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.

Step 1: Collecting Data

This step of cable sizing calculation involves collecting data about different parameters. This may include details of connected load such as:

  • single or three-phase
  • load types
  • power factor, etc.
  • besides this we also gather data about cable installation conditions and cable construction materials.

Step 2: Current Capacity of cable

When current flows through a conductor, it produces heat as a result of resistive losses. In this step, we find the maximum amount of safe current that the cable withstands to flow.

Please keep one thing in mind. That the more the cross-sectional area of the cable the less will be the resistive loses. So select a cable with a greater cross-sectional area.

Step 3: Voltage drop calculation

According to ohm’s law, when current flows through a conductor. A voltage drop occurs. There are two different reasons for voltage drop. Whether it is due to the current flow or due to resistance or impedance.

 Use the following Formula for voltage drop calculation.

V = I * R

Step 4 in cable sizing calculation: determining Short-circuit conditions

The fourth step in cable sizing calculation is to determine the minimum size of the cable for short circuit conditions. We know that short circuits have a large amount of current flow and it can damage the cable by overheating.

We can address the issue of the short circuit by selecting a cable with a more cross-sectional area. We use the following formula to select a minimum size.

A = √(i2t) / k

Where:

  • A, is the minimum size of the cable
  • I, is short circuit current
  • t, is the time for short circuit
  • k is short circuit temperature constant

Step 5: Earth fault loop impedance

Most circuits have earth fault protection. But in some cases, it misses out. So it is necessary that protective devices like fuse and breakers must trip at a certain time. For tripping to occur a fault current must exceed protective devices’ disconnection time. Us formula below.

IA = V0 / Z0

Example of choosing a cable

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

  • The second step is to 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

Selecting Copper, XLPE, 3*50+25 mm2 , it current is 185 A, Derating this current 185 * 0.93 = 172 A

Calculating voltage drop of this cable, VD = 0.715 MV/A/M, 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)

Important notes:

  • In this example we skipped short circuit calculations as it is much complicated and depend on many factors.
  • Voltage drop of the cable which is 0.715 MV/A/M, is in the valuable android app “CABLES TABLES“.
  • The current carrying ampacity of the cable is also in “CABLES TABLES” android app.

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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