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 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.
- Number of cable runs should be greater or equal to full load current and cable rated current ratio.
- Cable selection should be economically feasible.
In this article we will analyze a small distribution system depicted in Fig. 1. Let’s say that we need to choose the type of cable for 80 kW power and 600 volts Motor 2 connected to bus 4. Distance between bus 3 and bus 4 is 300 meters and allowable voltage drop is 5%. In Fig. 1 this motor is connected through nearly ideal cable, thus, we can see the full load current which is 96.509 Amps. Therefore, we need cable with rated current greater than 96.5 Amps. However, it is important to notice that rated current of a cable will depend on ambient conditions, because heat dissipation might not be equal under different conditions. To estimate the cable conductance under not rated ambient conditions derating factor K is used. It is calculated with equation:
K = K1 * K2 * K3 * K4 * K5 * K6 * K7
Meanings of coefficients are in Table 1.
|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)|
These coefficients are determined by present standards. If cable is buried in wet soil which temperature is 25 deg. and depth is approximately 1.5 meters then according to IEC 60364:2009 derating factor:
K = 1 * 0.89 * 1 * 1.13 * 0.96 * 1 * 1 = 0.965;
Derating factor K is used in the following formula:
Derated current = K * rated current;
In ESMOGrid you can choose from the wide variety of cables. Everything you need to do is to enter cable length and ESMOGrid will automatically calculate other parameters of it. You can also model overhead lines. For our purpose we can try NYY – J 1×35 600 V cable (Fig.2) which rated current is approximately 120 Amps according to standard IEC 60364.
Derated current of this cable is:
Id = 120 * 0.965 = 115.8 Amps.
And maximum short circuit current for 1 second is 4.02 k A (provided by manufacturer).
Load flow analysis results are depicted in Fig. 3 and short – circuit results are in Fig. 4.
As can be seen from load flow results, voltage drop at bus 4 is 4.526% (smaller than 5%), steady – state current is 100.766 Amps which is below derated current of cable we picked. Ratio 100.766 / 115.8 = 0.83 is less than 1, therefore, one parallel run is enough.
3 – phase short circuit current (which is always the biggest one) at bus 4 is 2.299 kA, therefore, the cable can withstand a short – circuit for at least 1 second.
As all criteria are met we can conclude that selected cable is good for our system.