Cable sizing 

Cable sizing is a common task for electrical system engineers. A proper cable is chosen by verifying several conditions that it needs to meet. These conditions are:cable

  • Cable rated current should be higher than a full load current of the load.
  • Voltage drop in the cable should be lower than a permissible voltage drop.
  • Cable should withstand a short circuit current for a sufficient amount of time until protection devices disconnects the line.
  • Cable selection should be economically feasible.

In this article we will provide verification case study for cable sizing with EA – PSM Electric. We will choose a suitable cable for 70 kW induction motor “Motor 1” which is depicted in Fig. 1.

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Fig. 1 Network with load flow results (cable between “system bus” and “bus 1” has low apparent impedance)

First of all, we will calculate full load current from “bus 0” to “bus 1”. For this calculation, we will connect “bus 0” and “bus 1” with a zero impedance cable so that nominal voltage (0.4 kV) at “bus 1” is maintained. In our case full load current is 109.822 A as shown in Fig. 1.

Now we have to choose a cable with rated current that is greater than this full load current. However, it is important to consider that rated current of the cable depends on ambient conditions. According to international standard (IEC 60364-5-52) influence of ambient conditions on a rated cable current is evaluated by derating factor. This factor is calculated:

K = K1 * K2 * K3 * K4 * K5 * K6 * K7
Where

Coeff. Meaning
K1 Temperature correction factor when cable is in the air
K2 Ground temperature correction factor
K3 Thermal resistance correction factor for soil (if known)
K4 Soil correction factor (if thermal resistance is not known)
K5 Cable depth correction factor
K6 Cable distance correction factor
K7 Cable grouping factor (depends on tray factor)

 

EA-PSM Electric has cable ampacity function that allows to calculate rated current and derating factors automatically. To perform this calculation user has to define cable installation conditions, conductor type, conductor material and insulation type as shown in Fig. 2.

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Fig. 2 Cable ampacity calculation window

We will calculate rated current of NYY-J 1×70 0.6kV cable by using EA-PSM Electric cable ampacity function and manually, to verify the results. Results of EA-PSM Electric cable ampacity analysis are already depicted in Fig. 2, estimated rated current is 143 A or 115 A if taking into account derating factors.

These results match standard requirements for 70 mm2 area cable conducted in the underground cable ducting from IEC 60364-5-52. Calculation according to the standard is shown in Table. 1

Table 1 Rated current calculation from the standard

Installation conditions Parameters for estimation Reference to the standard tables
Cooper cable, three loaded conductors 143 A B.52.4
Ground temperature: 15oC 1.05 B.52.15
Grouped cables: 3 Underground spacing: 0.25 m 0.8 B.52.19
Thermal resistivity: 3 K*m/ W 0.96 B.52.16
Rated current  = 143 A * 1.5 * 0.8 * 0.96 = 115 A

 

As our full load current is 109.822 A, this cable can be used for energy supply to induction motor at “bus 1”. Load flow results with this cable connected, is depicted in Fig. 3.

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Fig. 3 Load flow results with chosen cable

Voltage drop at “bus 1” is 1.37% which is within the required limits (usually 5% for low voltage networks).

However, from load flow results we can see that current flowing from “bus 0” to “bus 1” is depicted in blue color. EA-PSM Electric has functionality to estimate economical aspects of energy losses in the cable. The blue color means that it would be economically feasible to change an existing cable with other cable that has wider cross section area.

Reference

International standard IEC 60364-5-52. Third edition 2009.10