11kV or 33kV Cable Sizing Calculation

11kV or 33kV cab cable le sizin sizing g calcula calculation tion While working on a GIS ASS (Aux. Sub-Station) in one of an underground metro station, a regular question was how to decide the size s ize of a cable for 11 kV or 33kV line? We normally used 400 sq. mm cables for 33kV, but how how to know that this is the r ight option to use 400 sq. mm cable. As with increase in cross section of a ca cable, ble, the price of cable jumps to sky. To know the size of cable we must consider a couple of things, things, short circuit current curr ent and normal condition current carrying capacity. To get the minimum cross sect ion area (A) required of a cable based on short s hort circuit current.

Here is a formula (A)

= I x (√ t / K ) mm

2

So, I = (K*A)/ √ t Where I is the fault / short circuit current  in kA, t is the t ime of fault and K is a constant (K=0.094 for aluminum conductor with XLPE insulation) & (K= 0.144 for copper conductor XLPE insulation) [Electric Cables Handbook Page no: 152] Based on above equation we can calculate or relate between the cable size and the short circuit current Duration of short circuit current Time (t) second 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Area of Alu conductor Area (A) sq mm 10 16 25 35 50 70 95 120 150 185 240 300 400 500 630 800 1000

Short circuit current I (kA) = (K*A)/ √ t

I=(0.094 x A)/√(t)

kA 0.94 1.50 2.35 3.29 4.70 6.58 8.93 11.28 14.1 17.39 22.56 28.2 37.6 47.0 59.22 75.2 94.0

Area of Cu conductor Area (A) sq mm 10 16 25 35 50 70 95 120 150 185 240 300 400 500 630 800 1000

Short circuit current I (kA) = (K*A)/ √ t

I=(0.144 x A)/√(t)

kA 1.44 2.30 3.60 5.04 7.20 10.08 13.68 17.28 21.60 26.64 34.56 43.20 57.60 72.00 90.72 115.20 144.00