how to calculate cable size for electrical works


why we need to calculate cable sizing for electrical works. this questions most commonly asked by our electrical folks. The reason is when we want transfer electrical power from one place to other we need to use electrical cable. so the sizing of cable is very important one.

when we transfer power from one place to another there is chance for voltage drop due to cable resistance and reactance. so proper sizing is mandatory to keep the voltage constant in both the end.

condition for cable sizing

cable sizing is based on IEC 60502 standard, it should be satisfied three condition,

  1. Ampacity rating of the cable > full load current of equipment
  2. Voltage drop of the cable < 3%
  3. short circuit rating of the cable > short circuit rating of the equipment

the cable should satisfied the  above conditions.

Basics of cable

Mostly in electrical we been used two types of cable,

  1. Copper
  2. Aluminum

the two cables based on its insulations its further classified into 4 types

  1. XLPE Copper
  2.  PVC Copper
  3. XLPE Aluminum
  4. PVC Aluminum


Cable is having base current carrying capacity based on it cross sectional area (Sqmm.) But based on installations its consume some de-rating factor of its base currents that’s called Ampacity rating of cable.

Under ground installations
Correction factor of Ambient temperature (K1)
Correction factor of soil resistivity (K2)
Correction factor of depth of cable laying (K3)
Correction factor of grouping of cable together (K4)

Above ground installations
Correction factor of Ambient temperature (K1)
Correction factor of grouping of cable together (K2)



Criteria 1: Ampacity rating of cable

Now we will calculate Ampacity of 4sqmm cable, Ampacity = (base current x ambient temp factor at 40 Deg C x soil resistivity factor x depth of cable laying factor x grouping factor of cable)

These de rating factor values easily get from cable data sheet Ampacity = 56 x 0.81 x1.09 x 1 = 49.44 Amps After de-rating factor 4sqmm cable ampacity is 49.44Amps Full load of SDB = 40 Amps Ampacity of 4sqmm cable = 49.44 Amps > 40 Amps As ampacity criteria we need to choose 4sqmm Cu\XLPE\SWA\PVC coated cable for 40 amps full load of SDB feed from 100mtr distance



Criteria 2: Voltage Drop calculation

Formula for in coming and out going feeders circuit, KCmil = (2*K * I * L )/ (1000 * VD) for 1Ph supply

KCmil = (1.732*K * I * L )/ (1000 * VD) for 3Ph supply

K- resistance of the cable for Cu = 12.9 ohm at 75 Deg C and Al = 21.2 ohm at 75 Deg C

I = full load current of equipment

2 = number of wire or run of cable

L = distance VD = voltage drop

For the same example we need to find voltage drop for 4sqmm cable while installing in under ground with 100meter distance.

As per IEC 60502 STD. Acceptable voltage drop for feeder circuit is 3%

As per 3ph voltage drop formula, KCmil or AWG or SQMM = (1.732*K * I * L )/ (1000 * VD)

= 1.732 x 12.9 x 40 x 100 1000 x VD

Here VD = % VD x supply voltage = 3% x 415 = 12.45v

= 1.732 x 12.9 x 40 x 100 1000 x 12.45 = 89371.2 /12450 = 7.178 = 8 KCmil We need to select the cable as per 3 % voltage drop the cable size should be greater than 8 KCmil


As per KCmil table 10 KCmil is equal to 6Sqmm and 6Sqmm cable it is satisfied for ampacity and voltage drop conditions.

3. Criteria : Short circuit current rating

Short circuit current rating of cable should be grater than equipment short circuit rating

As per our example we select 4C x 6Sqmm\Cu\ XLPE\SWA cable for 100mtr distance UG installations to SDB rating 3ph 415V, 40A, 1KA @ 1Sec

So here system short circuit current = 1KA @ 1Sec

So our cable short circuit withstanding capacity should be greater than 1KA You can easily check with data sheet for 10sqmm copper XLPE armoured cable short circuit rating


So our selected 10sqmm cable is suitable for all 3 criteria,

Ampacity rating of cable 56A

Voltage drop of cable is < 3%

Short circuit rating of cable > 1KA


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