Answer / suresh kumar

The procedure for cable sizing is first to measure the
load in currents and voltage of load which is to be
connected in that particular cable. Then the length of
cable. Type of cable conductor whether copper or aluminium.
Medium of cable passing whether in opened cable tray (air),
closed cable tray, in open land, in cable trunch etc. The
usage of cable also to be taken care, ie like telephone
cable, control cable, power cable etc.
In an example the current rating of 10 sq. mm copper cable
in air is 40Amps. If the length of cable is shorter we can
use it and if the length of the cable more, we have to
calculate the voltage drop of that cable and may be have to
use 16 or 25 sq. mm cable instead of 10 sq mm.

Answer / surendra matharu

I really appreciate the efforts put by all.
I would like to throw some light on the matter.

There are three CRITERIA for cable sizing :
The criteria which gives the worst rating is adopted.

1. The maximum short circuit current which can pass through
the cable.
2. The maximum load current which the load can withdraw from
the system.
3. The maximum voltage drop allowed in the system.

Out of these, whichever criteria gives higher rating, that
is selected.

Hope, this satifies the query.

Answer / abhay

copper cable size for following

Panel Power Cable 63A 300mtr
15 HP VFD Cable x 325mtr
20HP VFD Cable x 325 mtr
10HP VFD Cable x 325 mtr
vfd panel 250 amp *50 meter
cable pass through pvc condut

Answer / raghavendran.r

Thanx to all for giving useful answer on that subject.I
would like to add that cable selection is also influenced
by certain factors like the Derating Factor.

A cables Current carrying capacity is reduced by the
following factors

a) Soil thermal resistivity.

b) Grouping Factor.

c) Ambient Temperature factor.

d) And the Depth at which Cable is to be laid.

Based on All these factor the current carrying capacity is
calculated and that also decides the number of runs of
cable to be laid.

Apart from this what Mr Matharu has stated is taken care of.

Formula to calculate SQMM is

Max short Ckt withstand current*sqrt[T(fault time)] /K.

for EX = 40 KA*sqrt(.25)/(.0928)

Answer / raul

the size of a cable depends upon the maximum dielectric
stress at the conductor surface.........
dielectric stress/potential gradient,g = V/xLOGeD/d
where V= system voltage; x= distance from conductor, D=
diameter of conductor and d= diameter of metallic
sheath....
so max. dielectric stress will b at conductor surface,i.e.
at g(max)= 2V/dLOGeD/d
d value of conductor diameter will be D/2.718 OR LOGe(D/d)
= 1

Answer / oarhe ize

We calculate cable sizes in order for us to know the amount
of load it can withstand (current rating)

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