How we find the size of cable if we know the current
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Answer Posted / 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
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