INSTRUCTION OF CHOOSING
LOW VOLTAGE WIRES & CABLES

LOW VOLTAGE WIRES & CABLES  

A/ SELECTION OF WIRE & CABLE: :

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 or ground temperature

Current rating :

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.

The current ratingand volt drop depends on the following factors:.

- Maximum operating temperature of conductor

- Ambient air temperature

- Ground temperature

- Soil thermal resistivity

- Depth of laying ( for cable laid direct in the ground)

- Conditions of the installatio

Voltage drop :

Another important factor for the determination of the conductor size is the voltage drop. In case of a too high voltage drop, it is necessary to choose a bigger conductor size. It depends on :

- The current carried

- The power factor

- The length of the cable

- The resistance of cable

- Reactance of the cable

According to IEE regulation 522-8, it is stipulated that the total voltage drop for any particular cable run must be such that the voltage drop in the circuit of which the cable forms a part does not exceed 2,5% of the nominal voltage.

For single phase circuit 220V, maximum voltage drop is 5,5V

For three phase circuit 380V, maximum voltage drop is 9,5V

I/ POWER CABLE

I.1/ PVC insulated power cable:

I.1.1./ Cables installed in air:

Basic assumptions and conditions of installation:

Ambient air temperature 300C

Maximum conductor operating temperature at rated current 700C

Current ratings and Voltage drop for Copper conductors, PVC insulated, unarmoured, PVC oversheathed cables, Installed in air

Table 1