Electrical example 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.
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 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
0.65
70
0.58
75
0.5
80
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
0.65
65
0.6
70
0.53
75
0.46
80
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
1.18
1.5
1.1
1.05
2.5
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
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
1.2
0.98
1.5
0.96
Cable Distance correction Factor (K6)
No of Circuit
Nil
Cable diameter
0.125m
0.25m
0.5m
0.75
0.8
0.85
0.9
0.9
0.65
0.7
0.75
0.8
0.85
0.6
0.6
0.7
0.75
0.8
0.55
0.55
0.65
0.7
0.8
0.5
0.55
0.6
0.7
0.8
Cable Grouping Factor (No of Tray Factor) (K7)
No of Cable/Tray
0.84
0.8
0.78
0.77
0.76
0.75
0.8
0.76
0.74
0.73
0.72
0.71
0.78
0.74
0.72
0.71
0.7
0.69
0.77
0.73
0.7
0.69
0.68
0.67
0.75
0.71
0.7
0.68
0.68
0.66
0.74
0.69
0.675
0.66
0.66
0.64
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
Lets select 3.5 core 70 Sq.mm cable for single run.
Current capacity of 70 Sq.mm cable is: 170 Amp,
Resistance = 0.57 /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 (Rcos + jsin) 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 its necessary to use next higher
size of cable.
Selection of cable Case #2
Lets 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 /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.
