This presentation is a brief introduction of cable sizing

1. CABLE SIZING CALCULATIONS
BY
SIDDHARTHA MUDGAL
ELECTRICAL ENGINEERING DEPARTMENT
MEERUT INSTITUTE OF ENGINEERING AND TECHNOLOGY,
MEERUT,UTTAR PRADESH
Dr. APJ ABDUL KALAM TECHNICAL UNIVERSITY(AKTU)
(Formerly known as UPTU)

2. What is Cable Sizing ?
The determination of conductor's parameters to get efficient, safe and economic
operation of transmission.
Importance of Cable Sizing
The proper sizing of an electrical cable(Which is bearing load) is important to
ensure that the cable can:
• Operate without any interruptions.
• Withstand the full load currents.
• Bear the voltage as well as transients .
• Minimize the initial capital investment.
• Provide effective and efficient transmission of electrical energy.

3. Major Solution providers for conductors in INDIA:-
• Havells India Ltd.
• Polycab Wires Pvt. Ltd.
• Sterlite Technologies Ltd.
• Finolex Cables Ltd.
• Diamond Power Infrastructure Ltd.

4. Techniqes of Cable Sizing:-
• Cable sizing methods do differ across international standards (e.g. IEC, NEC,
BS, etc) and some standards emphasise certain things over others. However
the general principles underlying any cable sizing calculation do not change.
As per IEC As per NEC

5. Common STEPS:-
• STEP 1:- Gathering the data about the cable, it's installation conditions and
the load which it needs to be supply.
• STEP 2:- Determine the minimum cable size based on the following:-
i)Current Carrying Capacity.
ii)Voltage Drop.
iii)Short circuit Temprature rise.
iv)Earth fault loop impedance.
• STEP 3:- Selection of the cable which have highest sizing based on the
above parameters

6. What we look for in STEP-1 ?
A)LOAD:-
• Type of load which is to be supplied.
• Maximum current required by the load.
• Voltage capacity of the system.
• Expected power factor of the system.
• Distance of load from the source.
B)CONTRUCTION OF CABLE:-
• CONDUCTOR MATERIAL:- Normally copper or aluminium.
• CONDCTOR TYPE:- Stranded or solid.
• CONDCUTOR SURFACE COATING:- Plain (no coating), tinned, silver or Ni.
• INSULATION TYPE:- PVC, XLPE, EPR.
• TYPE OF CORE:- Single core or multicore.

7. c)INSTALLATION CONDITIONS:-
• Installation medium:-
i)eg. Underground or overhead, soil thermal resistivity for underground cables,
etc.
• Arrangement:-
i)For underground cables whether it is placed directly or in a conduit, for
overhead wires whether it is installed supported by the walls, on the cable
towers etc.
ii)Depth of laying
iii)Bunching of cables, spacing b/w them.
• Atmospheric Temprature:-
i)Ambient of soil temprature.
ii)Soil thermal resistivity, in the case of undergrond cables.

8. What we look for in STEP-2 ?
A)Current carrying capacity:-
The current plays a vital role while deciding the sizing for a cable due to
following factors:-
i)Copper lossess.
ii)Thermal capacity of cable
iii)Reactive power.
iv)Insulation,bedding,armour and other components of cable
Thus depending upon these parameters we find out the minimum possible and
feasible size for the cable.

9. i)Base current ratings:-
Cable manufactuers maintains a table for base current rating for different type
of cables. These ratings are taken out in base environmental conditions and ths
these ratings are relevant for base installation conditions.
ii)Installed Current Rating:-
Whenever there is difference b/w the base conditions and actual instllation
conditions, then the derating factor is used in order to find out the actual
installed crrent ratings.

10. i) In ground Installation (Direct Burial)
C =Cs * Cg * Cd *Ct
Where, Cs = Rating factor for variation in ground temperature
Cg= Rating factor for Group of cables
Cd= Rating factor for Depth of laying
Ct=Rating for thermal resistivity
ii) Above Ground Installation
C = Ca * Cg
Where, Ca= Rating factor for variation in ambient air temperature
Cg= Rating factor for Grouping cables
Then installation current rating is obtained as:-
I(new) = I(base) * C

11. B)VOLTAGE DROP:-
Current flow through the cable – the higher the
current flow, the higher the voltage drop.
Impedance of the conductor – the larger the
impedance, the higher the voltage drop.
As, For a three phase system:-
For a single phase system:-
For a DC system:-
1000
)sincos(3
3
LXRI
V cc 



1000
)sincos(2
1
LXRI
V cc 



1000
2 LIR
V c
dc 

12. C)SHORT CIRCUIT TEMPRATRUE RISE:-
During a short circuit, a high amount of current
can flow through a cable for a short time. This
surge in current flow causes a temperature
rise within the cable.
k
ti
A
2

k
ti
A
2

Minimum Cable Size Due to
Short Circuit Temperature
Rise:-
The temperature rise constant is calculated
based on the material properties of the
conductor and the initial and final conductor
temperatures









i
if
k


5.234
1ln226









i
if
k


228
1ln148
(for copper conductors)
(for aluminum conductors)

13. D)EARTH FAULT LOOP
IMPEDANCE:-
EARTH FAULT LOOP IMPEDANCE
Zs = Zc + Ze
Where Zs is the earth fault loop
impedance (Ω)
Zc is the impedance of the active
conductor (Ω)
Ze is the impedance of the earth
conductor (Ω)
    



 
22
1000
ececs XXRR
L
Z

14. Implementation examples of Cable Selection process
EXAMPLE 1
EXAMPLE 2
 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 directly 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

15. Risk and Impact of Wrong Sizing
• High voltage drops, which increases reactive power.
• Sudden increase in cable temprature.
• Damage of cable components, such as armour, bed sheath, insulation etc.
• Damage of protective components.
• Poor power factor.
• Non continuous supply.
• Shorter Life span of cables.
• Increase in maintainance cost of the system.

16. Conclusion
Efficient, safe and economic operation of transmission can be obtained by
proper sizing and selection of cables
References
Wikipedia,
https://wiki.openelectrical.org/index.php?title=Cable_Sizing_Calculation
Electrical Engineering Portal,
https://electrical-engineering-portal.com/an-example-how-to-calculate-voltage-drop-and-size-of-electrical-
cable
My Electrical Engineering
https://myelectrical.com
Elements Of Power System,by U.A.BAKSHI

17. Thank You