How to find Cable size? Caterpillar generator produce (Two generators) 127*2=25

How to find Cable size?
Caterpillar generator produce (Two generators) 127*2=254 amp
at 11 kv .
What would be the cable size (XLPE) to supply 256 amp at 11
kv? Air or surrounding Temperature is 32 degree C. Cable
will go under ground at 2.5 feet depth. Group Rating Factors
is 3 cables. Clearance between cables is 7 cm. Soil
Temperature (Ambient) 28o C Soil Thermal Resistivity is 1.2
K.m/W

I would like to use 3 separate single core instead of using
3 core cable.

Some says I should use three separate single core 120RM cables.
Some says 70RM .some says 95.
I m totally confused.
Is there ant specific rule to determine cable size?
What would be the LT cable size to flow 500 Amp current?
Ifte
My e mail address is ifte13@yahoo.com
Anyone please send me the answer to my e mail address,
i will be very glad.

Question Posted / iftekhar mahmud

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How to find Cable size? Caterpillar generator produce (Two generators) 127*2=254 amp at 11 kv . W..

Answer / muhammad qasim

Dear, All those specification you described in your question
are cared while manufacturer design their product. Designing
contains many complex parameters to be cared while deciding
about the metallurgy, insulation. All good manufacturer care
worst operating conditions when they design their products.
When we go for their usage in industrial sector, main things
we need to assure are that:

1- Current Raging
2- Voltage Drop should not exceed the permissible level
3- Cable should have withstanding capability for transient
short circuit and thermal capability.

Normally, copper have current density of 1000 A/Sq inch
which becomes 1.8 A/Sq mm. and all cable and bus bars should
be designed 150% of operating rating for safety purpose.

Secondly, Conductor area is not affected with voltage level,
50 A at HT or LT shall require same conductor area but
insulation level shall be affected. For HT you need XLP and
for LT you need PVC-PVC cables.

Further if you get better answer please copy the same to
muazmi02@yahoo.com

Regards