An example how to calculate voltage drop and size of electrical cable

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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 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 Temperature Insulation PVC XLPE/EPR 10 1.22 1.15 15 1.17 1.12 20 1.12 1.08 25 1.06 1.04 35 0.94 0.96 40 0.87 0.91 45 0.79 0.87 50 0.71 0.82 55 0.61 0.76 60 0.5 0.71 65 0 0.65 70 0 0.58 75 0 0.5 80 0 0.41

Ground Temperature Correction Factor (K2)

 Ground Temperature Correction Factor: K2 Ground Temperature Insulation PVC XLPE/EPR 10 1.1 1.07 15 1.05 1.04 20 0.95 0.96 25 0.89 0.93 35 0.77 0.89 40 0.71 0.85 45 0.63 0.8 50 0.55 0.76 55 0.45 0.71 60 0 0.65 65 0 0.6 70 0 0.53 75 0 0.46 80 0 0.38

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

 Soil Thermal Resistivity: 2.5 KM/W Resistivity K3 1 1.18 1.5 1.1 2 1.05 2.5 1 3 0.96

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

 Nature of Soil K3 Very Wet Soil 1.21 Wet Soil 1.13 Damp Soil 1.05 Dry Soil 1 Very Dry Soil 0.86

Cable Depth Correction Factor (K5)

 Laying Depth (Meter) Rating Factor 0.5 1.1 0.7 1.05 0.9 1.01 1 1 1.2 0.98 1.5 0.96

Cable Distance correction Factor (K6)

 No of Circuit Nil Cable diameter 0.125m 0.25m 0.5m 1 1 1 1 1 1 2 0.75 0.8 0.85 0.9 0.9 3 0.65 0.7 0.75 0.8 0.85 4 0.6 0.6 0.7 0.75 0.8 5 0.55 0.55 0.65 0.7 0.8 6 0.5 0.55 0.6 0.7 0.8

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

 No of Cable/Tray 1 2 3 4 6 8 1 1 1 1 1 1 1 2 0.84 0.8 0.78 0.77 0.76 0.75 3 0.8 0.76 0.74 0.73 0.72 0.71 4 0.78 0.74 0.72 0.71 0.7 0.69 5 0.77 0.73 0.7 0.69 0.68 0.67 6 0.75 0.71 0.7 0.68 0.68 0.66 7 0.74 0.69 0.675 0.66 0.66 0.64 8 0.73 0.69 0.68 0.67 0.66 0.64

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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Jignesh Parmar

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.

1. 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.

2. Nkosinathi
May 08, 2018

Please help I have an assignment same as this format but given a single phase , please help me I need to find a diameter of the cable so I can find d area…to find voltage drop in the cable …and I’m stuck how can I choose ?
“You have been hired by the UMLAZI Construction Company to design, Electrical load
of 80KW, distance between source and load is 200 meters, system voltage 230V single
phase, power factor is 0.8, permissible voltage drop is 10% according to South African
Bureau of Standards (SABS) under SANS 10142 wiring code and demand factor is 1. 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. The
company will be choosing one of the model designs from this class and it is your job to sell
them your best design. You are expected to work in the group of TEN on your design, and
follow a strict timeline. Do the selection of proper cable following Specifications:

3. Hozan
Mar 27, 2018

Good explaintion, but is it as per IEC, if so please give us the source of these tables, K1,2,3,4,5,6

Feb 15, 2018

Dear Sir,
you mention system short circuit current capacity is 6KA. Please tell me how to find the system short circuit current capacity.

Regards

5. s.m shahiduzzaman
Jan 08, 2017

sir.I want to calculate the amount of ampere on the respect of cables dia meter or rm or mm.if you please help me.

