Cable Size Calculator AS/NZS 3008

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Parameters

• Standard
• Load
• Voltage drop
• Active cable
• Earth cable
• Installation
• Installation derating
• Short circuit
  • MCB
  • MCCB
  • Generic device

Definitions

• Single-core cable group

Calculations

• Current rating
• Derating
• Impedance
• Loop impedance
• Voltage drop
• Resistance
• Reactance
• Short circuit
• Conduit

See Also

• Maximum Demand Calculator AS/NZS 3000
• Arc Flash Calculator IEEE 1584

Standard parameters

• Standard: Select the relevant standard.
  • AS/NZS 3008.1.1:2025 Australian conditions.
  • AS/NZS 3008.1.1:2017 Australian conditions.
  • AS/NZS 3008.1.2:2017 New Zealand conditions.
  • 2025 New Zealand conditions have not been published by Standards New Zealand yet.
• The differences between New Zealand and Australian conditions are:

  	Air	Soil
  Australian conditions	40°C	25°C
  New Zealand conditions	30°C	15°C

• This results in different:
  • Cable ratings.
  • Air temperature derating factors.
  • Soil temperature derating factors.

Load parameters

• Phase: Select the phase arrangement.

  Three-phase AC 400V, 415V	Large loads and consumer mains.
  Single-phase AC 230V, 240V	Smaller loads and consumer mains.
  DC 12V, 24V, 110V
  Two-phase 120° 400V, 415V	Occasionally used in industrial and commercial lighting. Occasionally used as residential mains.
  Two-phase 180° 480V, 460V	Rural mains and large rural loads on SWER systems.

• Voltage (V): Select the voltage. If the voltage is not available in the select list, choose "Other" and specify any value in the ranges shown below.
• Specify voltage (V): Specify any voltage in the following ranges.

  	min	max
  Three-phase AC Two-phase 120° Two-phase 180°	1 V	1,000 V phase-to-phase
  Single-phase AC DC	1 V	1,000 V phase-to-neutral

• Rating: Specify the load rating in kW, kVA, A, or hp.

  When the rating is in ampere, enter the per‑phase line current for 3‑phase systems. For example, enter only 100 A in the following scenario.

  Phase 1 line current	100 A
  Phase 2 line current	100 A
  Phase 3 line current	100 A
  Enter only 100 A

  Note: It is the line current (not the delta winding current) for three-phase motors.

• 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 (%): Select the maximum allowable voltage drop at the load. Select Other to specify a custom value.
• Specify max. volt drop (%): Specify any voltage drop between 0.01 % and 100 %.
• Distance (m): The cable length in meters 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 is selected under Earth Cable Insulation.

  PVC		Max. operating temp.
  V-90	Standard. Most popular insulation. May be operated at 90°C temporarily, with no mechanical pressure on the insulation.	75°C
  V-90HT	High temperature. May be operated at 105°C temporarily, with no mechanical pressure on the insulation.	75°C
  V-75	Traditional (older) type PVC cables.	75°C
  XLPE
  X-90	Standard. Most popular XLPE insulation.	90°C
  X-HF-90	Fire rated.	90°C
  X-110	High temperature.	110°C
  X-HF-110	High temperature. Fire rated.	110°C
  Elastomer (rubber)
  R-EP-90	Standard.	90°C
  R-HF-90	Fire rated.	90°C
  R-E-110	High temperature.	110°C
  R-HF-110	High temperature. Fire rated.	110°C

• Cable type: Select the cable type as shown below.

  1 Phase AC or DC
  Multi-core 2C+E One live core. One neutral core. One earth core.
  Multi-core 2C (mains) One live core. One combined earth and neutral core.
  Single-cores 2x1C+E One live cable. One neutral cable. One earth cable.
  Single-cores 2x1C (mains) One live cable. One combined earth and neutral cable.
  3 Phase AC
  Multi-core 3C+E Three-phase cores. One earth core.
  Multi-core 4C+E Three-phase cores. One neutral core. One earth core.
  Multi-core 4C (mains) Three-phase cores. One combined earth and neutral core.
  Single-cores 3x1C+E Three-phase cables. One earth cable.
  Single-cores 4x1C+E Three-phase cables. One neutral cable. One earth cable.
  Single-cores 4x1C (mains) Three-phase cables. One combined earth and neutral core.

