Hello, I have 22 main cables going to the main house connection boxes of 22 buildings from the mini-sub station. The cable lengths are different for all the buildings ranging from (20 m (nearest to mini sub station) and approx 100 m (from mini sub station to the farthest building)). I have already calculated the maximum power demand of each building along with the circuit breaker current rating and the current carrying capaciyt of the each main cable (basically Ib, In & Iz). In my case, I have 2 building of same type (area, no. of apartments etc.) so eventually the power demand and all the 3 current values are same but the buildings are located 30 m and 60 m away from mini-sub station. According to my calculation, for the building, calculated minimum cross section is 25 mm2. Now, how will the cross section of the main cable from the mini sub station vary as the length of the cable differs ? Will it just be doubled as the length is doubled?
Dear Kranthi Ranadheer Moreddy
I=V/R By Ohms Law
and
R=Rho L/A Norte:Rho->Resistivity of the conductor.
I=V/Rho L/A
I=VA/Rho L
Therefore for the same load(I) as the length increases Area of cross section need to be increased Proportionately i.e diameter of the conductor.
Dear Mr. G.s. Vasantha Kumar,
Thank you for your answer. I knew all the equations that you have given in the above answer. I was just wondering on the cross sectionional area. However, your last statement ''Proportionately'' made my doubts clear.
Thank you!
Hello dear,
You can use the relationship below if you have the resistance of cable
R=rho*(L/A)
Where
R=resistance of cable
rho=constant
L=length of cable
A=cross-section area
Regards.
Hamzah
Hello Mr. Hamzah M. Marhoon ,
Thank you for your answer. However, my application of the wiring deals on the low-voltage distribution from the mini-sub-station to the main connection box (Mains) (can be outside the building or just near to the energy meter cabinet (MC). I know maximum load current (say; 100 Amps) that should enter the meter box so, accordingly I have calculated the cross-section area (CSA) (CSA according to norms for eg: 35mm² until a max. length of 19 meters) of the main cable from Mains to MC.
Now, my question is how should I choose the CSA of the cable based on the distance between mini-sub to Mains. The distance I have calculated is around 120 meters lets say, which CSA should I lay from mini-sub to mains so that my current is still 100 Amps after the mains (0.5 - 1% of voltage drop is allowed)?
Thank you in advance!
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