# How to choose high voltage cable size?

## 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 mm2

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:

1. 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.
2. 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 (ISC  )= 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.

4 step to calculate LT Cable Size and Voltage drop

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How to choose short circuit current

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