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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

Voltage parameters

  • Voltage (V): Select the phase arrangement, and the voltage. If the voltage is not available in the select list, choose Other and specify any value.

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.
  • 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 if used to calculate the voltage drop. Read more here.

Load parameters

  • Load: Specify the the load in kW, kVA, A, or hp.
  • PF: Specify the load power factor, cos(θ), when load is specified in kW or hp. Or when the Load Power Factor option in voltage drop options is selected as Specified.

Cable parameters

  • Cable type:
    Type Description
    Multi-core 2C+E 1 Phase AC.
    Multi-core with 3 cores. 1 Live, 1 neutral and 1 earth.
    Single-cores 2x1C+E 1 Phase AC.
    Three seperate single core cables: 1 live, 1 neutral and 1 earth.
    Multi-core 3C+E 3 Phase AC.
    Multi-core cable with 4 cores: 3 phase cores, and 1 earth core.
    Multi-core 4C+E 3 Phase AC.
    Multi-core cable with 5 cores: 3 phase cores, 1 neutral core and 1 earth core.
    Single-cores 3x1C+E 3 Phase AC.
    Fours seperate single-core cables: 3 phase cables and 1 earth cable.
    Single-cores 4x1C+E 3 Phase AC.
    Five seperate single-core cables: 3 phase cables, 1 neutral cable and 1 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
  • Insulation -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 mehanicall pressure on the insulation.
  • 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.
  • Live core type: Copper, Flexible copper or Aluminium.
  • Live core size: Select a cable size or select Auto. Auto will automatically select the smallest cable that meets the criteria for current rating, voltage drop, fault current rating, loop impeance, and protection device thermal trip current.
  • Earth core type: Only copper is currently supported for earth cores.
  • Earth core size: Select a cable size or select Auto. Auto will automatically select a cable based on AS 3000-2018, Table 5.1, "Minimum Copper Earthing Conductor Size".
  • User parallel cables: Select if required.
  • Number of parallel cables (multi-core) or Active cables per phase (single-core): Typically only one cable per phase, for single- or multi-core cables. More than one cable may be selected for high load scenarios. If the cable type is single-core, this parameter means sets of cables. That is, m number of sets (of two) for single-phase voltage. And x number of sets (of three) for three-phase.

Installation parameters

  • Cable installation: How the cable will be installed. Consider the worst case section of the cable installation.

Short circuit protection parameters -MCB

  • Protection device: MCB.
  • MCB curve type: The MCB tripping curve: B, C or D.
  • MCB rating: Select an MCB rating or select Auto. Auto 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 fault current (kA): Specify the prospective fault current on the primary side of the circuit breaker. This parameter is required when the source impedance method is selected as "Calculate".

Short circuit protection parameters -MCCB

  • Protection device: MCCB.
  • Make: Todo.
  • Model: Todo.
  • CB fault rating: Todo.
  • Trip unit type: Todo.
  • Trip unit rating: Select a trip unit rating rating or select Auto. Auto will automatically select the recommended rating. In Auto, the MCCB trip unit rating and and thermal trip level is selected for the load current. And then checked against the cable..
  • Thermal trip: Todo.
  • Magentic trip: Todo.
  • Three-phase fault: Todo.
  • Earth fault: Todo.
  • Source impedance: Specify the method to determine the external loop impedance.
    • Estimate: Estimate according to AS/NZS 3000-2018, i.e. assume 80% voltage available at the cable source during an earth fault.
    • Calculate: Calculate from the prospective fault current.
    • Measured: Specify the measured impedance in Ohm.
  • Prospective fault current (kA): Specify the prospective fault current on the primary side of the circuit breaker. This parameter shows when the current limiting parameter is selected as "no". Or when the source impedance method is selected as "Calculate".

Short circuit protection parameters -Generic

  • Protection device: Generic.
  • Trip current (A): The trip pickup current of the protection device.
  • Trip time (ms): The short circuit clearing time for the protection device.
  • Current limiting (yes/no): Specifiy if the circuit breaker or fuse can limit the fault energy. Typically fuses and MCCBs.
  • Let through energy (A2s): The let through fault energy I2t in A2s. The let-through energy is available on curves from the device manufacturer.
  • Source fault impedance: Specify the method to determine the external loop impedance.
    • Estimate: Estimate according to AS/NZS 3000-2018, i.e. assume 80% voltage available at the cable source during an earth fault.
    • Calculate: Calculate from the prospective fault current.
    • Measured: Specify the measured impedance in Ohm.
  • Prospective fault current (kA): Specify the prospective fault current on the primary side of the circuit breaker. This parameter shows when the current limiting parameter is selected as "no". Or when the source impedance method is selected as "Calculate".

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.

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

The cable sizing calculator supports the following conductors:

  • Copper (solid or stranded).
  • Flexible copper.
  • Aluminum.

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 calcuate the voltage drop.

This will result in a lower voltage drop.

It is usefull when the power factor of the load is known. And it is stable at full load. For example, electical 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.