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 (K3) for Soil (When Thermal Resistance of Soil is known):
|
Ther.Resi Correction Factor: K3 |
|
|
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
