Cables - impedance calculation


myCableEngineering Calculation Method

Calculation of cable impedance is reasonably involved.  Other notes give an insight into the individual calculations.  

With myCableEngienering, the calculation will typically follow the following procedure

  1. d.c. resistance - is determined using IEC 60278.  For any cases in which IEC 60227 is not suitable, the resistance is obtained using CENELEC CLC/TR 50480. See also Conductor Resistance.
  2. temperature adjustment - the calculated d.c. is adjusted so that its value is accurate relative to the operating temperature of the conductor. 
  3. a.c. resistance - the a.c. resistance is obtained by applying skin and proximity effect factors as defined in IEC 60277. See also Conductor Resistance.
  4. positive and zero sequences impedance - are calculated using the methods given in IEC 60909 and resistance values calculated above.

Resistance, Reactance

Positive and zero sequence impedance calculated above is in complex form.  The real part of the impedance gives the resistance and the imaginary part the reactance.

Capacitive Reactance

Currently, we do not calculate capacitive reactance.  This will likely be added as the application grows and develops the need for this calculation.

Impedance Equations

Cable Cores and Phase

Within myCableEngineering, the type of end load (regarding phases) a cable can be used on is dependant on the number of cores.

Single Core d.c., single phase a.c., three phase a.c., three phase + neutral a.c.
Two Core d.c., single phase a.c.
Three Core d.c., single phase a.c., three phase a.c.
Four Core three-phase a.c., three-phase + neutral a.c
Five Core three-phase a.c., three-phase + neutral a.c

* there may be additional variations within the application

Note: for armoured cables, the armouring is always considered as part of the earth return path. If the user opts to ignore cable armour, the equations for unscreened cable equations are used. The effect of any return path through the earth itself is considered. If multiple spare cores are available, only one can be used as an earth return path.  For any spare core being used as an earth conductor, its impedance is added in parallel with that obtained from the IEC equations.

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