Tutorial Video
Australian Cable Sizing Standard
This Cable Sizing Calculator can calculate minimum active, neutral, and earth cable sizes in compliance with Australian Standards AS/NZS 3008.1.1 and Wiring Rules AS/NZS 3000. It covers all cable types, installation methods, and derating factors in the standards.
This cable sizing standard applies to circuits up to 1,000 V AC or 1,500 V DC.
The calculator follows the cable sizing procedure outlined in Australian Standard AS/NZS 3008.1.1 and the rules related to cable sizing in AS/NZS 3000, which are discussed below in detail.
Assumed conditions:
- Ambient air temperature: 40°C
- Soil temperature: 25°C
- Soil thermal resistivity: 1.2 K·m/W
- Standard depth of burial: 0.5 metres (500 mm)
The tables' current-carrying capacities are based on this standard burial depth. If the actual depth differs, a rating factor for depth of laying must be applied. - Installation and grouping: Based on typical Australian practices and climate
Cable Sizing Overview
The minimum size of a low-voltage power cable is determined for a particular installation based on its ability to satisfy current-carrying capacity, voltage drop, and short-circuit performance/rating.
The minimum cable size will be the smallest cable that satisfies the three requirements.
⚠️There is also a minimum cable conductor size to meet mechanical strength requirements.
1. Current-carrying Capacity
Current carrying capacity, often called ampacity, is the maximum amount of electric current a cable can safely conduct without exceeding its temperature rating. This limit is determined by the cable's conductor material, insulation type, installation method, ambient temperature, and external factors. Constant constant current (100 % load factor) and constant ambient conditions are assumed.
ℹ️The current-carrying capacity dictates the active cable size for large load currents.
2. Voltage Drop
Voltage drop is the reduction in voltage that occurs as current travels through a cable due to the conductor's inherent impedance. This drop is influenced by factors such as the phases of the supply, the length of the cable, the cross-sectional area, the material, the operating temperature of the conductor, the amount of current flowing, and the power factor of the load it supplies.
ℹ️The voltage drop dictates the active cable size for long route lengths.
3. Short-circuit Rating
Short circuit rating is the maximum current a cable can safely carry for a very short duration during a fault condition, such as a short circuit, without sustaining damage. The minimum cable size to withstand the short-circuit current is determined based on the actual short-circuit current and the specified period (fault clearing time of the protection obtained from the time-current curve).
ℹ️The short circuit rating (rarely) dictates the active size for short cable runs carrying a small load current and where the supply’s fault level is high.
Minimum Conductor Size for Mechanical Strength
Conductors must have sufficient mechanical integrity to withstand handling, installation stresses, and environmental conditions without sustaining damage.
AS/NZS 3000:2018 provides guidelines to determine the nominal minimum cross-sectional area of conductors, as outlined in Table 3.3 below.
Cable Sizing Coordination with Protective Device
A protective device’s primary function is to protect cables from thermal damage due to overload and/or short circuits. Below is the image showing the damage caused by an improperly coordinated circuit protective device and power cables.
The required coordination between the cables and the circuit protection is shown in the figure and explained in the table below.
The following parameters are defined:
| Requirements | Notes | |
|---|---|---|
| ZONE 'A' | The protective device (PD) rating (IN) must be ≥ design current (IB) and ≤ current-carrying capacity (IZ) of the cable. | ℹ️The rating of fuses must not exceed 0.9IZ. |
| ZONE 'B' | The protective device's conventional tripping time (I2) must be ≤ 1.45IZ for circuit breakers and 1.6IZ for fuses. | ℹ️Standards mandate circuit breakers trip below 1.45IN, ensuring all breakers and fuses comply. |
| ZONE 'C' | The protective device fault breaking capacity (ISCB) must be ≥ ISC. | ℹ️If the overcurrent device only provides fault protection, IN may exceed IZ, and I2 may exceed 1.45 IZ. |
Cable Temperature Limits by Insulation Type
The current carried by a power cable system heats it. The conductor temperature is limited to avoid overheating the adjacent insulation layers.
The temperature limits of insulated cables, such as PVC (up to 75 °C continuous, 160 °C short circuit), XLPE (up to 90 °C continuous, 250 °C short circuit), EPR (up to 90 °C continuous, 250 °C short circuit), and Silicone Rubber (up to 180 °C continuous, 250 °C short circuit), are crucial for ensuring safety, performance, longevity, and regulatory compliance.
The continuous temperature limit of cable insulation determines a cable’s current-carrying capacity by setting the maximum allowable temperature rise during operation. The table below provides the temperature limits of cable conductors of common cable insulation types.
