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TABLE OF CONTENTS
1.0 INTRODUCTION 2
2.0 APPLICABLE STANDARDS 3
3.0 OBJECTIVE 4
4.0 CABLE CONTRUCTION 5
5.0 ASSUMTIONS AND CRITERIA 6
5.1 General 6
5.2 Conversion Factors for Cables in Air 6
5.3 Conversion Factors for Cables Lain in Ground 7
5.4 Derating for Cables Laid in Air 7
5.5 Derating for Cables Laid in Ground 8
5.6 General 9
5.7 Conversion Factors for Cables in Air 9
5.8 Derating for Cables Laid in Air 9
5.9 Other Assumptions 11
5.10
Other Considerations 11
5.11 Cable Connected in Parallel 11
5.12 Acceptance Criteria 12
6.0 ALGORITHMS 14
6.1 Ampacity Calculation 14
6.2 Motor Running / Static Load Voltage Drop 14
6.3 Motor Starting Voltage Drop 14
6.4 Percentage Voltage Drop 14
6.5 Short Circuit Thermal Withstandability 15
6.6 Cable Protected by Circuit Breaker 15
6.7 Cable Protected by Fuse 16
7.0 SAMPLE HAND CALCULATION 17
7.1 Calculation for 400V LV Motor – Test 1 17
7.2 Calculation for Cable from 11kV MV Switchgear to 20 MVA Transformer – Test 2 20
7.3 Calculation for 400V LV Motor Feeder – Test 3 22
7.4 Calculation for Cable Substation Incomer 11kV Transformer – Test 4 25
8.0 CONCLUSION 27
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1.0 INTRODUCTION
Start_your_text_here
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2.0 APPLICABLE STANDARDS
The cable electrical characteristics and cable current carrying capacity are based on vendor cable
specifications. The de-rating and conversion factors, including ambient conditions are based on the
following standards and recommendations:
(a) BS 7671:1992 IEE wiring regulations, Sixteen Edition.
(b) IEC 60287 Electrical Cables - Calculation of the Current Rating
(c) IEC 60502 Power Cables with Extruded Insulation and Their Accessories for Rated
Voltages
(The following part is an optional. Delete if not applicable)
(d) IEC 60331 Fire-resisting Characteristics of Electric Cables
(e) IEC 60332 Test on Electric Cables under Fire Conditions
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3.0 OBJECTIVE
The objective of this report is to state the criteria and methodology for cable sizing
calculation/checking for <NAME OF THE PROJECT> Project.
The calculated cable sizes shall be checked against the assumptions made in Section 5.0 of this
calculation report. This document is intended to establish the basis for calculating the cable sizes
and the calculation shall be updated as new information or assumptions used are updated.
Four hand calculations will be performed to validate the spreadsheet calculation, two calculations for
MV cable and two calculations for LV cable.
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4.0 CABLE CONTRUCTION
The cable construction shall be referred to Cable Specification <NAME OF THE PROJECT>
Document number The Low Voltage conductor shall in general be armoured, stranded copper with
XLPE insulation and the following ratings:
(a) LV CABLES (400V) : 0.6/1kV insulation voltage, flame retardant type.
(b) MV CABLES (6.6kV) : 6.3/11kV insulation voltage, flame retardant type.
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5.0 ASSUMTIONS AND CRITERIA
Calculations are based on standards and specifications stated in Section 2 and vendor cable data.
