How to calculate size of a cable for 11kV or 33kV line

Posted on Posted in Electrical, Study Material

There are three factors affecting the size of a cable

2. Length & Voltage drop
3. Short Circuit Current of the system

Now, we can consider these four points one by one and choose a cable which complies all these requirements.

1. Load – The load of circuit is available in kW or watts. For example if you are calculating the size of cable for a 1 kW single phase pump. Load on the circuit is 1 kW.

Now, calculate the current that will be running in the circuit due to 1 kW load of the pump.

P=Volt X Current X Cos α

(Where Volt of single phase system – 240 Volt, Cos α – power factor = 0.8)

Therefore –

1000 watts = 240 X Current X 0.8

Current = (1000/240)/0.8

Current = 5.2 Amps

Now we know the current rating of the system = 5.2 Amps, we can choose suitable size of cable from below table. Note : the given table is for copper cable (XWY).

 Sr. No. Conductor Cross section in sq.mm Current rating in Amperes 1 1.5 22 2 2.5 30 3 4 36 4 6 47 5 10 62 6 16 79 7 25 108 8 35 132 9 50 162 10 70 198 11 95 240 12 120 276 13 150 318 14 185 366 15 240 426 16 300 480 17 400 530

So from above table you can choose 1.5 Sq mm Cu Cable for 1 kW load.

For larger loads you should consider the de-rating factor of cables, for more info on de-rating you can go to https://en.wikipedia.org/wiki/Ampacity. Or our blog De-rating Factor of Cables.

1. The second factor which you should consider before choosing a cable size is the length of the cable and the permissible voltage drop of the system.

For example, if your cable length is too large, there will be a larger voltage drop. If you supply 240 Volts single phase at the source, your load might receive 195-210 Volt due to voltage drop in the cable. And due to voltage drop there will be heating losses in the cable.

To know how to calculate the voltage drop in a cable you can read our blog How to create cable schedule for a project.

1. Next comes the short circuit current of the system, for a 240 Volts single phase system, design short circuit is 6kA, for 11kv it is up to 25 kA and 33 kV it is up to 70 kA. So if you are detailed engineering we would suggest considering rated short circuit current for cable also.

To know the size of cable we must consider a couple of things, short circuit current and normal condition current carrying capacity.

To get the minimum cross section area required of a cable based on short circuit current.

Here is a formula

–    I X √t / K

where I is the fault/ short circuit current in kA, t is the time of fault and K is a constant (K=0.094 for aluminium conductor with XLPE insulation)

Based on above equation we can calculate or relate between the cable size and the short circuit current

 Duration of short circuit current Area of Al. conductor Short circuit current (kA) Time (t) Area (A) I=0.094 x A/sq.rt (t) 1 70 sq mm 6.58 1 95 sq mm 8.93 1 120 sq mm 11.28 1 150 sq mm 14.1 1 185 sq mm 17.39 1 240 sq mm 22.56 1 300 sq mm 28.2 1 400 sq mm 37.6

Hi Reader, hope below information covers up whatever you are looking for. Please post comment below if you need more info on any electrical related queries. Or mail us at pratham6138@gmail.com

5 thoughts on “How to calculate size of a cable for 11kV or 33kV line”

sir can u pls send me table of standards for calculation of every type of cable wither we are calculating it for room or industry etc God bless

2. ZXpress says:

Every state has its own works department which publishes SOR – schedule of rates and technical specification every year. This specification is based on IS/NEC/ BIS codes applicable. So you can search electrical SOR/ tech spec of your state over google. If you cant find it yet, let us know your state and country.. we will mail you one
For internal wiring (Copper)
* 1.5 sq mm for point wiring – 2.5 sq mm for it’s circuit ( less than 800 watt per ckt)
* 2.5 sq mm for 5 A light plug – max 3 plug points in one ckt – 4 sq mm ckt
* 4 sq mm ckt for 16/5 A (max 2 sockets)
* Sub-main / main ckt as per load calculation
* Earthing – earthing wire should be copper wire of same size as phase wire

3. nagu says:

sir how to calculate the load for main breakers.if breaker is 400A

4. Abu Hanif says:

What will be constant value K= for copper

5. Sir,I didn’t get the formula for calculating a cable size based on the short circuit current.