This document discusses the sizing and selection of cable systems for electrical installations. It outlines factors to consider such as electrical loads, voltage drop, short circuit current, cost optimization through switchgear positioning and cable material selection. The key steps are determining the current carrying capacity and nominal current rating, applying derating factors for temperature, grouping, insulation, and protective devices, calculating the voltage drop based on route length, and selecting a cable with an adequate current rating.

1. Sizing And Selection of Cable
Systems
Introduction
•The design of a building or Factory environment electrical installation
involves broad Electrical Loads and ascertaining their characteristics and
selecting type size and location and routing of the cable network.
•In Sri Lanka we Follow the IEE Regulations latest edition according to
legislative obligations.
•As learnt the circuits distributed and the designer decides to place the
most suitable locations to place the Distribution Board. And the 4%
restricted voltage drop to be assured in the connection point of the
equipment or the circuit end point.
•Prospective Short Circuit Current to be decided at the installation to see
the adequacy of the fault capacity of the equipment and protective
devices.

2. Cost Considerations
• Minimum cross section conductor to be
determined (However this method is not
the cost optimization on a specific period
of operations)
• Switchgear positioning will play a overall
cost optimization.
• Selection of PVC and XLPE also plays a
role in cost optimization.

3. Determination of minimum cross
sectional areas of phase
conductors
• Determine the Current carrying by the conductor (Ib)
and the nominal current of the O/C device (In )
• Then the De-rating Factors to be decided
1. Ca – expected ambient temperature
corrections
2. Cg – Grouping factor how many cables per
phase to be used in parallel
3. Ci - If there is a thermally insulated material
• In > I b
• Determine Route length and then the voltage drop
• Determine Cf if the protective device used is according
to semi enclosed fuses to BS3036 and Over load with
or without short Circuit

5. • Then
• Calculate 1 / Ca Cg Ci Cf
• Then Select the cable I t from the tables
which is greater than 1 / Ca Cg Ci Cf
• Reiterating methods can be used in vise
versa steps on calculation .