__Calculation of nominal cross sectional area of the high voltage cable __

__Calculation of nominal cross sectional area of the high voltage cable__

High voltage cable

The cross sectional area of high voltage cable where calculated shall not be less the value determined by following formula:-

** S = (I _{SC }*√t ) / K**

Where,

S= Nominal cross sectional area of the conductor in mm^{2 }

I _{SC }= Value of fault current or short circuit current in KA

T=Fault clearing time in seconds (1 second or 3 second)

K= Factor taking account of the resistivity, temperature co-efficient, heat capacity of the conductor material, initial and final temperature. Value of K (as per BS 7671) shall be taken as **0.094** for XLPE insulated **Aluminium** cables, **0.143** for XLPE insulated **Copper** cables and **0.076** for **PVC** Insulated Aluminium cables.

**Note:**–

- Wherever the application of the formula results in a non standard size a conductor to the next higher standard cross section area shall be used.
- Short circuit duration for cables must be commensurate with the tripping time of the protective deceives associated with feeder.

**Let’s go through a example:**

__Data Available/ Require :-__

Suppose Short circuit current (I_{SC )}= 50 KA

Tripping time for system is 1 second.

**Solution:**

XLPE insulated **Aluminium **cables

As per above formula:-

S= (50× √1)/0.094 = 531.91 = 532 sqmm

So select in near size of cable like 3 core 630 sqmm .

XLPE insulation **Copper** cables

As per above formula

S= (50× √1)/0.143 = 349.65 = 350

So select in near size of cable like 3 core 400 sqmm .

So Finally we found two different size of high voltage cable and Cable will select as per system require. Copper cost is higher than aluminium.

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