In order to choose the right power cable, one has to consider:
the current rating
the voltage drop
the short circuit rating
the installation methods
the ambient temperature
the frequency and harmonic current
maximum safe length at short circuit
When electric current flows through the conductor of a cable, the electrical resistance of the conductor generates heat. When a temperature greater than that allowed is reached by the cable due to heat generation, a larger conductor size (with lower electrical resistance) has to be selected. Other important considerations are methods of installation of the cable and ambient temperature. Calculation which takes into account all criteria are described in IEC 60287 and BS7671, and are rather complex. In general, preferences is given to standard current rating tables which are issued by national standardization bureaus.
The current rating are based on the following standard conditions of the installation.
1. Maximum operating temperature of conductor = 90OC
2. Ambient air temperature = 30OC
3. Ground temperature = 15OC
4. Soil thermal resistivity = 1.2OC m/w
5. Depth of laying (For cable laid direct in the ground) = 0.5m
Another important factor for the determination of the conductor size is the voltage drop. The voltage drop of the cable at a given current is caused by losses in the cable. In case of a too high voltage drop, it is necessary to choose a bigger conductor size. The voltage drop in a cable demotes the difference in voltage at the beginning and at the end of the cable.
It depends on:
the current carried
the power factor
the length of the cable
the resistance of the cable
reactance of the cable
The permissible voltage drop is usually stated as a percentage of the circuit voltage.
Selection of Cable based on Voltage Drop and Current using Tables
Since the actual power factor of the load is usually not known, the most practical approach to the question of the voltage drop is to assume the worst conditions, i.e. power factor equal to one and the conductor is at maximum operating temperature. The voltage drop values given in the tables are based on these assumptions.
The values of the voltage drop (Vd) are tabulated for a current of one Ampere for a 1 meter run, the value of voltage drop needs to be multiplied by the length of the run, in meter, and by the current, in Ampere that the cables are to carry.
V=Vd x l x L
Where V = Voltage Vd = Approximate Voltage drop/Ampere/meter I = Current in Ampere per phase L = Route length in meters
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