Calculate Cable Size and Voltage Drop
February 8, 2014 59 Comments
Calculate Cable Size and Voltage Drop:
Calculate Voltage Drop and Size of Electrical cable for following data.
 Electrical Details: Electrical Load of 80KW, Distance between Source and Load is 200 Meter, 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. No of Cable per Trench is 1. No of Run of Cable is 1 Run.
 Soil Details: Thermal Resistivity of Soil is not known. Nature of Soil is Damp Soil.
Calculation:
 Consumed Load= Total Load x Demand Factor
 Consumed Load in KW= 80 x 1 =80KW
 Consumed Load in KVA= KW/P.F
 Consumed Load in KVA =80/0.8=100KVA
 Full Load Current= (KVAx1000) / (1.732xVoltage)
 Full Load Current= (100×1000) / (1.732×415) = 139Amp.
 Calculating Correction Factor of Cable from following data :
 Temperature Correction Factor (K1) When Cable is in Air is
Temperature Correction Factor in Air :K1 

Ambient Temp© 
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 Temp© 
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):
Ther.Resi Correction Factor: K4 

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):
Soil Correction Factor:K4 

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):
Cable Depth 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):
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 
(Cable Grouping factor K7 )==No of 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 are
 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 De rating Factor= k1x k2 x k3 x K4 x K5 x K6 x K7
 Total De rating Factor= 0.93
Selection of Cable:
 For selection of Proper Cable following Conditions should be satisfied
 (1) Cable De rating Amp should be higher than Full Load Current of Load.
 (2) Cable Voltage Drop should be less than Defined Voltage drop.
 (3) No of Run of Cable >= (Full Load current / Cable De rating Current).
 (4) Cable Short Circuit Capacity should be higher than System S.C Capacity at that Point.
Selection of Cable Case (1):
 Let’s Select 3.5Core 70 Sq.mm cable for Single run.
 Current Capacity of 70 Sq.mm cable is 170Amp,Resistance=0.57Ω/Km and Reactance=0.077 mho/Km
 Total De rating Current of 70 Sq.mm Cable= 170×0.93 =159 Amp.
 Voltage Drop of Cable= (1.732x Current x (RcosǾ+jsinǾ) x Cable Lengthx100) / (Line Voltage x No of Runx1000)
 Voltage Drop of Cable= (1.732x139x(0.57×0.8+0.077×0.6)x200x100)/(415x1x1000)=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 No’s of Run than Voltage drop is 2.8% which is within limit (5%) but to use 2 no’s of Run of cable of 70 Sq.mm Cable is not economical so It is necessary to use next higher size of Cable.
Selection of Cable Case (2):
 Let’s Select 3.5Core 95 Sq.mm cable for Single run, S.C Capacity =8.2KA.
 Current Capacity of 95 Sq.mm cable is 200Amp,Resistance=0.41Ω/Km and Reactance=0.074 mho/Km
 Total De rating Current of 70 Sq.mm Cable= 200×0.93 =187 Amp.
 Voltage Drop of Cable= (1.732x139x(0.41×0.8+0.074×0.6)x200x100)/(415x1x1000)=2.2%
 Voltage Drop of Cable=2.2%
 To decide 95Sq.mm Cable, Cable selection condition should be checked.
 (1) Cable De rating 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)No of Run of Cable (1) >= (139A / 187A =0.78) =O.K
 (4) Cable S.C Capacity (8.2KA) is higher than System S.C 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
dear sir,
thanks for all hard work u took for excellent work which u have undertook in educating and making perfect electrical engineer like us. Dear sir i wanna purchase electrical notes in pdf form kindly let me know how can i get it .
thanks and regards.
