DEVELOPMENT OF A SOFTWARE PACKAGE FOR 
CALCULATING CURRENT RATING OF MEDIUM 
VOLTAGE POWER CABLES 
A Thesis presented to 
the department of Electrical Engineering 
University of Moratuwa, Sri Lanka 
In partial fulfillment of the requirement for the degree 
Master of Engineering 
BY 
D.G.A.K. Wijeratna 
December 2002 
Project Supervisors: Prof. J.R. Lucas, Dr. J.H.C. Peiris, Dr. H.Y.R. Perera 
University of Moratuwa 
78482 
7 8 *82: 
ABSTRACT 
CableAmp, a Windows based software has been developed to calculate the cable 
ampacity of medium voltage power cables laid in free air or directly buried in the 
ground. The highly graphical user interface allows easy usage. The user inputs the 
required data of the cable design as well as the cable installation method. CableAmp 
then calculates the continuous current rating (at 100% load factor) of the power cables 
laid in free air or directly buried in the ground. It is applicable to extruded solid 
insulation, rated from 6 kV to 33 kV. An increased peak current rating, called the 
cyclic current rating, can also be obtained from the software package for a specified 
load profile. The calculation procedure is in accordance with International Standard 
IliC 287. 
The report presents the theory for the software, and also validates the results obtained 
b> the software by comparison with manufacturers current ratings for standard cables. 
IV 
DECLARATIONS 
1 certify that this thesis has not been previously prepared in whole or part 
to any University or Institution for a higher degree. 
A · k ·~~ 
D.G.A.K. Wijeratna 
December 2002 
Prof J.R.Lucas 
Supervisor 
-
~ 
II 
ACKNOWLEDGEMENT 
I wish to express my deepest gratitude to the project supervisors Prof. J. R. Lucas, Dr. 
Jchan Peiris and Dr.H. Y. R. Perera. I am grateful for them for the guidance and 
enthusiasm they have shown and the valuable advices offered. 
My special thanks to the staff members of the department of Electrical Engineering, 
Lniversity of Moratuwa for the help given in making my project a success. 
1 dso wish to thank Mr. Chandana Samarasinghe of CEB for supporting me to obtain 
the required IEC standards. 
Thanks are due to Mr. K. Sooriyabandara of Kelani Cables (Pvt.) Ltd. for the 
a~-;istance he has given. 
Fi1ally I appreciate the supports given by Dr. K.M. Liyanage and my colleagues at the 
Computing Centre, University of Peradeniya in completing my project successfully. 
-
Ill 
ABSTRACT
CableAmp, a Windows based software has been developed to calculate the cable 
ampacity of medium voltage power cables laid in free air or directly buried in the 
ground. The highly graphical 
required data of the cable design as well as the cable installation method. CableAmp 
then calculates the continuous current rating (at 100% load factor) of the power cables 
laid in free air or directly buried in the ground. It is applicable to extruded solid 
insulation, rated from 6 kV to 33 kV. An increased peak current rating, called the 
cyclic current rating, can also be obtained from the software package for a specified 
load profile. The calculation procedure is in accordance with International Standard 
IEC 287.
The report presents the theory for the software, and also validates the results obtained 
by the software by comparison with manufacturers current ratings for standard cables.
interface allows easy usage. The user inputs theuser
iv
TABLE OF CONTENTS 
l.t) Introduction 
2 1) 
3 1) 
4.1) 
5 
6 I ) 
The fundamentals of calculating current carrying capacity 
2.1 Continuous current rating 
2.2 Cyclic rating 
2.2.1 Cyclic rating factor 
2.3 Jmponant parameter which affect ratings 
Main components of a power cable 
3.1 Conductor 
3.2 Insulation 
3.3 Metallic sheath/screen 
3.4 Bedding 
3.5 Armour 
3.6 Outer serving 
Power cable losses 
4.1 Conductor 12R losses 
4.2 Sheath/screen losses 
4.3 Armour losses 
4.4 Dielectric losses 
Methodology 
5.1 The main procedure 
5.2 The system of equations 
5.2.1 Temperature difference 
5.2.2 Dielectric loss 
5.2.3 A.C. resistance 
5.2.4 Sheath/screen loss factor 
5.2.5 Armour Joss factor 
5.2.6 Calculation thermal resistance 
5.2.6.1 Thermal resistance between one 
conductor and sheath 
5.2.6.2 Thermal resistance between 
sheath and armour 
-
5.2.6.3 Thermal resistance of outer serving 
5.2.6.4 External thermal resistance 
5.3 
5.2. 7 Pern1issible constant cutTent rating of the cable 
5.2.8 Cyclic rating 
Assumptions made in the current rating calculation 
Procedure 
The software 
6.1 An overview 
6.2 The operation 
6.2.1 General details input 
6.2.2 Conductor data input 
v 
Page 
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3 
4 
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25 
70 
6.3 
6.2.3 Insulation data input 
6.2.4 Sheath/screen data input 
6.2.5 Bedding data input 
6.2.6 Armour data input 
6.2.7 Outer serving(Jacket) data input 
6.2.8 Installation data input 
6.2.9 Continuous current rating calculation 
6.2.1 0 Cyclic current rating calculation 
6.2.11 Reporting 
6.2.12 Error handling 
6.2.13 Programme justification 
System requirements 
Conclusion 
7.1 Suggestions for the improvement of the software 
-References 
