, red, white, and blue phase single-core cables.

For three-phase applications, A cable group includes three phase cables, the neutral cable (if applicable), and the earth cable. It can be installed in a flat or trefoil formation.

For single-phase applications, A cable group includes a phase cable, a neutral cable, and an earth cable.

Each set is counted as one separate cable group.

  • Do not count the three phases.
  • Do not count the neutral.
  • Do not count the earth cable.

Example: Two parallel single-core cable groups.

Example: Two parallel multi-core cables.

Cable current rating calculation

The current ratings are selected from Tables 4 to 21 in AS/NZS 3008-2017. It is based on cable type, insulation type and the cable installation method.

For Australian conditions, Tables 4 to 21 are based on an ambient temperature of 40°C and a ground temperature of 25°C.

For New Zealand conditions, Tables 4 to 21 are based on an ambient temperature of 30°C and a ground temperature of 15°C.

The cable sizing calculator supports the following conductors:

  • Copper (solid, stranded and flexible).
  • Aluminum (solid, stranded and flexible).

Cable current derating calculation

The current derating for the cables has been implemented according to AS/NZS 3008:2017.

Cable impedance calculation

The impedance is calculated as:

\(Z_c = \sqrt{R_c^2 + X_c^2}\)

Where,

Loop impedance calculation

The maximum loop distance is calculated as:

\(L_{max}=\dfrac{0.8 \cdot V_{1\phi} \cdot 1000}{I_{min} \cdot Z_{c} }\)

Where:

  • V is the single phase voltage.
  • Imin is the minimum alowable tripping current of the MCB or other protection device.
  • Zc is the cable impedance in Ohm/km.

Voltage drop calculation with worst case load power factor

This is the default method in the calculator.

Its the simplest. The most conservative. And the most often used.

The worst-case power factor is when the cable and load power factors are the same.

The voltage drop formulas are shown below.

1-phase AC \(\Delta V_{1\phi}=\dfrac{I \cdot L \cdot 2 \cdot Z_{c}}{1000}\)
3-phase AC \(\Delta V_{3\phi}=\dfrac{I \cdot L \cdot \sqrt{3} \cdot Z_{c}}{1000}\)
DC \(\Delta V_{dc}=\dfrac{I \cdot L \cdot 2 \cdot R_{c\_ph}}{1000}\)

Where,

  • I is the load current in ampere (A).
  • L is the cable distance on meters (m).
  • Zc is the cable impedance in Ω/km.
  • Rc is the cable resistance in Ω/km.

The impedance Zc for the worst case power factor is calculated as:

\(Z_c = \sqrt{R_c^2 + X_c^2}\)

Where,

Voltage drop calculation with specified load power factor

The specified load power factor is used to calculate the voltage drop.

This will result in a lower voltage drop.

It is useful when the power factor of the load is known. And it is stable at full load. For example, electrical motors.

The formulas are shown below.

1-phase AC \(\Delta V_{1\phi} {=} \dfrac{I {\cdot} L {\cdot} 2 {\cdot} [R_{c} {\cdot} \cos (\theta) {+} X_{c} {\cdot} \sin (\theta)]}{1000}\)
3-phase AC \(\Delta V_{3\phi} {=} \dfrac{I {\cdot} L {\cdot} \sqrt{3} {\cdot} [R_{c} {\cdot} \cos (\theta) {+} X_{c} {\cdot} \sin (\theta)]}{1000}\)
DC \(\Delta V_{dc}=\dfrac{I \cdot L \cdot 2 \cdot R_{c\_ph}}{1000}\)

Where,

  • I is the load current in ampere (A).
  • L is the cable distance on meters (m).
  • Rc is the cable resistance in Ω/km.
  • Xc is the cable reactance in Ω/km.
  • θ = arccos(pf), and pf is the specified load power factor.

Cable resistance

The calculator selects the resistance, Rc, values from Table 34, Table 35 and Table 37 in AS/NZS 3008.

The selection is based on the "Conductor Temperature" selection under the voltage drop options.

Conductor temperature Resitance selection
Calculated

The conductor temperature is calculated.

And the nearest (higher) resistance is selected from the tables.

This is described in section 4.4. in AS/NZS 3008.

The cable resistance is smaller at lower conductor temperatures.

Maximum

The maximum allowable insulation temperature is assumed.

And the relevent resistance values is selected from the tables.

This is the most conservative option.

Table 36 (shaped conductors) is not used. Table 35 (circular conductors) is used instead. It is more conservative.

AS/NZS 3008 does not specify the DC resistance. The specified AC values are used.

Cable reactance

The calculator selects the reactance values, Xc, from Table 30 and Table 31 in AS/NZS 3008.

Short circuit calculation

The short circuit capacity of the cables is calculated according to AS/NZS 3008-2017 as

\(I^2t = K^2S^2\)

Where:

  • I is the short circuit current capacity in amperes,
  • t is the short circuit duration in seconds.
  • S is the cross-sectional area of the conductor.
  • K is a constant that is selected from Table 52 in AS/NZS 3008-2017.

The fault constant, K, is based on the insulation material, the initial conductor temperature, and the final conductor temperature.

The calculator uses the maximum allowable operating temperature as the initial conductor temperature. For example, 75°C is used for PVC insulated cables, 90°C is used for XLPE insulated cables, and 110°C is for XLPE 110°C.

The maximum allowable short circuit temperature from Table 53 in AS/NZS 3008-2017 is used as the final conductor temperature i.e. 160°C for PVC and 250°C for XLPE.

The calculator selects the K value from Table 52 in AS/NZS 3008, based on the initial conductor temperature and the maximum allowable short circuit temperature.