You can buy at any time through ONLINE either in Indian Rupees or US$. Go to “Electrical Notes” Page
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Dear Sir,
How you define cosǾ & sinǾ value
Thanks to u & all the best
thanks for all your hard work, sir if you mention with references of electrical codes that could be more useful.
thanks,
I always indicate Reference Code Where it is available or Applicable. Reset are from manuals or Books.
thank you sir for this good topic
EEE CPTC CHENNAI INDIA
LIKE
CPT ElectricalEngineers LIKE ALL EEP OUT PUTS
Thanks for such a detailed description of cable selection guide. I was looking for such article from long time. Each and every thing is explained properly and with examples.
Dear *Jignesh* Sir,
Thanks a lot to share your knowledge with us.
I need busbar calculation formula for control panels applications. I have tried more to find out exact formula for it but i couldn’t get it, So Kindly help to get me out from this problem.
*How can I calculate busbar size on behalf of Fault Current in Control Panels???*
Kindly Expedite
*Regards,*
*VIVEKANAND JHA (**Design & Costing Engg.)*
Dear sir,
i am azeez Electrical Engineer from chennai tamilnadu. now i am working as a Electrical Site Engineer in construction field in Saudi Arabia sir help me to survive in this position i need to know the calculation voltage drop and current rating and short circuit current.
Thank you sir.
Dear Jignesh , Thanks a lot …. Thanks for sharing valuable information in detail…
Thanks Mr jignesh for your good thinking of sharing the knowledge!, and it is very useful for technicians like me! All the information and technical things has to be transparent and to be made easy to understand, so that the beginners and technicians can enter easily into the subject to explore new things and to make better & safe the existing! This can be done only by the experts like you , which will make our country advance in all fields to become self sufficient !
I once again thank you for all your sincere efforts!
Rgds,
A.S.Y.RAJA..
Hi..
Thank you very much Jignesh sir for your all hard work and perseverance that you have put to make all electrical engineers knowledgeable and aware about the practical Electrical enegry or power sector…
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IS STANDARDS will be more helpful and fruitful to the engineers like us to be a real Electrical engineers..
Thanking you in anticipation and for your valuable knowledge..
It is totally illegal to Put/ Upload/Share IS standard on net. we have to purchase IS code for study.
that is the reason, I have upload some abstract of IS code.not IS code itself.
thank you very much for your great support,
Thanks for the handy reference.
Thanks a lot sir for your efforts to put us through. But please in this calculation of: Full Load Current= (KVAx1000) / (1.732xVoltage). Please can you explain this (1.732) value stands for.
Sir.. its really nice blog to learn…
Hello sir,
Myself M.a.Hussain, am an electrical engineer graduate i have a doubt in selection of cable section where just wrote current carrying capacity for the 70 sq mm cable as 170 amps without any formula or derication. i like to know how do you calculate cable current carrying of a cable
Thank You
Regards
M.a.Hussain
Dear Jignesh sir,
Can you pls explain how 100 in the nominator( next to cable length) & 1000 in the denominator (next to no of runs ) arrives in the formula.
(1.732x Current x (RcosǾ+jsinǾ) x Cable Lengthx100) / (Line Voltage x No of Runx1000).
I SEARCH LOT OF PAGES FOR CABLE SELECTION. UR PAGE IS VERY VERY HIGHLY SUPERB. I PROUD ABOUT UR KNOWLEDGE SIR .THANK U
Sir please give the correct formula how to calculate the size of the cable
sir how can we calculate inductance of power cable?
Sir how can we calculate reactance of cable.
Sir,
I am very thankfull to you in knowing the cable size calculation from the site and i want to bring to your notice regarding a typing mistake.
In case study to it was written as “Total De rating Current of 70 Sq.mm Cable= 200×0.93 =187 Amp”. as we are using 95sqmm cable it should be “Total De rating current of 95 Sq.mm cable=200*0.93=187 Amp”.
Dear sir,
I want to know about how to decide or calculate Electrical equipment capacity.Please provide basic formula for the same.
