Cable Basic Parameters Resistance, Inductance and Capacitance

Cable Basic Parameters Resistance, Inductance and Capacitance

As a part of daily work of cable engineer or system protection engineer it is very much essential to calculate and find out the basic cable parameters like DC resistance, AC resistance, Inductance and Capacitance of you cable. You can get some ready made data from cable manufacturer’s supplied catalog. But for accurate control setting you need more accurate data which not possible to mentioned in catalog. By using some formula you easily can calculate this data. Today we are going to introduce such formula.

Typically the values of conductor DC resistance given by manufacturer’s catalog are based on standard 20°C temperature that you can find just open the temperature table in technical data and cables parameters of your cable vendor. But in your cable vicinity is not possible to keep at all in said standard temperature 20°C. You can calculate the accurate resistance as per cable ambient temperature. In case the DC resistance is required at any other temperature the following formula can help you to calculate.

DC & AC Resistance Calculation for Cable:

Rd =R20[1+ a(d -20)]   ohm/km

Where
Rd =Conductor DC resistance in ohm/km at d°C;
R20=Conductor resistance in ohm/km at 20°C;
d=Operating temperature in°C;
a=Resistance temperature coefficient in 1/°C;
= 0.00393 for Copper
=0.00403 for Aluminum

To get AC resistance of the conductor at it operating temperature please use the following formula:  

RAC=Rd(1+Yp+ Ys)   ohm/km

Where
Yp =factor of proximity effect of the cable;
Ys =factor of screen effect of the cable;
These effects are depends on operating frequency and cable spacing.
km- kilo meter, °C- degree centigrade.


Calculation of Cable Inductance:
The self and mutual inductance are formulated as below where you should consider the total number of wire that formed the cable main core to carry the load, the conductor diameter, also consider the type of cable formation like trefoil or flat and spacing between them.

L= K + 0.2 ln(2s/d)  mh/km

Where
L= Inductance in mh/km;
K= Constant depends on the conductor’s number of wire;
d= Conductor diameter in mm;
S= Axial spacing in mm between cables in trefoil formation;
S= 1.26x axial spacing in mm between cables in flat formation;

mh-mili henry, mm-mili meter, km-kilo meter.


Calculation of Cable Capacitance:

The capacitance is formulated as follow

C = Er/[18ln(D/d)]  μf/km

Where
C= Capacitance in μf/km;
Er= Relative permeability of insulation material;
D= Diameter in mm over insulation;
d= Conductor diameter in mm.
μf-micro farad, mm-mili meter, km- kilo meter,

Hope this three basic cable parameters will help you for cable operation and maintenance work. To get more information keep in touch. 


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