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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)

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.

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.

About Author //

author-pic

Jignesh Parmar

jiguparmar - Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & Knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

78 Comments


  1. esam
    Sep 25, 2016

    Good morning
    please advise how can I calculate voltage drop for circuit contains three phase and single phase load : example,, main cable 300 mm2 and connected to MDB , from MDB there is sub DB connected with 3phase load 80KW and SUB DB connected with 1phase loads average 3 KW (10 No’s).


  2. Palanikalvarayan
    Sep 07, 2016

    Hello I am working in Oman New job construction electrical working here total 5 floors each pilot 1 st floor 5 rooms apartment total room 25. apartment total watts 350 kw cable size and mdb smdb mcc pdb ldb how many cable size details diagrams ground floors snapping mall please need yours helped so I am waiting for your reply as soon as possible thanking you


  3. ummer
    Aug 09, 2016

    i want to know 220VAC,50Hz motor will work with 220VAC,60 Hz without any problem.


  4. Liyakat ali
    Jul 29, 2016

    Dear sir
    1. How can i detremine Power Factor Pannel.
    Supose our load 500 kva. Than which type of KVAR pannel install.
    2. How Can i check cable size


  5. Brenton B
    Jul 11, 2016

    i want to size a cable for a class project with a voltage drop of 2.5% 220volts 2000amps at 15meter run
    whats the best formula to use?


  6. selvamani
    Jul 05, 2016

    Dear sir,
    How can calculate power cable AMP capacity? Wich is formula use this calculation standard.
    For example 185sqmm 4core cable what is AMP capacity how calculate?
    Please give me feed back in my mail sir.

    Thanks


  7. manas pal
    Jun 29, 2016

    What is the code you have followed can you please mention? Please send me snap of the code.


  8. MANIK CH.BHANDARY
    Jun 08, 2016

    Dear sir,
    I would like to know how can calculate power cable AMP capacity? Wich is formula use this calculation standard.
    For example 50sqmm 4core cable what is AMP capacity how calculate?
    Please give me feed back in my mail sir.

    Thanks


  9. MAHARSHI
    May 06, 2016

    Sir, I have a query about the 4th condition regarding cable short circuit capacity and system short circuit capacity. In case 2 you have taken system short circuit capacity(6.0KA). how to find it?


  10. lusekelo
    Apr 26, 2016

    very helpfull information Thank you


  11. Murali
    Apr 25, 2016

    Very good..informative topic.. basic information for all electrical engineers..


  12. VISHAL
    Jan 19, 2016

    I HAVE A DOUBT IN VOLTAGE DROP OF CABLE.
    1.In that formula cable length is multiplied with 100
    2.why we multiply that?
    3.if we are using it for converting meter in to millimeter means we should use 1000?
    please clear my doubt


    • ANIL GHILDIYAL
      Mar 30, 2016

      It might be % voltage drop. That’s why 100 is put there for %.


    • shyam sharma
      Apr 29, 2016

      this 100 is used for %age VD calculation and not to convert cable length in mm.


  13. sumith ranaweera
    Jan 15, 2016

    Thanks..this topic its very useful for us.


  14. Niraj
    Nov 30, 2015

    Dear Sir,

    I have some doubts related to cable sizing,
    1- Why we do cable sizing calculation on connected load ?
    2- If we use the APFCR can i do the calculation with .95 pf.


  15. marko peter
    Oct 15, 2015

    I have draw the 350 dc current , so what is the size of the cable i can use? give me answer with formula?


  16. Tlou
    Oct 11, 2015

    Dear sir

    Please clarify me 3.5 core. How can i identify it?


  17. inderpal
    Sep 17, 2015

    Dear sir,

    What should be the size of cable used for connection of lightning rods,
    Can you please suggest us with some example.


  18. Vijay
    Sep 17, 2015

    Hi,

    I would like to know how to calculate the cable sizing for UNDERWATER application (submerged in deepwell of 220m) load details are Electrical Induction Motor, 540Amps, 0,85PF, 440V, ThreePhase, Voltage drop 3% ,Water Temperature 56 deg.C, 3 cables tied to steel column pipe of 200m.


  19. NARENDRA MISTRI
    Sep 08, 2015

    Dear sir,
    I would like to know how can calculate power cable AMP capacity? Wich is formula use this calculation standard.
    For example 50sqmm 4core cable what is AMP capacity how calculate?
    Please give me feed back in my mail sir.

    Thanks


  20. parthasarathy chellappan
    Aug 30, 2015

    Sir, i am working in saudi arabia. i want to purchase your electrical notes. i can send the money through the money transfer agencies. i want the sending details


  21. sohail malik
    Aug 17, 2015

    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


  22. Sachin
    Aug 15, 2015

    Excellent , sir


  23. Syed Asif
    Aug 01, 2015

    Dear Sir,

    My Q is why did u use 139 A to calculate Volt drop formula for 70,95sqmm cables??.
    why dint you apply the actual current rating for the cables viz 170,200 A respectively.I mean these are the cables we are selecting so why not use their rating in the formula??
    Pls explain


  24. JAIPAL SINGH
    Jul 17, 2015

    KINDLY CONFIRM HOW TO DO THE HIGH VOLTAGE TESTING OF 33KV CABLE OF 16KM LENGTH.