• 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 (or cable groups): Enables the use of parallel mutli-core cables, or parallel single-core cable groups in a circuit for high-current installations. This means more than one one live core per phase. The minimum allowable size for parallel cables is 4 mm², as specified in AS/NZS 3000.

  Parallel cables are not supported for "Exposed to Sun" installations by AS/NZS 3008.

• Number of parallel cables (or cable groups): The number of parallel multi-core cables or parallel single-core cable groups in this circuit.

  This is when you want to use parallel multi-core cables or single-core cable groups in a circuit to increase the current rating or reduce the voltage drop.

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

  Parallel cables are not supported for "Exposed to Sun" installations by AS/NZS 3008.

  Examples:

  Three parallel multi-core cables

  

  Three parallel single-core cable groups

  

Earth cable parameters

• Earth cable insulation: Applicable to single-core cables. It can be different from the active cable insulation. The options are the same as the active cable insulation options.
• Flexible earth: Applicable to single-core cables. Select if a flexible earth cable is used.
• Earth 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 core size (mm²): Select an earth cable size or select Auto.

  Multi-core cables

  Only Auto is supported for multi-core cables.

  The calculator will select the minimum copper earth size from Table 5.1 in AS/NZS 3000. These sizes match commercial copper multi-core cables.

  For aluminium cables the calculator will select an earth size equal to the active size. This matches most commercial aluminium cables.

  The fault current rating and loop impedance requirements are checked if Check fault rating and Check loop impedance are enabled. It does not increase the earth size.

  Parallel multi-core cables

  The earth fault rating is checked per earth core, i.e. no earth fault current split is considered.

  The loop impedenace is checked on the combination of parallel active and earth cores, i.e. an equal earth fault current split is considered.

  Single-core cables

  Single-core cables support Auto and Manual selection of the earth size.

  To select a specific earth size, select a specific active size first.

  In Auto, the selection process is as follows:

  • Step 1: The calculator begins with the minimum recommended earth size Table 5.1 in AS/NZS 3000.
  • Step 2: If Check fault rating and Check loop impedance are enabled, the calculator increases the earth cable size until both the fault current rating and loop impedance requirements are met.
  • Step 3: If no suitable earth size is found, the active size is used. If you need a larger earth cable, select it manually.

  If the earth core size is selected manually, the selected earth size is still checked against Table 5.1. The fault rating and earth loop impedance are also checked when these options are selected.

  Table 5.1 does not specify aluminium earth sizes. They are calculated based on the impedance and fault rating of the equivalent copper earth cables. Click here for more details.

  Parallel single-core cables

  By default, the earth fault rating is checked per earth cable, i.e. no earth fault current split is considered.

  However, in high current installations, the option Combined fault rating can be enabled, and earth fault rating will be based on an equal split of fault current across parallel earth cables. See Combined fault rating for more information.

  The loop impedance is checked on the combination of parallel active and earth cables, i.e. an equal earth fault current split is considered.

  For parallel active and parallel earth cables, the combined active size is calculated as:

  \(S_{active\_combined} = \dfrac {S_{active} \cdot m}{n} \)

  Where,

  • S_active is the size of one of the active cables in mm².
  • m is the number of parallel phase cables.
  • n is the number of parallel earth cables.

  The combined active size is then used to select the minimum earth size from Table 5.1.

  For multiple earth cables, the calculator checks that each earth cable is equal to or larger than the minimum earth cable required for one individual active conductor.

  For example, in Table 5.1 the minimum earth cable size for a 240 mm² active cable is 95 mm². If two earth cables are selected, the minimum individual earth cable size is still 95 mm².

  If the combined active size is larger than 630 mm², the earth size is calculated according to Table 5.1 as follows:

  • Copper: Minimum 25% of the combined active size.
  • Aluminium: Minimum 40% of the combined active size.

  For high current installations, the option Ignore table 5.1 can be enabled, and Table 5.1 will be enforced up to 120 mm² (copper) or 240 mm² (aluminium) only. See Ignore table 5.1 for more information.

