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Home / Technical Articles / An example how to calculate voltage drop and size of electrical cable

Input information

Electrical details:

Electrical load of 80KW, distance between source and load is 200 meters, system voltage 415V three phase, power factor is 0.8, permissible voltage drop is  5%, demand factor is 1.


Cable laying detail:

Cable is directed buried in ground in trench at the depth of 1 meter. Ground temperature is approximate 35 Deg. Number of cable per trench is 1. Number of run of cable is 1 run.

An example how to calculate voltage drop and size of electrical cable
An example how to calculate voltage drop and size of electrical cable (photo credit: 12voltplanet.co.uk)

Soil details:

Thermal resistivity of soil is not known. Nature of soil is damp soil.


Ok, let’s dive into calculations…

  • Consumed Load = Total Load · Demand Factor:
    Consumed Load in KW = 80 · 1 = 80 KW
  • Consumed Load in KVA = KW/P.F.:
    Consumed Load in KVA = 80/0.8 = 100 KVA
  • Full Load Current = (KVA · 1000) / (1.732 · Voltage):
    Full Load Current = (100 · 1000) / (1.732 · 415) = 139 Amp.

Calculating Correction Factor of Cable from following data:

Temperature Correction Factor (K1) When Cable is in the Air

Temperature Correction Factor in Air: K1
Ambient TemperatureInsulation
PVCXLPE/EPR
101.221.15
151.171.12
201.121.08
251.061.04
350.940.96
400.870.91
450.790.87
500.710.82
550.610.76
600.50.71
6500.65
7000.58
7500.5
8000.41

Ground Temperature Correction Factor (K2)

Ground Temperature Correction Factor: K2
Ground TemperatureInsulation
PVCXLPE/EPR
101.11.07
151.051.04
200.950.96
250.890.93
350.770.89
400.710.85
450.630.8
500.550.76
550.450.71
6000.65
6500.6
7000.53
7500.46
8000.38

Thermal Resistance Correction Factor (K4) for Soil (When Thermal Resistance of Soil is known)

Soil Thermal Resistivity: 2.5 KM/W
ResistivityK3
11.18
1.51.1
21.05
2.51
30.96

Soil Correction Factor (K4) of Soil (When Thermal Resistance of Soil is not known)

Nature of SoilK3
Very Wet Soil1.21
Wet Soil1.13
Damp Soil1.05
Dry Soil1
Very Dry Soil0.86

Cable Depth Correction Factor (K5)

Laying Depth (Meter)Rating Factor
0.51.1
0.71.05
0.91.01
11
1.20.98
1.50.96

Cable Distance correction Factor (K6)

No of CircuitNilCable diameter0.125m0.25m0.5m
111111
20.750.80.850.90.9
30.650.70.750.80.85
40.60.60.70.750.8
50.550.550.650.70.8
60.50.550.60.70.8

Cable Grouping Factor (No of Tray Factor) (K7)

No of Cable/Tray123468
1111111
20.840.80.780.770.760.75
30.80.760.740.730.720.71
40.780.740.720.710.70.69
50.770.730.70.690.680.67
60.750.710.70.680.680.66
70.740.690.6750.660.660.64
80.730.690.680.670.660.64

According to above detail correction factors:

– Ground temperature correction factor (K2) = 0.89
Soil correction factor (K4) = 1.05
– Cable depth correction factor (K5) = 1.0
– Cable distance correction factor (K6) = 1.0

Total derating factor = k1 · k2 · k3 · K4 · K5 · K6 · K7

– Total derating factor = 0.93


Selection of Cable

For selection of proper cable following conditions should be satisfied:

  1. Cable derating amp should be higher than full load current of load.
  2. Cable voltage drop should be less than defined voltage drop.
  3. No. of cable runs  (Full load current / Cable derating current).
  4. Cable short circuit capacity should be higher than system  short circuit capacity at that point.

Selection of cable – Case #1

Let’s select 3.5 core 70 Sq.mm cable for single run.

