Cable Sizing Calculation, Step by Step Example

Cable Sizing Calculation, Step by Step Example
cable sizing calculation

Why is cable sizing calculation important?

To ensure safe operation and work on full load without being damaged cable sizing calculation is very important. It helps us to provide suitable voltage for the load.

Besides this proper sizing help 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 curse we will give a simple example to choose a cable.

Cable Sizing Calculation Steps

The process of the size calculation method consists of six steps.

  1. In the very first step, we collect data about cable, load, and environmental conditions.
  2. The second step is to determine the minimum cable size for continuous current carrying capacity.
  3. The next step is to determine the minimum size of the cable based on voltage drop.
  4. Determine the minimum size of cable for short circuit conditions.
  5. The fifth step includes determining the minimum size of the cable in case of earth fault loop impedance.
  6. The final step is to select the minimum size of cable in the previous steps.

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Now we will discuss each step in detail to find the correct size of the cable.

Example of cable sizing step by step

Assume we have:

  • power of load, P = 80KW
  • The load is at 200 meters distance of source
  • Three phase, V = 415V
  • power factor is, pf = 0.8
  • 5% allowable voltage drop
  • direct buried in ground
  • 1 meter burial depth
  • 35 °C Ground temperature
  • one cable per trench

Ok, 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

Determine correction factors

From derating factor tables we find the derating factors of the example conditions.

For more details about derating factors read our article What is derating factors of underground cables?

  • correction factor of ground temperature = 0.89
  • correction factor of soil = 1.05
  • 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 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 in 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.

Important notes:

  • In this example we skipped short circuit calculations, as it’s much complicated and depend on many factors.
  • Voltage drop and The current carrying ampacity of the cable which are 0.715 MV/A/M, and 185A, are on my android app “CABLES TABLES“. I made it my self.

Conclusion

  • Choosing power cable of electrical loads depend on load current, cable ampacity, installation conditions etc.
  • Cables currents tables are in “CABLES TABLES” android app.
  • Derating factors should be applied to the cable current.
  • Voltage drop value should be in the accepted limits.
  • If the cable voltage drop is higher than the limits, then we choose the larger sized cable.

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