• Number of earth cores or cables:

  Multi-core cables	Matches the number of parallel cables. Cannot be changed.
  One single-core cable group	One or two earth cables.
  Multiple single-core cable groups	Any number of earth cables between one and the number of parallel cable groups.

• Advanced options: Select to show the following two options.

• Ignore table 5.1 sizes: Ignores earth sizes specified in Table 5.1 in AS/NZS 3000. Earth size will be based on earth fault current and loop impedance only.

  This allows for more economical earth cable sizes.

  This option is only available for:

  • Three-phase, single-core cables, excluding mains.
  • Number of parallel cable groups > 1.
  • Check fault rating and Check loop impedance are enabled.
  • The calculated earth size is ≥ 120 mm² (copper) or ≥ 240 mm² (aluminium).

• Combined earth fault rating: The fault rating of the earth cables is based on an equal split of earth fault current across the parallel earth cables.

  This allows for more economical earth cable sizes.

  This option is only available for:

  • Three-phase, single-core cables, excluding mains.
  • Number of parallel cable groups > 1.
  • Number of parallel earth cables > 1.
  • Check fault rating and Check loop impedance are enabled.
  • The calculated earth size is ≥ 120 mm² (copper) or ≥ 240 mm² (aluminium).

  Important: To ensure safety, it is essential that the parallel earth cables:

  • are the same length,
  • follow the same route,
  • are mechanically protected,
  • insulated,
  • and separated from the live cables to prevent direct faults between phase and earth cables.

Installation parameters

• Formation: Trefoil or laid flat.
• Installation method: Select the installation method. Consider the worst-case section of the cable installation. Which is the installation section with the lowest current rating.

Installation derating parameters

• 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 (or cable groups): The number of multi-core cables or single-core cable groups in this installation. This includes parallel cables in this circuit and cables from any other circuits included in the same installation.

  The standard refers to this parameter as the Number of Circuits. And the standard treats parallel cables as multiple circuits. jCalc uses the names "Number of cables" and "Number of cable groups" instead of "Number of Circuits" to make it more intuitive, especially when parallel cables are used.

  Note that the parameter number of parallel cables (or cable groups) under Cable Type specifies the number of parallel cables in this circuit only. This parameter considers the parallel cables in this circuit, and cables from any other circuits in the same installation.

  All cables in the installation must be considered, because heat from adjacent cables affects the current rating and voltage drop.

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

  Multiple cables (or cable-groups) are not supported for "Exposed to Sun" installations by AS/NZS 3008.

  Examples:

  Multi-core (number of cables = 3)

  

  

  Single-core (number of cable groups = 3)

  

  

  

• 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

• Number of tiers: Number of tiers.

  • Number of horizontally tiered racks: Tiers of racks or ladders installed above each other. The minimum vertical space between racks is 300mm.
  • Number of horizontally tiered unperforated trays: Tiers of trays installed above each other. The minimum vertical space between trays is 300mm.
  • Number of horizontally tiered perforated trays: Tiers of trays installed above each other. The minimum vertical space between trays is 300mm.
  • Number of vertical back-to-back perforated trays: One or two vertical back-to-back trays. The minimum horizontal space between back-to-back trays is 230mm.
• Air temperature (°C): For cables installed in air. The defaults (no derating) are.
  • Australian conditions: 40°C
  • New Zealand conditions: 30°C
• Soil temperature (°C): For cables installed underground. The defaults (no derating) are:
  • Australian conditions: 25°C
  • New Zealand conditions: 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.

Conduit parameters

• Calculate conduit: Check this box if you want to calculate conduit sizing. The option will only show for installations with enclosures. Conduit sizing is based on Tables C10, C11 and C12 in AS/NZS 3000:2018. Click here for more details.

• Conduit type: Select the conduit type. Click here for more details.
• Conduit size (mm): Select a conduit size or select Auto. Click here for more details.

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, 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 method
  MCB	Estimated, Calculated or Measured
  MCCB	Calculated
  Generic device	Calculated

• Protection: Select the 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 3200 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.

  MCBs are manufactured according to AS/NZS 60898.1, which specifies trip current ranges for the MCB curve types.