6. santhakumar
Jan 04, 2017

Sir,
I appreciate your service with my whole heart. I am also an Electrical Engineer. I have one small doubt on this regarding the condition for deciding the cable size,

the point number one is correct, accepted.
but the point number three is not correct i believe. because, once we accept the first point and third point, then, the number of cable run will be always one. there is no way to get more than a run at any circumstance. but i have seen many projects and I myself installed more than a run, esp. transformer to LV panels there will be min 2 or more runs of single core, and many other areas.

so, I request you to reconfirm it and edit if I am right.

thank you,

thank you,
santhakumar
+96897882004
santhakumar.ind@gmail.com

7. Chris Wilken
Nov 06, 2016

Good morning

I would like to know how to accurately calculate cable diameter for DC systems.

Regards

Chris

8. 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).

• engr gd warraich
Nov 09, 2017

first of all calculate distance i think under 100feet no voltage drop ocuure
if ur distance more then 100 feet then divivde it to dividions

calculate total amount of current with three phase formula and for single phase it is nessory take in account for phase balancing

9. 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

10. ummer
Aug 09, 2016

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

11. 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

12. 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?

13. 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

14. manas pal
Jun 29, 2016

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

15. 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

16. 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?

17. lusekelo
Apr 26, 2016

18. Murali
Apr 25, 2016

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

19. 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?

• 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.

• Teekay
Aug 29, 2017

Also, the divsion by 1000 is for converting the impedance of the cable / km to a value corresponding to 200m of cable length.

How did you find the short ckt capacity of the cable at the load point as 6.0kA?
Could you show the steps to it please?

20. sumith ranaweera
Jan 15, 2016

Thanks..this topic its very useful for us.

21. 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.

22. 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?

23. Tlou
Oct 11, 2015

Dear sir

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

24. 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.

25. 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.

26. 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

27. 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

28. 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

29. Sachin
Aug 15, 2015

Excellent , sir

30. 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

31. JAIPAL SINGH
Jul 17, 2015

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

32. 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.

33. Mustafa Kaydawala
Jun 09, 2015

Dear Sir.

I would like to know how to size a Transformer.

34. 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?

Apr 21, 2015

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

36. 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.

37. kishore chandra sahu
Apr 04, 2015

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

Thank you

38. 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.

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.

40. Deepak kr. Mishra
Mar 30, 2015

Very much interesting & useful

41. 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

Mar 22, 2015

thank you

43. 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

44. Samira
Feb 20, 2015

Hi Jignesh,
This is Samira, Electrical engineer. I read your comments on how to calculate cable size. Thank you.
• 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.
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.

45. prathap
Feb 06, 2015

how to calculate derating facter please mail me

46. 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.

47. MOhan
Nov 11, 2014

Thank you for the information

48. D K Nagpal
Nov 05, 2014

Dear Sir,

49. 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

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.

51. Kashif
Oct 18, 2014

52. 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

53. 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

54. 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 .

55. 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.

56. 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

57. turbold
Jul 29, 2014

Thank you for your information..From mongolia

58. iman
Jul 27, 2014

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

59. Ajay John
Jul 27, 2014

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

60. kiranreddyk
Jul 23, 2014

Good article

61. 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%

62. 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.

63. Javed Akhter
Jul 13, 2014

Thanks Mr. Parmar for sharing the knowledge.

64. gnanavel
Jul 10, 2014

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

65. 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,………

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.

67. piyush
Jul 03, 2014

good

68. 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

• Ameer Bakhsh
Mar 28, 2018

Dear sir I want to know about the formula you have used to calculate the voltage drop. My question is that how have you calculated resistance and reactance (or conductance mho/km) 0.57ohm/km and 0.077 mho/km respectively? What values have you taken for calculation? kindly tell me I’m very confused and also how have you calculated the value of short circuit capacity which is 8.2KA you have not mentioned the formula and calculation. kindly reply thank you
Best regards;
Ameer Bakhsh

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.

Jul 02, 2014

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

71. mohamed gamal
Jul 01, 2014

it is very inmportante data

72. 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) .

73. 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.

74. OKK
Jun 30, 2014

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

75. felix sandoval
Jun 30, 2014

Congrat, great information and issues. Very helpful.
Thanks