Introduction
In electrical engineering design and installation, choosing the right cable size is important.
It’s not just about ensuring power flows efficiently, but also about safety, compliance, and reliability.
Opt for a size too small, and it’s non-compliant and unsafe; go too big, and it’s simply a waste of resources.
This is where AS/NZS3008, the Australian Standard for electrical installations, comes into play.
AS/NZS3008 provides comprehensive guidelines for cable sizing, taking into account various factors such as current carrying capacity, voltage drop, short-circuit temperature limits, and more.
In this blog, we’ll look on the different methods of cable sizing using AS/NZS3008’s principles, as a bonus we will also cover temperature rise in cables.Let’s GO!
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Cable Sized based on Current Carrying Capacity
The first method we will look at is cable sizing based on current carrying capacity. This method is mostly used for short cable runs. The steps are as follows.
- Firstly, determine the current requirements of the circuit, this is determined by the connected load and characteristics of the circuit. The following condition should hold true;
Where IB is the maximum demand of the circuit and IZ is the current rating of the cable.
If you are not sure how to calculate the maximum demand please do read our blog on understanding maximum demand or simple use our maximum demand calculator.
- Secondly, check the environment where the cable is to be installed. Consider factors such as the type of soil, laying depth, and soil thermal resistivity. Each of these elements has its own derating factors. To obtain these derating factors, refer to Table 27 through Table 29 of AS/NZS3008. If more than one derating factor applies, multiply them all to arrive at one overall derating factor
- Next, divide the maximum demand of the circuit by the overall derating factor. This will provide you with the derated current requirements of the circuit
- Finally, refer to Tables 4 through 21 of AS/NZS3008 to find the type of cable, along with the required core and insulation, that can carry the derated current.
In a nutshell these are the steps to follow when sizing a cable based on ampacity.
Cable Sized based on voltage drop
On longer cable runs, voltage drop predominates and is the main driving factor in determining cable size. The steps for sizing using voltage drop are as follows;
- Determine the maximum demand current (I) of the circuit, as with the previous method.
- Determine the actual cable length (L), not just the ‘as the crow flies’ route.
- Look up the voltage drop limits (Vd); in Australia, this limit generally stands at 5%, but it can vary from 1% to 7% based on project specifications. For further information, refer to our blog post, ‘Understanding Voltage Drop.’
- Calculate the voltage drop (Vc) in millivolts per ampere meter (mV/A.m) using the equation.
- Finally, refer to Tables 40 through 51 of AS/NZS3008 to identify the type of cable, including the required core and insulation, that possesses a tabulated value (mV/A.m) closest to, but not exceeding, the value derived in point 3 above. This cable is considered the smallest one meeting the voltage drop limits used in the calculation.
Cable Size based on let through energy
The final method we will discuss is the adiabatic cable sizing method. In this method, it is assumed that all the heat generated during a short-circuit is retained within the conductor, without any heat dissipation to the surrounding environment.
This assumption allows for straightforward calculations, as it simplifies the thermal analysis.
While the adiabatic method is convenient for quick estimations, it tends to provide a conservative result.
In reality, some of the heat generated during a short-circuit would escape from the conductor and transfer to the surrounding insulation.
Therefore, while the adiabatic method can be useful for initial assessments, more detailed and accurate methods like the ones we have looked at are recommended.
We won’t go into the mathematics of it here as we have already covered it, and we’ve also created a calculator specifically for you. Please do check it if you haven’t done so already
Cable Temperature Rise
When an electric current flows through a conductor, it encounters resistance.
This resistance is a natural property of the material, and it results in some of the electrical energy being converted into heat energy. The conductor temperature can be estimated using the equation;
Where:
- Io is the operating current,
- IR is the cable rated current,
- θO is the cable temperature when carrying a current of IO,
- θR is the cable temperature when carrying a current of IR, and
- θA is the ambient soil or air temperature.
All temperatures are measured in degrees Celsius and current in amperes.
The cable operating temperature, θO, is determined by examining the cable insulation.
For more details on how to determine it, refer to the ‘Understanding Cable Insulation’ blog post. Care should be taken not to exceed the cable’s maximum rated temperature limit.
Rounding Up
That’s all we have for you today on this topic. If you’ve learnt something new from this post, you might also be interested in other articles we offer on related subjects available on our blog’s homepage.
While you’re here, take a moment to explore our collection of electrical engineering calculators. Cheers!