  Calculations in AS/NZS 3000 and jCalc is based on the mean values of the ranges.

  Curve type	Trip current AS/NZS 60898.1	Trip current AS/NZS 3000 and jCalc
  B	3-5 x rated current	4 x rated current
  C	5-10 x rated current	7.5 x rated current
  D	10-20 x rated current	12.5 x rated current

• MCB rating: Select a specific rating or select Auto.

  In Auto, sizing is according to AU/NZS 3000, i.e. the MCB rating is selected for the load current, and then checked against the cable rating, to satisfy:

  I_b ≤ I_n ≤ I_z

  Where,

  • I_b = load current.
  • I_n = the MCB rated current.
  • I_z = cable current rating.
• MCB fault rating: Used to determine the typical MCB fault let-through energy (I²t). Applicable when "Check short circuit rating" is selected. See MCB let-through energy.
  • The MCB must be current-limiting i.e. reduce the peak and limit the let-through energy to less than a symmetrical half-cycle.
  • The prospective fault current may be higher when back-up protection is used.
  • The actual fault current flowing through the MCB may be lower, due to a lower prospective fault current, back-up protection limiting, or location of the fault in the cable.
  • The fault current rating of the MCB is used as the worst-case scenario to simplify the calculator.
• MCB let-through energy: Specifies the source of the MCB fault let-through energy (I²t). Applicable when "Check short circuit rating" is selected. See MCB fault rating.
  • Typical: Based on Schneider iC60/C120 MCBs, 0-415V AC and DC. DC let-through typically lower when wired to OEM requirements, e.g. two-pole fault interruption. Above 415V, use Custom let-through energy.

    Fault rating	MCB ratings
    	1-8A	10-63A	80-125A
    4.5kA	iC60	iC60	C120
    6kA	iC60	iC60	C120
    10kA	iC60	C120	C120
    15kA	iC60	C120	C120

  • Custom: User specified let-through energies are used. User must manually read from OEM curves.
• 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: Select between Schneider and Terasaki TemBreak Pro.
• Model: Select the model. Note that the number in the model name represents the current rating of the circuit breaker. For example, a Schneider NXS100 is rated for 100A, and a Terasaki P250 for 250A.
• CB fault rating: The fault current rating of the circuit breaker. This affects the let-though energy.
• Trip unit: Select between Thermal Magnetic, Micrologic 2.3 (NSX) or Micrologic 2.0. (NS)
  • 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 is selected manually.
    • Magnetic trip: The magnetic trip setting can be selected if the trip unit rating is selected manually.
  • Micrologic 2.3 (NSX) and Micrologic 2.0 (NS)
    • 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 is selected manually.
    • Short-time trip: The short-time trip setting can be selected if the trip unit rating is selected manually.
    • Instantaneous trip: The instantaneous trip setting can be selected if the trip unit rating is selected manually. Note that there is no instantaneous trip on the Micrologic 2.0 (NS) trip units in the calculator. It is modelled as a short-time trip instead. The catalogue refers to the trip as instantaneous. However, it behaves like a short-time trip when compared to other electronic trip units.
• 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. When Check short circuit rating is selected, it is also 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.
• 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. It is used to check the phase fault current rating of the live and neutral cores of the cable if the current limiting option is not selected.
• Phase fault let-through energy (A²s): It is checked against the fault energy rating of the live and neutral cores of the cable if the current limiting option is selected The let-through energy (I²t) is available on current-limiting curves from the device manufacturer. It is a function of the prospective phase fault current.
• 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 if the current limiting option is not selected.
• 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 used to calculate the external earth loop impedance. It is used to check the earth fault current rating of the live and earth cores of the cable if the current limiting option is not selected.
• Earth fault let-through energy (A²s): It is checked against the fault energy rating of the live and earth cores of the cable if the current limiting option is selected. The let-through energy (I²t) is available on current-limiting curves from the device manufacturer. It is a function of the prospective earth fault current.
• 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 if the current limiting option is not selected.

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 or Tables 3.9 to 3.26 in AS/NZS 3008-2025.

The current ratings are based on the cable type, insulation and the installation method.

In AS/NZS 3008:2025, dedicated current ratings are defined for direct current (DC) applications.