⚠️The current carry capacity tables of AS/NZS 3008.1 are based on the ‘Normal use’ temperatures.
Derating Factors
The Australian Standard AS/NZS 3008.1 provides derating factors in Tables 22 to 29.
Refer to this page for a complete list of all derating factor tables from AS/NZS 3008.1.1.
- It’s simple to use the derating factors:
- Obtain the current rating based on the standard conditions.
- Determine the factors from the derating factor tables for your installation conditions.
- Multiply all derating factors together and then multiply this value by the cable's standard (tabulated) current rating to obtain the derated cable current rating.
The Derating Wizard of this calculator includes all of the derating factors from the Australian Standard.
Voltage Drop Calculations
Excessive voltage drop can cause equipment malfunction and failures. Therefore, the standards dictate maximum permissible voltage drop limits. Voltage drop often significantly impacts the minimum cable size and must be calculated accurately.
✅Accurate voltage drop calculations will result in smaller cable sizes.
Accurate Voltage Drop Equation
Accurately calculating voltage drops results in lower voltage drops, which leads to smaller cable sizes and saves money. The accurate voltage drop equation for three-phase systems is shown below.
ℹ️For DC voltage drop calculations, the value for reactance X is zero.
Voltage Drop Limits
The voltage drop between the point of supply for the low-voltage electrical installation and any point in that installation must not exceed 5 % of the nominal voltage of the point of supply. Where the point of supply is the low-voltage terminals of a substation located on the premises containing the electrical installation and dedicated to the installation, the permissible voltage drop may be increased to 7 %.
Stand-alone systems are designed so that the combination of the output voltage from the source and the voltage drop within the installation does not result in the utilisation voltage of the low-voltage equipment falling more than 11 % of the nominal supply voltage under normal conditions.
| Any low-voltage electrical installation | Must not exceed 5% voltage drop |
|---|---|
| Where there is a substation located on the premises | Must not exceed 7% voltage drop |
The following voltage drop limits can be used as a guide to assist with design.
| Consumers mains | allow 0.5% voltage drop |
|---|---|
| Sub-mains | allow 1.5-2% voltage drop |
| Final subcircuits | allow 2.5% or up to the prescribed limit, considering the upstream circuits |
The limit for voltage rise is 2 % from the inverter to the connection point as per Standard AS/NZS 4777.1:2016. The maximum DC voltage drop limit is quoted in AS/NZS 5033:2014 as 3%.
| Voltage rise (AC) | Must not exceed 2% |
|---|---|
| Voltage drop (DC) | Must not exceed 3% |
Power Factor Affects on Cable Sizes
Power factor can significantly affect voltage drop and cable sizes, especially for large cables with high reactance. Below are the results of an example calculation.
Example calculation: Three-phase load of 300 A. The run length is 200 m. The 400 V supply voltage drop limit is 2.5 %. The cable type is 3C+E, PVC-insulated (V-75), with copper conductors. The installation is unenclosed in the air and spaced from a surface.
| Minimum cable size | Cost | Power factor | Voltage drop | Ratio of load current to CCC | Conductor temperature |
|---|---|---|---|---|---|
| 240 mm2 | $57,000 | 1 (unity) | 2.37 % (9.48 V) | 0.728 | 58.56 ̊C |
| 300 mm2 | $70,065 | 0.95 | 2.41 % (9.62 V) | 0.636 | 54.14 ̊C |
| 400 mm2 | $92,426 | 0.9 | 2.21 % (8.82 V) | 0.551 | 50.64 ̊C |
ℹ️Standard rules require users to maintain a power factor of 0.9 lagging or above.
Unbalanced Multiphase Circuits
In an unbalanced system, current will be flowing through the neutral conductor as illustrated by the phasor diagram in the figure below:
There are two ways of dealing with voltage drop for unbalanced three-phase circuits:
| Where: | Voltage drop calculation method | |
|---|---|---|
| 1 | Out-of-balance conditions are intermittent | Assume balanced three-phase load conditions and use the heaviest loaded phase current. |
| 2 | Out-of-balance conditions are consistent | Calculate voltage drop on a single-phase basis by summing the voltage drop in the heaviest loaded phase and neutral. |
Conductors in Parallel
Running conductors in parallel is a standard method of increasing a circuit's current-carrying capacity without resorting to excessively large conductors. This technique involves using two or more conductors in parallel to share the electrical load, but it requires careful consideration to ensure safety, efficiency, and compliance with standards.