  • Current capacity of 70 Sq.mm cable is: 170 Amp,
    Resistance = 0.57 Ω/Km and
    Reactance = 0.077 mho/Km
  • Total derating current of 70 Sq.mm cable = 170 · 0.93 = 159 Amp.
  • Voltage Drop of Cable =
    (1.732 · Current · (RcosǾ + jsinǾ) · Cable length · 100) / (Line voltage · No of run · 1000) =
    (1.732 · 139 · (0.57 · 0.8 + 0.077 · 0.6) · 200 · 100) / (415 · 1 · 1000) = 5.8%

Voltage drop of cable = 5.8%

Here voltage drop for 70 Sq.mm Cable (5.8 %) is higher than define voltage drop (5%) so either select higher size of cable or increase no of cable runs.

If we select 2 runs, than voltage drop is 2.8% which is within limit (5%) but to use 2 runs of cable of 70 Sq.mm cable is not economical, so it’s necessary to use next higher size of cable.


Selection of cable – Case #2

Let’s select 3.5 core 95 Sq.mm cable for single run, short circuit capacity = 8.2 KA.

  • Current capacity of 95 Sq.mm cable is 200 Amp,
    Resistance = 0.41 Ω/Km and
    Reactance = 0.074 mho/Km
  • Total derating current of 70 Sq.mm Cable = 200 · 0.93 = 187 Amp.
  • Voltage drop of cable =
    (1.732 · 139 · (0.41 · 0.8 + 0.074 · 0.6) · 200 · 100) / (415 · 1 · 1000) = 2.2%

To decide 95 Sq.mm cable, cable selection condition should be checked.

  1. Cable derating Amp (187 Amp) is higher than full load current of load (139 Amp) = O.K
  2. Cable voltage Drop (2.2%) is less than defined voltage drop (5%) = O.K
  3. Number of cable runs (1) ≥ (139A / 187A = 0.78) = O.K
  4. Cable short circuit capacity (8.2KA) is higher than system short circuit capacity at that point (6.0KA) = O.K

95 Sq.mm cable satisfied all three condition, so it is advisable to use 3.5 Core 95 Sq.mm cable.

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author-pic

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

191 Comments


  1. SAGEESHRAJ
    Apr 24, 2019

    Dear sir,
    Sir I am working in Bahrain, I am an Electrical supervisor.Sir can you explain to me how to calculate the cable size as per the total load


  2. Kamal
    Apr 20, 2019

    2.2% voltage drop calculated in this article for 95 sq.mm cable is wrong. Please recalculate & correct it. It comes out to be 4.32% & not 2.2%.

    Thanks.


    • NITIN RAJPUT
      Aug 30, 2019

      The voltage drop for 90 sqmm cabe 2.2% is wrong it should be 4.3 %


    • lokeshkumar
      Sep 03, 2019

      you are correct i got 4.3 too


  3. Ramsha Nawal Alvi
    Apr 08, 2019

    Dear Sir,

    Thanks for the beneficial information. I have no electrical background,i am a chemical engineer and have a keen interest to understand about the cable sizing criteria but have needed some leads from your side which would be very helpful for me. Sir regarding your work explained above, i want some clarifications from you on the given below listed points:
    1 On what basis (assumptions) you have let the size of cable ” 3.5 core 70 sq mm” & “3.5 core 95 sq mm”. please explain.
    2 From where you get the value of current capacities of 170 Amp & 200 Amp?
    3 From where you get the value of resistance & reactances?
    4 From where you get the value of theeta ?
    5 From where you get the value of short circuit capacities of 8.2KA & 6.0KA?
    Waiting for your kind response sir.
    Thank you.


  4. hang chetha
    Mar 22, 2019

    Can you tell me why you multiple Resistance and Reactance to 0.8 and 0.6?

    Please


    • Himanshu Sachan
      Jun 25, 2019

      Cos$ is equal to the power factor and value of power factor is 0.8 from here we can get the vale of theta and then we can calculate Sin$ which is equal to 0.6


  5. NL Chauhan
    Mar 19, 2019

    I have one cable of 3 1/2 core .Actual size/rating in sqmm is not known to me.Diameter & perimeter of the cable is known.Please specify how to calculate the size in sqmm. Similarly if the cable is single core than also how to calculate the size if these two parameters are known


  6. Hardeepsingh
    Mar 11, 2019

    Dear Sir,
    In above calculation you have consider 200 mtr cable length. Please inform, it is 200 mtr distance between source terminal and Load terminal or you consider 100 mtr line cable and 100 mtr nutral cable as 200 mtr total length.