  25. Waqas
    Jun 14, 2015

    Thats a really good writeup, clears many misconceptions regarding cable sizing. For all those asking for standardized tables for wire gauge and ampere capacity I recommend to search websites of your local cable manufactures, I am in Pakistan so I downloaded the tables from Pakistan cables website.


  26. Mustafa Kaydawala
    Jun 09, 2015

    Dear Sir.

    I would like to know how to size a Transformer.


  27. Aqil
    May 28, 2015

    Sir, can we take the values for resistance and reactance for cable from the specs Manual of the cable to be used?


  28. Eng.mohammad Alamawi
    Apr 21, 2015

    May i ask how did you chose the Amp capacity of the cable in both cases ??


  29. Arnie A. Bañaga
    Apr 14, 2015

    Good Day! Where did you get the tables? Please let me know where do you find this standard tables that you used. Thanks!


    • Sushil Kumar
      Apr 14, 2015

      Dear Sir, I don’t have the tables for resistance and reactance specified for under ground cable of different dia. So please attach the table also.


  30. kishore chandra sahu
    Apr 04, 2015

    Dear sir,
    please give the electrical motor cable calculation details .
    please send me my mail id.

    Thank you


  31. Murugeshwari
    Apr 04, 2015

    Dear Sir,

    I thoroughly understand the above expalined calculation. But i have one doubt ( where u took resistance , reactance & current carrying capacity Of cable ) if it has any tabulation pls send it to my email ID.


  32. Nabeel Ahmad
    Apr 03, 2015

    Sir I’m Nabeel ahmad I want to ask how did u get capacity of cable like 70sq.mm is 170A &resistance of cable . Tell me also about derating factor.
    Email me.nabeelahmad912@yahoo.com


  33. Deepak kr. Mishra
    Mar 30, 2015

    Very much interesting & useful


  34. Emile
    Mar 30, 2015

    With regards to the statement:
    “If we select 2 runs, than voltage drop is 2.8% ….”

    How did you derive at 2.8%
    Could you provide the formula , step for step calculation used


  35. saad
    Mar 22, 2015

    thank you


  36. Mahesh
    Feb 23, 2015

    Dear Jignesh,
    I am mechanical engineer from VJTI, Mumbai; I want to install 100 cameras at 100 mtrs distance on 6mtrs high poles around the agriculture land. Each camera will be operating at 12 volts and carrying 1 amps; please can you guide me about the cable size to supply power to all 100 poles. The circumference of the field is about 10 kms.


    • Kristian Malihan
      Feb 26, 2015

      HI Jignesh,

      May i ask what type of camera is it IP or Conventional Camera(coaxial)?
      is your are you using AC 12V or DC 12V?

      I think i could help you with this just provide me some information. you could email me also.

      Kristian Malihan


  37. Samira
    Feb 20, 2015

    Hi Jignesh,
    This is Samira, Electrical engineer. I read your comments on how to calculate cable size. Thank you.
    I have a couple of questions for you; can you please help with the answers?
    • Can you please explain voltage drop calculation in single-phase system as well?
    • What is the source of these tables? I need to have a handy copy of the reference please for my projects. Some of my cables are in the air in an indoor warehouse.
    • Can I ask how did you get current capacity of 70 mm cable? Also I have same question about its resistance & reactance.
    Please email me on ‘samira.namin@gmail.com’.
    Thank you,
    Samira


    • Samira
      Feb 26, 2015

      I got my answers via investigation on net & your webiste, thank you


      • sri venkatesh
        Mar 10, 2015

        Can I ask how did you get current capacity of 70 mm cable? Also I have same question about its resistance, reactance, short circuit capacity of the cable.
        please give me answer to this mail id- venkatesh5476@gmail.com


  38. prathap
    Feb 06, 2015

    how to calculate derating facter please mail me


  39. singaraj
    Dec 22, 2014

    these grouping factor ,cable correction factor & distance factor you shown the table here
    how do you prepared (0.6,0.8,1,1.2)like this , what datas you have to prepare these things
    can you post.
    give me a reply.


  40. MOhan
    Nov 11, 2014

    Thank you for the information


  41. D K Nagpal
    Nov 05, 2014

    Dear Sir,

    I Need sofftwre for cable size calucations. Please advise.


  42. saravanan
    Oct 25, 2014

    Sir,
    Thanks a lot for sharing your experience ……….I want you to share your experience on factory electrical cable selection , DSL bus bar selection , cable size , current carrying capacity of cable , mcb selection


  43. Engr Muhammad Tanveer
    Oct 22, 2014

    Sir,
    Thanks a lot for sharing your experience ……….I want you to share your experience on building electrification of high rise buildings and what are the factors that as electrical engineer you should be considered.


