Answer / aju

We can choose the cable according to the current rating
after considering overload and voltage drop factors

Usually cables are loaded upto 75-80 % of the rating
specified by manufacturer ..

For instance, if you need to choose a cable for a current
rating of 25 A, we can simply choose 6 sq.mm cable since it
can carry 35 Amps. It is desirable not to use 4 sq.mm cable
which has a current rating of 28 A since it has no
sufficient spare capacity for overload condition.

Again, the cable size also depends upon the voltage drop
which depends upon the length of the cable....

usually voltage drop must be within 3-5 % range.If it
exceeds this limit then next higher size must be used until
it satisfies the 3% range


Eg. for calculation of voltage drop in 25A, 240V single
phase feeder having 150M length

Voltage drop/Ampere/Metre for 35 Sq.mm cable=2.1mV

Voltage drop= (2.1x25x150)/1000 =7.88V

% Voltage drop= (7.88x100)/240 = 3028%

Since it exceeds the 3% range, take next cable size

Voltage drop of 50Sq.mm cable= (1.6x25x150)/1000 =6V

% Voltage drop =(6x100)/240 =2.5%


Hence, we can select 50Sq.mm cable

Answer / reddy

cable size can be selected like ,suppose if u have any
motor with u of 110kw and its full load current is 190 then
when it is in dol stater starting torque is high up to 2
times of that full load so we have to use 300sq mm.if it is
in star delta 240 sq mm is enough because in a star
starting current is low it means 70 to 60 percent of full
load .after that some time delay delta pick ups

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