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Cable Sizing Calculations Steps

The document outlines the steps required for cable sizing calculations, including factors such as soil thermal resistivity, allowable voltage drops, and cable length. It details the process for both low voltage (LV) and high voltage/medium voltage (HV/MV) cable sizing, emphasizing the importance of continuous current carrying capacity and derating factors based on installation conditions. Additionally, it provides formulas for calculating voltage drop and conductor cross-section to ensure safety and efficiency in electrical installations.

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Sabuj Nath
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32 views1 page

Cable Sizing Calculations Steps

The document outlines the steps required for cable sizing calculations, including factors such as soil thermal resistivity, allowable voltage drops, and cable length. It details the process for both low voltage (LV) and high voltage/medium voltage (HV/MV) cable sizing, emphasizing the importance of continuous current carrying capacity and derating factors based on installation conditions. Additionally, it provides formulas for calculating voltage drop and conductor cross-section to ensure safety and efficiency in electrical installations.

Uploaded by

Sabuj Nath
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Input required for cable sizing

STEPS TO BE FOLLOWED FOR CABLE calculations:


SIZING CALCULATIONS. 1. Soil thermal resistivity
2. Short circuit study report
3. Allowable voltage drops
4. Cable length
5. Site conditions (Ambient
temperature)
6. Cable installation details
LV CABLE SIZING HV/MV CABLE SIZING

1. As per continuous current carrying capacity 1. As per continuous current carrying capacity

2. As per allowable voltage drop 2. As per allowable voltage drop

3. Checking of SCPD operation for end of cable fault 3. Checking of conductor cross-section
to withstand the fault current

Step 1: As per continuous current carrying capacity

No.of runs required= (Load current)


((Cable current carrying capacity) x (Total derating factor))

a) calculate the load current


b) find the current carrying capacity of selected cable
c) calculate the de-rating factor of cable as per installation type
d) calculate no of runs as per above formula

Step to calculate the derating factor of cable

i) Cable running above ground in cable tray


a) derating factor as per ambient air temperature factor
b) derating factor as per grouping factor of cable as per no. of cable and no. of trays running together

Derating factor above ground= a x b

ii) Cable running below ground in cable trench


a) derating factor as per ground ambient air temperature factor
b) derating factor as per cable grouping factor
c) derating factor as per cable depth of laying factor
d) derating factor as per soil thermal resistivity

Derating factor below ground = a x b x c x d

Step 2: As per allowable voltage drop:

check the voltage drop as per design basis / standard and ensure that running and starting voltage drop is less than
allowable voltage drop:

Voltage Drop= 3× I x L x (( R Ccos ) + (X sin ))


N×1000

Where,
I =Full load current (Amp)
cosΦ =power factor
R =Cable resistance (Ohm/km)
X =Cable reactance (Ohm/km)
L = Cable length (metre)
N = No. of cable runs

Step 3: Checking of conductor cross-section to withstand the fault current


Calculate the conductor cross-section to withstand the fault current as per below formula

S= (I t)
k
Where,
S = Cross-sectional area, in mm2
I = Prospective fault current for a fault of negligible impedance in Ampere
t = Operating time of protective device for automatic disconnection, in second
k = Factor dependent on the material of the conductor

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