cable size calculation?

To ensure minimum of cross section of the cable is still remaining in case of short circuit occured.

Regards,

Sittichai B.

what is K ?

short circuit calculations are required not only for HT but also MV it will guide you for cable sizing ,it will indicate the max current that can flow in event of a fault.

The capability of the selected cable size to withstand the short circuit current is applicable for all cables whether HV, MV or LV.

Most of the time the LV Cables from LV feeders are from current limiting type MCCB or a switch fuse combination. Since the operating time t of MCCB or fuse is very less and within thermal stress allowed by the cable hence the verification for short circuit withstand capability is not required.

However, in case the LV feeder is from eg an ACB then it would require calculation for shct withstand capability.

It is required for all power cables HT mv or LV to ensure the cable can withstand the short circuit current if when it flows however brief the time may be.

1. In case of MV cables, if the minimum cross-section required is 350 sqmm ( for say aluminium, XLPE cable for a given short time fault current ) and due to ampacity/voltage drop requirement, it needs min 500 sqmm - is it OK to select 2 runs of 300 sqmm or each run has to meet the sckt criteria and hence i should use 2 runs of 400 sqmm cable ?

or in simple terms - in case of multiple runs of cables - does each run of cable meet the minimum c/s required as per sckt ?

2. For cables laid in ground - what is the base ampacity to be selected before applying derating ? - is it the current capacity in ground or in air ?

Regards,