Haw to measure 521 cable size
hj
how to measure HT and LT cable depend upon load voltage drop in HT and LT cable
is same
thank u sir for a great article, but it is very difficult to calculate the resistance and reactance of the cable…….sir plz add the formulas for how to calculate the reistance and reactance of the cable
Sir, i will be very grateful if you please let me know from where do i collect this system S.C. capacity . and there is another thing i would like to point out, this cable S.C. capacity which you have given as 8.2 KA, is this correct?
Total curent rating 500A lenght ofcable 100mtr 400v wat size of cable using
sir can you explain what is 0.125m,0.25m ,0.5m in cable distance factor
Can you check this calculation did in cable selection 2… it seems to be wrong..
Voltage Drop of Cable= (1.732x139x(0.41×0.8+0.074×0.6)x200x100)/(415x1x1000)=2.2%
for me it’s coming 4.3%
Dear Sir can we calculate voltage drop for Flexible wires
sir, your articles are superb,If i was in touch with you physically, we would have discuss more, I appreciate your work. I will suggest why put them in hard copy people will buy them in a hurry.
please can you send me problems associated with 11kv feeder, and ways of proffering solutions. It is a project i am working on.
Can you tell me which cable is use in your calculation ?? copper or aluminium ?? And how you calculate the current capacity of those cables ?? Kindly reply me with your formula/equation of those cable’s phase and ground
it’s excellent.
Dear Sir, i have two Que.
1) I have 3.5 core 35sqmm Alu. armed Cable. so pls ask me what is my per phase Amp. capacity and what is total capacity of 3.5 core 35 sq mm alu armed cable.
2) I have 63 amp 4 pole MCB so what is my per phase current capacity. and total capacity of mcb
Dear, Mr.Jignesh Parmar
As mentioned, Current Capacity of 70 Sq.mm cable is 170Amp,Resistance=0.57Ω/Km and Reactance=0.077 mho/Km
how to calculate the resistance and reactance for the cable ?
if there is a chart, kindly post it…
Dear sir
Please let me know about allowable voltage drop between PCC panel to MCC panel?
(Cable S.C Capacity (8.2KA) is higher than System S.C Capacity at that Point (6.0KA) =O.K)
My question is how to calculate Short circuit capacity at that point ? as short circuit capacity of the cable is taken from the table
Plz Sir clear my concept about my sent issues. which are sent to your email by me
how to cable calculate in panel
520 kw cu cable suitability sq mm of cable
all the cable cos and sine value is same pls ans me sir,
you are really Great Sir
Dear Sir…thanQ very much for the valuable info about cable designing.Kindly let me know the correction factor for cables to be laid inside PVC ducts embedded in concrete.
Thanks
is it correct if we derive voltage drop as V = I x Z, where Z = (sqrt(sumsq(r2,x2))?, in above calculation you have calculated Z = R cos phi + j sin phi, i think here Z is a complex number with some angle to X axis, say Z = R*0.8 + X*0.6, this gives me a magnitude of Z at 36 degree to X axis. please help on this issue whether which is correct calculation of Z to arrive at actual voltage drop i.e. V = I*Z?
My load is 4836a three phase tell me standrd size of cable for each phase length is 100feet
Sir ,
nice explanation.It helped me a lot.
Dear Jignesh Sir,
How can we select the size of flexible wire for power & lighting loads?
Hi, I’m trying to ascertain the size of 3 core wired armoured cable I will need to keep voltage drop to a minimum level to ensure an existing 100am 240V 18KVa supply is still maintain at that level 300m away from the supplies supply….online calculators suggest I might need a 125mm/6 inch cable? Kind regards Mr Hibbert
Electrical Load of 2.34KW, Distance between Source and Load is 1000 Meter, System Voltage 415V Three Phase, Power Factor is 0.8,Permissible Voltage drop is unknown, Demand Factor is 1. What would be the cable size in laying under the soil with ambient temperature.
Sir can you share the voltage drop formula for single phase? how we can derived it??