# 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

## How can I improve this calculator?

• Voltage (V): Specify the voltage and select the phase arrangement: 1 Phase AC, 3 Phase AC or DC.
• Load (kW, kVA, A, hp): Specify the the load in kW, kVA, A, or hp.
• PF: Specify the load power factor (cos&Phi) when load is specified in kW or hp.
• Max. volt drop (%): The maximum allowable voltage drop at the load.
• Distance (m): The cable length in meters from the source to the load. The return lenght is automatically included by the calculator.

## Short circuit protection parameters

• Protection device: Select one of the following:
• 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
• Generic: MCCBs, Air circuit breakers (ACBs), Vacuum circuit breakers (VCBs), or fuses.
• 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".

## Active cable parameters

• Cable type: The number of cores in the cable. Ignore the earth conductor in three phase cables.
• Insulation type: The type of insulation. Typically "Thermoplastic (PVC), 75°C" or "Thermoset (XLPE), 90°C". In special cases "Thermoset (XLPE), 110°C" is used.
Note that there is no option for "Thermoplastic (PVC), 90°C" (V-90) cables in AS/NZS 3008 and in the calculator. In this case "Thermoset (XLPE), 90°C" can be selected in the calculator. However, bear in mind that V-90 cables cannot be exposed to high mechanical stress at 90°C. Refer to AS 3008 for more details.
• Core type: Copper or Aluminium.
• Core size: Select a cable size or select Auto. Auto will automatically select the smallest cable that meets the three criteria for current rating, voltage drop, and fault current rating.
• Cables per phase: 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, x number of sets (of two) for single-phase. And x number of sets (of three) for three-phase.

## Earth cable parameters

• Earth core type: Only copper is currently supported.
• Earth conductor 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".

## Installation parameters

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

## 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:

• Solid or stranded copper.
• Aluminum.

Flexible cables are not supported yet.

## Cable current derating calculation

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

## Cable impedance calculation

The impedance is calculated as:

$$Z_c = \sqrt{R_c^2 + X_c^2}$$

This method calculates the impedance for the worse case power factor, i.e. when the cable and load power factor is the same.

The cable sizing calculator uses the resistance Rc from Table 35 in AS/NZS 3008-2017 .

The reactance for single core cables is selected from the flat, touching column in from Table 30 in AS/NZS 3008. This is the worse case scenario.

The reactance for multi-core core cables is selected from the circular conductors column in from Table 30 in AS/NZS 3008. This is the worse case scenario.

## 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

The three phase AC voltage drop is calculated as:

$$V_{d3\phi}=\dfrac{I L (\sqrt{3}Z_c)}{1000}$$.

The single phase AC voltage drop is calculated as:

$$V_{d1\phi}=\dfrac{I L (2 Z_c)}{1000}$$

Where I is the load current, L is the distance, and Zc is the cable impedance in Ohm/km.

## 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.

K It is based on the insulation material, initial conductor temperature and final conductor temperature.

The calculator assumes that the initial conductor temperature is the maximum allowable operating temperature for the insulation type, i.e. 75°C for PVC, and 90°C for XLPE 90°C, and 110°C 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 following K values are used.

• 111 for PVC 75 degree rated cables.
• 143 for XLPE 90 degree rated cables.
• 132 for XLPE 110 degree cables.