Introduction
The cable sizing calculator of Cable Pro Web software is explained in this tutorial. The calculator for the Australian and New Zealand cable sizing standard AS/NZS 3008 which is applicable for low voltages up to 1 kV AC or 1.5 kV DC. Note there is also another calculator that complies with the standard BS 7671.
All the cable types, installation methods and derating factors from the standard are included. There are references to the tables and columns used from the standard in the results and the reports.
The user interface is divided into several logical input sections and a results section.
Load section
Starting with the Load section.
a) The Load of the equipment is entered as a current, power or in horsepower. You can enter very large loads and the software will automatically calculate the cable size or you can fix the cable size and number of parallel conductors to check compliance.
b) Load current is used for the minimum current rating of the cable and for determining the actual voltage drop.
c) Power factor of the load is entered here and affects voltage drop. Note power factor has a big impact on voltage drop for large cable sizes and not so much on small cable sizes.
Cable Pro Web software uses the accurate method for calculating voltage drop. Refer to this article with the accurate voltage drop equations.
d) Derating factor can be entered, and this is multiplied by the standard current rating of the selected cable size. Derating factors are used for when your actual installation conditions differ from the standard conditions assumed by the standard.
e) There is a button called Derating Wizard which opens a window to help you work out the derating factors for your installation.
Supply section
Next is the Supply section.
a) The Phases can be selected as single phase, three phase, DC or 2 phase. This selection will affect cable size and voltage drop calculations. It will also change some of the options available in the other sections.
b) Changing Phases will update the Voltage input. There are default voltages for each of the phase settings for example 230V for single phase. Additionally, these default voltages can be changed in Settings.
c) Voltage drop is entered as a percentage of the supply voltage. This is the maximum allowed voltage drop and changing this input may determine the final cable size. You can switch between voltage drop or voltage rise calculations. This doesn’t really change the calculations but affects the wording of the load current and the wording in the reports.
Refer to our article on voltage drop limits for LV installations for the Standards rules.
d) The Length of run is the distance from the point of cable supply to the load which affects the voltage drop calculations. You can enter very long runs into the software.
e) Fault level is the prospective fault current level at the point of supply of the cable, which affects the short-circuit rating of the cable. The resultant cable size is the minimum size to withstand the fault current. Note that it is meant to be the actual fault level for your installation and that it is not the rated breaking capacity of the circuit breaker.
It can be quite difficult to get an accurate value for fault level, but it should be provided by the power authority.
Sometimes confusing results are seen for small cable sizes when the length of run is short, and the fault level is overly high – in these cases the fault level can be the dictating factor for minimum cable size. To check, just reduce the fault level. Refer to our help desk article.
Cable section
Next are the Cable inputs.
a) All the Insulation types are available including PVC, XLPE, Fire rated, Flexible cords, mineral insulated and Aerial cables or conductors.
b) From the Conductor drop down you can select Copper, Copper Flexible or Aluminium conductors.
Note that there are also Aluminium Flexible conductors available from the drop down.
c) From the Cores menu you can select single core or multicore cables.
d) Your selections in Supply and Cable will affect the Installation table which is the next section.
Installation section
The available installation methods are displayed in the centre of the screen in the Installation section.
a) The table number from the standard and from which the installation methods are taken is displayed for reference.
b) Most input combinations allow cables to be installed in air unenclosed or enclosed, exposed to sun, surrounded by thermal insulation, buried direct or buried in ducts.
c) Whenever the installation method is changed, or any input is changed, then the cable size results are instantly updated in the results section.
Derating Wizard
Now having a look at the Derating Wizard.
a) Once the installation method has been selected it’s a good time then to use the Derating Wizard to calculate the Derating factor.
Refer to this article which provides tables of Current Rating Derating Factors.
b) There are two different sequences of steps with the derating wizard depending on whether the cables are installed in air or buried. Notice if I switch to another installation method which is buried then the Derating Wizard changes. I will use the buried installation as an example.
c) Stepping through the Derating Wizard windows you will be asked to complete each section. If the installation conditions of your project differ from the standard conditions, then a derating factor will be calculated for each condition.
d) At the end of the wizard an overall derating factor will be calculated and updated in the main window. The complete results for derating factors will be added to the final report.
Protection section
The protection section provides an important set of inputs.
a) There are different Devices including MCB’s, HRC fuses, adjustable MCCB’s and ACB devices as well as custom devices available.
b) For an adjustable device the Rating is the nominal trip rating and Thermal setting is the actual current trip setting of the device which should be less than or equal to the trip rating.
c) The rating or the thermal setting of the selected device must be greater than the load current in order to be able to properly protect the cable.
d) The rating or thermal setting will be used to calculate the minimum cable current rating.
e) Note that if the protective device is unknown then the earth cable size will be based on Table 5.1 from AS/NZS 3000, otherwise earth size will be based on fault loop impedance from the protective device settings.
f) The Fault time is the time required for the protective device to trip during a short-circuit condition and is used for calculating the short-circuit rating of the cables.
g) The trip multiplier is a characteristic of the protective device and is set by default but should be taken from the manufacturer datasheet.
h) The trip current is the current required to cause the device to trip within the specified earth fault time and is equal to the rating multiplied by the trip multiplier.
i) A residual current device (RCD) can also be added that will further affect the fault loop impedance of the circuit.
Results explained
Next, let’s look at the Cable Size results.
The cable size results are updated live with every change you make. The minimum Active size is automatically calculated along with the number of circuits. The active size and the number of circuits can also be set manually, and compliance will be checked.
The spare capacity is the difference between the current carrying capacity of the circuits as per the standard, and the protective device rating.
Neutral size is automatic or can be manually set for multiphase circuits. Refer to this article that explains Neutral Conductor Sizing.
Earth size is automatically calculated based on fault loop impedance and short-circuit current or the earth size can be specified. The earth cable can also be turned off. Refer to this article which explains Fault Loop Impedance and how it relates to earth cable size.
The actual voltage drop is shown and should be less than the voltage drop limit under Supply. If it isn’t, it will be highlighted.
The fault loop impedance is shown and if it is an issue will also be highlighted.
Conduit size is calculated and shown for any Installation methods which are in conduits.
Under the Details tab you will find more information which you can check such as:
– The standard table and column number references for current rating and those for voltage drop.
– Short-circuit currents which were calculated are given under Short-Circuit Performance.
– Details of the fault loop impedance calculations are provided.
Report
It’s easy to generate a detailed report.
You can include your own company logo in the reports by uploading an image using the Settings menu. You can email the report to one or several colleagues, customers, and so on. You can generate the PDF report and download it onto your computer.
The report includes all your inputs, the results including Pass or Fail as well as all the details of the derating factor calculations and Table No. and Column number references to the standards.
