UNIVERSITY OF SANTO TOMAS
FACULTY OF ENGINEERING
España, Manila
POWER CABLE DESIGN AND SYSTEMS – MV, HW, AND EHV
I.
Basic Electric Theory Applicable to Cable Design
Jose Lorenzo Z. Ladisla
In engineering, every option must be considered to be able to meet a certain standard. The
point where practical, economical, and technical aspects meet is the optimum solution to a
problem. For instance, the utilization of underground power distribution system isn’t the best
option for every areas in the country. Though the underground power distribution has its
advantages, it is still better to use the overhead distribution system in some places.
Cables have a wide variety of designs to meet various requirements. Since cables are placed in
different locations, different properties are also needed so that the performance of the cable is
at its best. Different insulations, armors, and protective finishes are incorporated to the cable
to protect it from harsh environment.
Current Carrying Capacity
There are various factors that affect the current carrying capacity or ampacity of a cable. This
section of the journal will discuss some of the factors affecting the ampacity of air cables and
underground cables.
Conductor Size
-
The cables are installed and spaced from a wall on a tray in trefoil formation. The
size of the cable is directly proportional to its ampacity. The graph below shows how
conductor size greatly affects the ampacity of the cable.
Figure 1.1
Ampacity
ratings with varying conductor sizes
Solar Radiation Intensity
PE4| Christian L. Ranada
UNIVERSITY OF SANTO TOMAS
FACULTY OF ENGINEERING
España, Manila
-
Exposure to solar radiation results to a higher temperature of the cables thus
increasing the resistance and decreasing its ampacity. According to IEEE standards,
solar intensity heavily relies on the type of atmosphere the area has. The table
below shows how exposure to sunlight affects the cable’s ampacity.
Table 1.1 Influence of solar radiation intensity on ampacity when surface absorption
coefficient is 0.8
Ambient Air Temperature
-
Ambient air temperature has a huge influence on the cable’s ampacity. This factor is
primarily considered in both IEC and IEEE standards.
Figure 1.2 Ampacity ratings with varying air temperature
Conduit
-
Enclosing a cable inside a conduit results to added thermal resistance of the
conduit wall and raised temperature of enclosed air, this leads to lower ampacity
ratings
for
cables. The
graph below
shows
the
effects
of
enclosing a
cable inside
a
conduit
and
also
how
cable
size plays a
role on the
effects of the
conduit.
PE4| Christian L. Ranada
UNIVERSITY OF SANTO TOMAS
FACULTY OF ENGINEERING
España, Manila
Figure 1.3 Effect of enclosing a cable inside a conduit to its ampacity with varying
conductor size
Groups of Cables
-
Grouping cables results to hotter cables, and lesser cable ampacity. The cable
diameter and the distance between circuits influences this effect, if the distance
between groups overrides the critical ratio then the thermal proximity effect can be
ignored.
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- Spring '18
- Kenneth Sinson
- UNIVERSITY OF SANTO TOMAS, power cable