For Australian conditions, current ratings are based on an ambient temperature of 40°C and a ground temperature of 25°C.

For New Zealand conditions, current ratings 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).
• Aluminium (solid, stranded and flexible).

Cable current derating calculation

The current derating for the cables has been implemented according to AS/NZS 3008 (2017 and 2025).

Cable impedance calculation

The impedance is calculated as:

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

Where,

• R_c is the cable resistance in Ω/km.
• X_c is the cable reactance in Ω/km.

Loop impedance calculation

There are three methods to calculate the maximum loop distance, depending on how th source impedance is determined.

Where:

• V_1Φ, single-phase-to-earth or DC-to-earth voltage.
• I_min, minimum trip current of protection device.
• I_f, prospective earth fault current.
• Z_s, measured source impedance in Ω.
• Z_max, maximum allowable total loop impedance in Ω.
• Z_p, phase core impedance in Ω/km.
• Z_e, earth core impedance in Ω/km.
• L_max, maximum allowable cable distance in meters.

For parallel cables, the phase core impedance (Z_p) is divided by the number of phase cores (m), and the earth core impedance (Z_e) is divided by the number of earth cores (n).

Voltage drop calculation

The voltage drop is calculated according to AS/NZS 3008 as:

Where,

• I is the load current in ampere (A).
• L is the cable distance in meters (m).
• R_c is the cable resistance in Ω/km.
• X_c is the cable reactance in Ω/km.
• Z_c is the calculated cable impedance in Ω/km. See calculation below.
• θ = arccos(pf), and pf is the specified load power factor.

The impedance Z_c can be calculated in two ways under Advanced Options:

1. Worst-case load power factor.

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

   The worst-case power factor is assumed.

   Theoretically when the cable and load power factors are the same.

   Simplest, most conservative, and the most often used method.

2. Specified load power factor.

   \(Z_c = R_c {\cdot} \cos (\theta) {+} X_{c} {\cdot} \sin (\theta)\)

   The specified load power factor is used to calculate the impedance.

   This will result in a lower voltage drop.

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

Cable resistance

The calculator selects the resistance (R_c) values from Tables 34 to 37 in AS/NZS 3008-2017 or Tables 4.7 to 4.11 in AS/NZS 3008-2025.

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

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

DC resistance values are not specified in AS/NZS 3008-2017. The specified AC values are used.

DC resistance values are specified in AS/NZS 3008-2025 in dedicated tables.

Cable reactance

The calculator selects the reactance values (X_c) from Tables 30 and 31 in AS/NZS 3008-2017 and Tables 4.1 and 4.2 in AS/NZS 3008-2025.

Short circuit calculation

The short circuit capacity of the cables is calculated according to AS/NZS 3008 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 defined in Table 52 in AS/NZS 3008-2017 or Table 5.1 in AS/NZS 3008-2025.

The fault constant (K) is based on:

• Insulation material.
• Initial conductor temperature. The calculator uses the maximum allowable operating temperature. For example, 75°C for PVC cables and 90°C for standard XLPE cables.
• Final conductor temperature. The calculator uses the maximum allowable short circuit temperature from Table 53 in AS/NZS 3008-2017 or Table 5.2 in AS/NZS 3008-2025. For example, 160°C for PVC cables and 250°C for XLPE cables.

Conduit calculation

Conduit size calculations are based on Tables C10, C11 and C12 in AS/NZS 3000:2018

The tables specify the maximum number of cables for a specific cable size, cable type, consuit type and conduit size.

The tables cover only PVC and XLPE insulation. Not elastomer (rubber) insulation.

• Note a: The cable size for single-core cables is based on the largest cable in a cable group. For example, a 3x1C+E single-core group (16 mm² actives, 6 mm² earth), is calculated as a 4 x 16 mm² cables.
• Note b: Flat multi-core cables are not currently supported in jCalc.
• Note c: PVC and XLPE insulated cables are specified separately in Table C12. They have minor differences. jCalc uses the more conservative PVC data for both XLPE and PVC cables.

Tables C10, C11 and C12 covers four conduit types.

Here is an example of how the maximum number of cables is specified in Table C12.