Cable sizing is the process of selecting the appropriate sizes for electrical power cable so that the chosen cable works efficiently. Cable sizes are typically described in terms of cross sectional area or in terms of Standard Wire Gauge (SWG) according to the geographic region. It is important to chose the apt size of cable for all applications so that it is ensured the cable satisfies the following requirements;
a) The cable should be able to operate continuously under full load with out any damage.
b) The cable should be able to provide the load with suitable / required voltage, avoiding any excessive voltage drops
  c) The cable should be able to withstand any short circuit current passing through.

GENERAL STEPS FOR CABLE SELECTION

Cable sizing methods differ with different standards adopted viz. IEC, BS etc. But the general principle that underpins the cable selection methodology shall be as mentioned below;
1) Gathering Data - Installation, load details etc.
2) Determine the minimum size of the cable based on the ampacity.
3) Determine the minimum size of the cable based on the voltage drop calculations.
4) Determine the minimum size of the cable based on the short circuit current calculations.
5) Choose the maximum cable size based on the calculations above.

GATHERING DATA

The initial step is to gather data regarding the cable viz, its construction, application and installation.

Constructional Data - Details such as the type of cable (i.e if it is an Aluminium or Copper Cable), the type of insulation of the cable (i.e PVC or XLPE etc), number of cores in the cable (i.e single core or multi core)

Application Data - Details of the type of load of the cable viz, number of phases of the supply ( Single phase or Three Phase), Full load current, Length of the cable from source to load etc. 

Installation Data - Details of where and the cable is being is installed, i.e cable tray or ladder etc. the temperature at site were the cable is being installed, details of cable grouping, cable spacing etc.

AMPACITY CALCULATION 

Current flowing through a cable will be generating some heat and implies resistive losses in conductor. A cable's insulation must be capable of handling the heat emanating from the cable. The ampacity of the cable is the maximum current it can carry with out damaging the insulation. Cable with larger conductor cross sectional area can carry larger current and hence have greater ampacity. Say, a 16sq.mm cable has more ampacity that that of what a 4 sq.mm cable has.

By referring to the cable manual, details regarding the ampacity can be obtained. The cable manufacturer provides with details of ampacity based on the construction of the cable. These values will be specific to ideal conditions and / or may not be suitable for all site conditions. The ampacity would differ based on the conditions at site. The manufacturer would also provide details of derating factors for a range of installation conditios such as ambient temperature, grouping of cables etc. A base derating factor is obtained by multiplying all the given derating factors.

Say, It - derating factor for ambient temperature and Ig - derating factor for grouping, and Ib - base derating factor is obtained as; Ib = It X Ig

The derated ampacity can be obtained by multiplying the ampacity with the derated ampacity. 

Based on the current required by the load, a minimum size of the cable is chosen with the apt ampacity. 

VOLTAGE DROP CALCULATION

A Cable can be seen as an Impedance, and hence a voltage drop will be associated with each and every type and size of cable. The impedance of the cable is dependent on the cross sectional area of the cable and the length of the cable. The voltage drop will be higher if the current flowing through the cable is high and if the cable has higher impedance. 

The reactance and resistance values of a cable will be provided by the manufacturer. 

The voltage drop can be calculated as; 

 Vr = 1.732 X If (Rf Cos Er + Xf Sin Er) / Number of Runs

Where, Vr = Voltage Drop during running condition
If = Full load current
Rf = Resistance of the cable
Xf = Reactance of the cable

Using the above equation, the voltage drop can be calculated in %. The allowable voltage drop may be considered with in 3% to 5% depending upon the specifications. The minimum size of the cable with significantly lower voltage drop shall be chosen.

SHORT CIRCUIT CURRENT CALCULATION

A short circuit can cause high amount of current to flow through the cable. This surge in current flow causes a temperature rise within the cable which can degrade the condition of the cable. The minimum cable size to withstand the short circuit current can be calculated as below;

A = Sqrt(i^2 X t) / k

where, A = minimum cross sectional area of the cable (in mm )
i = short circuit current (in A)
t = duration of short circuit current (in seconds)
  k = short circuit temperature rise constant