Cable Resistance

First of all I doubt whether each 240 sq.mm Aluminium cable will carry 30o Amperes continuously. Assuming that it is a PVC Insulated cable, the standard current rating for multicore Alu. cable is 275A in ground and 280A in air. But, this is only under standard conditions of laying. In practice, one cannot follow the standard conditions of laying; hence one has to apply a lot of de-rating factors. Thus, the 'practical' continuos current rating of the cable comes down drastically. Please cross check your cable sizing.

Now, one way of calculating the losses is the way you have shown. (i.e.) Current carried by each cable squared x AC Resistance of each cable per km x actual length of cable in km.

Other way is: Aggregate of the currents flowing in all the cables squared x net resistance of the paralleled cables per km x actual length of cable in km. In both ways you must get the same calculation.

It is true that a cable has got resistance plus inductive reactance plus capacitive reactance, whose combination is called it simpedance. But, remember losses happen only in the resistive component and thus it is enough if you take only the AC resistance into account while calculating the cable losses.

I assume you know cable sizing calculation. If not, then say thru PM i'll send you, specimen. I agree with EE65. for curiocity i m asking why do you want to calculate cable power loss? Depending on this I can help you some more.

dear sir,

we are analysing 6.6KV drive vs 690V drive system.in this case we go for 690V drive.our cable loss will be more.based on that we have to caculate the payback period of 6.6KV cost.

we are providing 4 runs of 300Sq.mm cable for 1200A.Kindly advise me.

regards,

N.Ganesan.

If you are considering only active power loss then you didn't need to worry about inductance and capacitances of the cable.

For 4 runs of cable you need to find the parallel combination of the 4 cables to find its effective resistance. Generally for 3X240 sq.mm 1.1KV XLPE cable Al cable, dc resistance at 20 degree C is 0.125 ohm/Km. So now you can calculate its copper loss.

And if you want to calculate its reactive power loss, then for 3X240 sq.mm Al cable approx capacitance (microfarads/Km) 1.1KV XLPE cable is 0.66.

And its reactance at 50Hz(Ohm/Km) for 1.1KV XLPE cable is 0.0710, so now you can calculate its reactive power loss.

And moreover its AC resistance at 90 degree C comes out to be 0.16 (ohm/Km) {please check once by calculating}, so you can also calculate its reactive power loss.

While calculating these values i have considered the cable as to be XLPE.