To ensure safe operation and work on full load without being damaged cable sizing calculation is very important. It helps us to provide a suitable voltage for the load.
Besides this proper sizing helps us to withstand the worst short circuit current, and ensure device safety during operation.
In this article, we will discuss methodologies and other related parameters regarding cable sizing calculation. And of course, we will give a simple example of choosing a cable.
Cable Sizing Calculation Steps
The process of the size calculation method consists of six steps.
- Collect data about cable, load, and environmental conditions.
- Determine the minimum cable size for continuous current carrying capacity.
- Determine the minimum size of the cable based on voltage drop.
- Determine the minimum size of cable for short circuit conditions.
- Choosing the minimum size of the cable in case of earth fault loop impedance.
- The final step is to select the minimum size of cable in the previous steps.
Now, let’s discuss each step in detail to find the correct size of the cable.
Example of cable sizing step-by-step
Assume we have:
- 80KW load
- The load is 200 meters distance from the source
- Three phases, V = 415V
- power factor is, pf = 0.8
- 5% allowable voltage drop
- directly buried in the ground
- 1-meter burial depth
- 35 °C Ground temperature
- One cable per trench
Okay, let’s choose the cable step-by-step
Load current I = (Power in watt) / (1.732*V*pf )
= (80*1000) / (1.732*415*0.8)
= 139 A
To better understand The Critical Role of Cable Size in Electrical Efficiency read my in-depth article.
Determine correction factors
From derating factor tables, we find the derating factors of the example conditions.
For more details about derating factors and tables read my article What is derating factors of underground cables?
- A correction factor of ground temperature = 0.89
- A correction factor of soil = 1.05
- A correction factor of cable burial depth = 1.0
Total derating factor = 0.89 * 1.05 * 1.0 = 0.93
From cables ampacity tables. Selecting Copper, XLPE, 3*50+25 mm2 , its current is 185 A, Derating this current 185 * 0.93 = 172 A
Calculating the voltage drop of this cable, VD = 0.715 MV/A/M, from voltage drop tables.
VD = 0.715 *distance*load current = (0.715/1000)*200*139 =19.877 V
The allowable voltage drop = 415 * 5% = 20.75V.
This means that the cable is accepted because the voltage drop is within limits (5% = 20.75 V)
Finally, we can use a cable of Copper, XLPE, 3*50+25 mm2 for the load and laying conditions mentioned above.
Read my article Voltage Drop Woes? Here’s How to Choose the Perfect Wire Size
Important notes:
- In this example, we skipped short circuit calculations, as it’s much more complicated and depends on many factors.
- Voltage drop and The current carrying ampacity of the cable which are 0.715 MV/A/M, and 185A.
Conclusion
- Choosing the power cable for electrical loads depends on load current, cable ampacity, installation conditions, etc.
- Derating factors should be applied to the cable current.
- The voltage drop value should be within the accepted limits.
- If the cable voltage drop is higher than the limits, we choose the larger cable.
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