  44. Kashif
    Oct 18, 2014

    Kindly Provide your email


  45. Kashif
    Oct 18, 2014

    Dear Sir, I would like to be the Member of Institute of Engineers India. Kindly I need your reference for Membership


  46. vikash kumar singh
    Oct 09, 2014

    Sir,
    Thanks a lot for sharing ……….I have a doubt about how to calculate the fault level of Ht cable ……..plz reply


  47. Ateeq
    Sep 21, 2014

    Dear Sir , kindly Clear my Doubt for Applying Correction Factor to the size of the cable Why the Full load current is reducing when we apply correction factor .


  48. Marikkani
    Sep 18, 2014

    Dear Jignesh,

    Thanks for your contribution and its sure an good material for every persons.And let me request to please understand that the how the selection of current capacity vs cable size happens
    (3.5core 75 sqmm for 170A & 3.5 core 95 sqmm for 200A ) .let me request to please share 1 core .5sqmm to maximum cable 4 core size selection chart( as per Indian & International standards) in excel by which I shall go for my use.


  49. satheesh
    Aug 17, 2014

    Let me know ,in CASE 1 calculation how did you know the cos(pi) and sine(pi) value as 0.8&0.6.

    Any one plz clarify my doubt.


    • abhishek
      Nov 19, 2014

      you have power factor .8 , so cos(pi) will be .8 , then calculate value of angle(36.8698)
      By this again calculate value of sin 36.8698, which is 0.6


  50. turbold
    Jul 29, 2014

    Thank you for your information..From mongolia


  51. iman
    Jul 27, 2014

    Thanks, but I think this site is very basic and simple calculations are
    Thanks again for taking the time


  52. Ajay John
    Jul 27, 2014

    I wants to how this short circuit capacity of 8.2 and 6 kA are got? Regards


  53. kiranreddyk
    Jul 23, 2014

    Good article


  54. Umesh Charola
    Jul 16, 2014

    There is a small typo mistake.. Instead of 139, It should be 159 A.

    (1.732 · 139 · (0.57 · 0.8 + 0.077 · 0.6) · 200 · 100) / (415 · 1 · 1000) = 5.8%


  55. Melvin
    Jul 16, 2014

    sharing your experience with debuting engineers like me will be of great help. I hope to remain in contact with your scientific touches.


  56. Javed Akhter
    Jul 13, 2014

    Thanks Mr. Parmar for sharing the knowledge.


  57. gnanavel
    Jul 10, 2014

    I would be thankful to you for giving the best things in electrical and I appreciated for the same.


  58. N R VENKATESAN
    Jul 06, 2014

    i want to say thanks to Mr. Jignesh Parmar bcause i gained lot of practical knowledge bcoz his publication in EEP website…. thaks and thanks lot sir,………


  59. Ahmad Monajemi
    Jul 05, 2014

    W/O looking at the calculation details, it looks like the VD calculated in using the load PF. Power Factor is primarily determined by load whereas line VD must be using the line conductors impedance.


  60. piyush
    Jul 03, 2014

    good


  61. Czarjeff
    Jul 02, 2014

    Hi,

    Great article, same calculation is used here in oil & gas industry.

    Btw, just for your correction and so as not to confuse the readers. Please correct the typo error on this paragraph. The 70 Sq.mm in the second bullet should be 95 Sq.mm.

    “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%


    • hari
      Jan 05, 2016

      Dear sir,
      I would like to know how can calculate power cable AMP capacity? Wich is formula use this calculation standard.
      For example 300sqmm 4core cable what is AMP capacity how calculate?
      Please give me feed back in my mail sir.

      Thanks


  62. Sadiqkhan
    Jul 02, 2014

    Very nice to see the data.
    GREAT GOD will help you for better work for this n coming generation. Do some thing for India, in where the power supply company fault may come out with knowledge to them where they lost the electric current when supplying. Specially in village.


  63. Muhammad Junaid
    Jul 02, 2014

    can we neglect thermal correction factor (K4)???


  64. mohamed gamal
    Jul 01, 2014

    it is very inmportante data


  65. Michael
    Jul 01, 2014

    Thanks for this priceless Technical information. I want you to shed more light on the relationship between, design current, total derating factor, derating current and cable current carrying capacity. According to IEEE & BS 7671 (2011), derating current = design current Ib / total derating fator Kt. So, in the above calculation, I expect derating current to be = 139/0.93 9 (i.e 149.5A) .


  66. Irfan Ansari
    Jul 01, 2014

    its economical to use 3.5 core instead of 4 core. Because, in balance 3 phase network the current in neutral conductor is zero.


  67. OKK
    Jun 30, 2014

    Amazing that you can let this slip through without any proofreading!


  68. felix sandoval
    Jun 30, 2014

    Congrat, great information and issues. Very helpful.
    Thanks


  69. Muhammad Junaid
    Jun 30, 2014

    Why we use 3.5 core cable,instead of 4 core cable?

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