Using multiple smaller conductors in parallel significantly boosts the circuit's overall current-carrying capacity. Smaller conductors are easier to install, route through conduit, and navigate around obstacles. Parallel conductors provide redundancy; if one conductor fails, the others can continue to carry the load, albeit at a reduced capacity.
⚠️Parallel conductors require more cables and installation costs than larger or fewer parallel cables.
Requirements
To avoid overheating one or more parallel cables, it is essential to ensure that the grouping of cables does not affect the cooling of each parallel cable or group and that there is equal load current sharing between them. This is achieved as follows:
Conductors must be made of the same material and have a cross-sectional area.
Cables follow the same route and have the same length.
Each parallel cable or group of conductors is effectively bonded at both ends.
The relative position of active and neutral conductors in and between parallel groups considers the mutual impedances (i.e. separation is symmetrical and consistent, and the phase arrangement is optimal).
The Australian/New Zealand Wiring Rules AS/NZS 3000:2018 provide guidelines and specifications for using conductors in parallel. This standard requires the cables to have a minimum size of 4 mm2.
Optimal Phase Arrangements
Proper phase arrangements of multiple single-core cables of multi-phase systems are essential for ensuring balanced load current sharing. The figure below from the standard AS/NZS 3008.1 references the optimal phase arrangements for up to four parallel circuits.
Neutral Cable Sizing
The neutral conductor is connected to a system’s neutral point and is intended to carry current under normal conditions. A neutral conductor is required for each primary circuit, and the Standards have rules for its size. This page summarises the requirements of Standards AS/NZS 3000:2018.
The neutral conductor of circuits designed to Australian Standard AS/NZS 3000 must have the same current-carrying capacity (same size) as the associated active conductors.
When there are substantial harmonics, the neutral conductor size may need to be larger than the active conductors. In the case of a multi-core cable, the neutral currents may dictate the active size.
An exception is for multi-phase circuits feeding predominantly multi-phase loads (balanced phases). In this case, the neutral conductor size may be smaller than the active conductor size, but must be sized to carry at least the out-of-phase current.
| Type of Circuit | Neutral conductor size requirement | |
|---|---|---|
| Single phase two-wire - One active conductor - Multiple active conductors Multiphase - Consumer mains, submains and final subcircuits | Neutral conductor size is not less than active conductor size. | Sneutral = Sactive |
| Multiphase - 3rd harmonic currents are substantial | Neutral conductors may be required to be larger than associated active conductors. Refer to IEC 60364-5-52. | Sneutral ≥ Sactive |
| Multiphase - Feeds predominantly multiphase loads | The neutral conductor must be capable of carrying the maximum out-of-balance current, including any harmonic component. | Sneutral ≤ Sactive |
Earth Cable Sizing
In low-voltage (LV) electrical installations, the earth conductor is critical in ensuring safety and compliance with AS/NZS 3000 standards. Its primary purpose is to provide a low-resistance path to the ground, which helps protect both people and equipment from electrical faults. When a fault occurs, such as a short circuit or insulation failure, the earth conductor directs the fault current safely to the ground. This action facilitates the immediate operation of protective devices like circuit breakers or fuses, reducing the risk of electric shock, fire, and equipment damage. Proper installation and maintenance of the earth conductor are essential to maintaining the integrity and safety of the entire electrical system.
How to Size the Earth Cable
The earth cable size needs to be sufficient to ensure:
- Appropriate earth fault loop impedance (Zs).
- Adequate current-carrying capacity for prospective earth fault currents for a time at least equal to the tripping time of the associated circuit protection (adiabatic equation).
- Adequate mechanical strength.
According to section 5.3.3 of AS/NZS 3000:2018, the earth conductor size is determined by either:
(a) Table 5.1 in AS/NZS 3000:2018 provides conservative earth cable sizes about the largest active cable size (or summation where there are parallel circuits); OR
(b) By calculation, the protective device details must be known. The appropriate earth fault loop impedance must be ensured when calculating earth cable size.
ℹ️The earth cable size should be determined by calculation because it provides a more accurate and often economical result.
Table 5.1 of AS/NZS 3000
Below is the look-up table extracted from the standard AS/NZS 3000:2018. The industry commonly uses this simple table, but it is inaccurate compared with calculations and often results in oversizing.
Cable Sizing Calculation Examples
Understanding the fundamentals of performing cable sizing calculations by hand is important.
You should understand the cable selection process outlined by the Australian Standard AS/NZS 3008.1.1.
You should also understand how to perform neutral and earth cable sizing, which are covered by the Australian Wiring Rules AS/NZS 3000.
Refer to the step-by-step tutorial on cable sizing calculations for Australian Standard AS/NZS 3008.1.