  7. Kamal Naser
    Feb 16, 2019

    I need the excel calculation sheets and correction factors
    thanks


  8. kaushal pandey
    Feb 06, 2019

    sir i want your excel sheet then pls send me


  9. Satish Deore
    Feb 05, 2019

    Respect sir. I guide cable size 1200 ft lengthfrom 63 transformer.15 hp 5 stage submersible motor .


  10. Ali Allahverdizadeh
    Jan 15, 2019

    Would you please let me know if there is any de-rating factor which should be applied for the feeder cables with more than one cable in each phase to compensate current unbalance in each individual conductor. Please mention any standard for this.
    I have seen in some cable sizing document that we should apply 1.15 factor for sizing of cables for feeders with more than one cable in each phase, but couldn’t find any reference standard for that.
    Please share your knowledge in this case with me and email it to [email protected].
    Thanks.


  11. PAUL
    Nov 15, 2018

    WHAT IS THE MINIMUM SIZE OF ARMOUR CABLE TO FEED 630AMPS DISTRIBUTION PANEL?


    • NEERAJ YADAV
      Nov 20, 2018

      copper cable of 3core 150sqmm 2runs cable.


    • Naval Tayde
      Jan 29, 2019

      4Cx240mm2 XLPE + 1Cx120 mm2 will be sufficient to feed 630Amp.


  12. rasty
    Oct 21, 2018

    could you attach cable capacity table and resistance/Km it will be so helpful


  13. Md. Samsul Arefin
    Aug 07, 2018

    How can we calculate the Reactance of a 3.5 core 95 Sq.mm cable . Here you have mentioned the reactance 0.074 mho/Km. Can you explain the formula to calculate reactance for any core or size cables ?
    Thank you.


  14. Paresh
    Jul 27, 2018

    I wanted to calculate underground armourered cable size in aluminium, our society/colony is having 3 lanes.

    How do i calculate the cable size depending on Amper rating.

    Current i am calculating:

    Month unit per home – assumed 200 Kwh , in KW 20 , i.e for per day approx 1 KW. total KW for 40 Homes , 40 KW. for single phase voltage , 40000w / 230 = 173.91 amper & for 3 phase voltage 40000W/440 = 90.9 ampere.
    do i need to consider the voltage drop in %ge & add in the ampere rating.

    Please confirm correct calculation to choose proper underground cable, its urgent.


  15. Joseph Gyebuni
    May 12, 2018

    please I need help to tackle a voltage drop issue in my company. We have installed a steam turbine generator of 1MW and voltage of 400V. We use about 40% of the capacity to feed our 30tons/hr palm oil mill and we have extended the power to our residencies which is about 1Km from the generation point. At the distribution point for the residencies we have a voltage of 330-345V. we used 120 sq.mm aluminium. we have also used 3 changeovers in the line. now some of our fridges and metal halides bulbs are not working properly. what can we do to improve the voltage? some suggestion came that we should installed a capacity bank at the residents distribution point and another suggested that we should used transformers to step up at the generation site and step down at the residents distribution point. please I need help urgently. I am an electrical engineer under training. thank you very much for helping.


    • ROY
      May 25, 2019

      Hi Joseph
      I am just an electrician.not a part of EEP..I see that this was a post from a year ago,so I hope you have by now solved your problem..To me your cable,without doing the calculations sounds to be undersized..Copper would probably have been a better choice than aluminium.I don’t think transformers to step up and down would be the answer as I believe the criteria for the cables burying depth and protection would be different for MV than LV and also the cost of the 2 transformers could be prohibitive..presumably your voltage live to neutral is just 190volts …….I think at the generator the voltage could probably be increased to 440 volts without too mush of a cost..which would give you a slight increase at the residence end…..How many residences are supplied by this cable?…….Would it be possible that you could convert to single phase …doubling up on 2 pairs of cables (if there is a distributed neutral),though that might cause quite an imbalance on phases back at the factory…Even a temporary change to confirm the volt drop is acceptable.then you could run in another KM of similar sized cable to double up.


    • S k pattnaik
      May 27, 2020

      High drop in voltage is because of 1km cable length. Lay 240sqmm 2 runs to get with in 5% drop.

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