Appendix A -Flowcharts showing the calculation procedure 
of each parameter in the current carrying 
25 
26 
27 
27 
28 
28 
28 
29 
30 
30 
30 
30 
31 
31 
32 
calculation formula 33 
Appendix B -Specifications of the cables considered in the 
programme 46 
Appendix C -Fommla for calculating Ei(x), G, and ~es 48 
Appendix D -Thermal resistances, electrical resitivities and 
other coefficients used in the programme 51 
VI 
Fi Jurc 2.1 
l·isure3.1 
Figure 3.2 
F ..!Ure 3.3 
F15ure 5.1 
F15ure 6.1 
f iJUrC 6.3 
I I:Sure 6.3 
h gure 6.4 
Figure 6.5 
Fl..!Ure 6.6 
ri~ure 6.7 
Fqure 6.8 
ft ~ure 6.9 
List of Figures 
-Thermal circuit model of a 3-core metal-sheathed 
power cable 
-Cross section of a single core metal sheathed, wire 
armoured power cable 
-Copper tape shielded power cable 
-Aluminum sheathed cable 
-Flow chart showing the main calculation procedure 
-Main programme window 
-Window for entering the conductor details -
-Window for entering the insulation details 
-Window for entering the sheath/screen details 
-Window for entering the bedding details 
-Window for entering the armour details 
-Window for entering the outer serving details 
-Window for entering the installation details 
-Window showing details of continuous current rating 
calculation 
Page 
2 
6 
8 
8 
l l 
25 
25 
26 
26 
27 
27 
28 
28 
29 
Fi ~ure 6.10 -Window for entering details of daily load cycles in order 29 
to calculate cyclic rating 
VII 
A 
c 
D 
D.t' 
D • 
D 
D <)C 
D1t 
D 
F 
B 
l c, nt 
l l\c lic 
L 
l\ 
P. Q 
R 
R' 
R , 
R 
R 
'J 
., 
J 
'J 
t 
\\ 
\\ d 
:\ 
:\" 
LIST OF SYMBOLS 
Cross sectional area of the armour 
Capacitance per core per unit length 
Mean diameter of the sheath/screen 
External diameter of the arn10ur 
External diameter of cable 
External diameter of insulation 
The diameter of imaginary coaxial cylinder which just touches 
the crest of a corrugated sheath 
The diameter of imaginary coaxial cylinder which just touches 
the inside surface of troughs of a corrugated sheath 
External Diameter of sheath 
Coefficient defined in 5.1.7.1 
Intensity of solar radiation 
Continuous current in one conductor 
Cyclic current in one conductor -
Distance from the surface of the ground to the cable axis 
Number of conductors in the cable 
Coefficient defined in 5.1.5 
a.c. resistance of conductor at maximum operating temperature 
d.c. resistance of conductor at maximum operating temperature 
a. c. resistance of armour at maximum armour temperature 
d.c. resistance of conductor at 20°c 
The resistance of sheath or screen per unit length of cable at its 
maximum operating temperature 
Thermal resistance between the conductor and sheath 
Thermal resistance between sheath and the armour 
Thermal resistance of the outer serving 
Thermal resistance of the surrounding medium 
Voltage between conductor and screen or sheath 
Total joule losses per cable at maximum operating temperature 
Dielectric Loss per unit length per phase 
The reactance and resistance per unit length of the sheath or 
screen per unit length of cable 
Mutual reactance per unit length or cable between sheath of 
an outer cable and the conductors of the other two 
c Distance between the axis of one conductor and the axis 
(mm2) 
(F/m) 
(mm) 
(mm) 
(m) 
(mm) 
(mm) 
(mm) 
(mm) 
(W/m2) 
(A) 
(A) 
(mm) 
(0/m) 
(0/m) 
(0/m) 
(0/m) 
(illm) 
(K.m/W) 
(K.m/W) 
(K.m/W) 
(K.m/W) 
(kV) 
(W/m) 
(W/m) 
(illm) 
(0/m) 
of the cable (mm) 
d Mean diameter of sheath or screen (mm) 
d , Mean diameter of the armour (mm) 
d External diameter of insulation (mm) 
d External diameter of conductor (Including screen if any) (mm) 
r System frequency (Hz) 
h Heat dissipation coefficient 
k , Factor used in calculating xp(proximity effect) 
k Factor used in calculating x5(skin effect) 
r Radius of the circle circumsbring the two sector shaped conductors (mm) 
:-. Distance between conductor axes (mm) 
VIII 
t, 
t2 
t ~ 
ts 
y. 
Thickness of insulation between conductors 
Thickness of insulation between conductor and sheath 
Thickness of bedding 
Thickness of the outer serving 
Thickness of sheath 
Skin effect factor 
Yr Proximity effect factor 
tq Sheath loss factor 
i. ~ Armour loss factor 
0 
~(I 
\ !Is 
8l :x::) 
(J 
8 
() : 
(t) 
f. 
" 211 
p 
p, 
P 11 
Prl 
PI J 
Pr~ 
Maximum operating temperature 
Temperature difference 
Excess of cable surface temperature above ambient temperature 
Conductor steady state temperature rise above ambient 
Absorption coefficient of solar radiation for the cable surface 
(This value is taken as I 03 for most latitudes) 
Equivalent thickness of armour (AJ(ndA) 
Soil thermal diffusivity (taken as 0.5* 10-6 m2/s) -
Angular frequency of the system (2 n f) 
The relative permitivity of insulation 
Constant mass temperature coefficient at 20°c per Kelvin 
Relative permitivity of the steel tape (Usually taken as 300) 
Electrical resitivity of sheath material at operating temperature 
Thermal resitivity of insulation 
Thennal resitivity of bedding material 
Thermal resistivity of outer serving material 
Thermal resistivity of soil 
IX 
(mm) 
(mm) 
(mm) 
(mm) 
(mm) 
(>C) 
(K) 
(K) 
(K) 
(mm) 
(IlK) 
(Om) 
(K.m/W) 
(K.m/W) 
(K.m/W) 
(K.m/W)