The cables are calculated based on cable construction indicated in Section 4. The following are the
assumptions made in the calculation:
SAMPLE ONSHORE ASSUMPTIONS AND CRITERIA (Delete this section i f not applicable)
5.1 General
5.2 Conversion Factors for Cables in Air
(a) Ambient Temperature in Air 40 º C
(b) LV Grouping Factor – Single core
i. Installation
ii. Spacingiii. Grouping Factor
iv. No. of Circuits
1 Vertical / 6 Horizontally
S=D0.15 m touching
6
(c) LV Grouping Factor - Multicore
i. Installation
ii. Spacing
iii. Grouping Factor
iv. No. of Circuits
1 Vertical / 6 Horizontally
S=D
0.15 m touching
6
(a) Maximum design air temperature 40 º C
(b) Minimum design air temperature 14.5 º C
(c) Air temperature 40 º C
(d) Ground temperature 30 º C
(e) Specific thermal resistance of soil 2.5 K.m/W(f) Depth of cable laying 1.00 m
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5.3 Conversion Factors for Cables Lain in Ground
(a) Ambient Temperature in Ground 30 º C
(b) MV Grouping Factor – Single core
i. Installationii. Spacing
iii. Grouping Factor
iv. No. of Circuits
1 Vertical / 2 HorizontallyS=D
0.15 m touching
2
(c) MV Grouping Factor - Multicore
i. Installation
ii. Spacing
iii. Grouping Factor
iv. No. of Circuits
1 Vertical / 2 Horizontally
S=D
0.15 m touching
2
5.4 Derating for Cables Laid in Air
LV Criteria Parameter/ Units References Derating SINGLE MULTI
Temperature 40 º C A1, Pg 13, Tb 16 1f 1.00 1.00
T R A Y
Grouping
Factor
1 Vertical /
6 Horizontal
A1, Pg 13, Tb 15 2f 0.80 0.70
Derating Factor 21 f f × 0.80 0.70
11kV** Criteria Parameter/ Units References Derating SINGLE MULTI
Temperature 40 º C A2, Pg 24, Tb 34 1f 0.86 0.86
T R A Y
Grouping
Factor
1 Vertical /
2 Horizontal A2 2f 0.81 0.80
Derating Factor 21 f f × 0.70 0.69
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5.5 Derating for Cables Laid in Ground
LV Criteria Parameter/ Units References Derating SINGLE MULTI
Temperature 30 º C A1, Pg 13, Tb 16 1f 0.96 0.96
Dept Laying 1.00m A2, Pg 24, Tb 35 2f 0.98 0.98
Thermal
Resistivity2.5 K.m/W A2, Pg 24, Tb 36 3f 0.73 0.76
T R E N C H
Grouping
Factor
1 Vertical /
6 Horizontal A1, Pg 13, Tb 15 4f 0.85 0.80
Derating Factor 4321 f f f f ××× 0.58 0.57
11kV** Criteria Parameter/ Units References Derating SINGLE MULTI
Temperature 30 º C A2, Pg 24, Tb 34 1f 0.89 0.89
Dept Laying 1m A2, Pg 24, Tb 34 2f 0.98 0.98
Thermal
Resistivity2.5 K.m/W A2, Pg 24, Tb 34 3f 0.73 0.76
T R E N C H
Grouping
Factor
1 Vertical /
2 Horizontal A2 4f 0.80 0.80
Derating Factor 4321 f f f f ××× 0.51 0.53
* Assume arrangement of cables S=D / Touching
** 11 kV Grouping factors is in Appendix 2 on page 26 Table 39 for single core and page 28,
Table 42 for multicore.
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SAMPLE OFFSHORE ASSUMPTIONS AND CRITERIA (Delete this section i f not applicable)
5.6 General
5.7 Conversion Factors for Cables in Air
(a) Ambient Temperature in Air 40 º C
(b) LV Grouping Factor – Single core
i. Installation
ii. Spacing
iii. Grouping Factor
iv. No. of Circuits
1 Vertical / 6 Horizontally
S=D
0.15 m touching
6
(c) LV Grouping Factor - Multicore
i. Installation
ii. Spacing
iii. Grouping Factor
iv. No. of Circuits
1 Vertical / 6 Horizontally
S=D
0.15 m touching
6
5.8 Derating for Cables Laid in Air
LV Criteria Parameter/ Units References Derating SINGLE MULTI
Temperature 40 º C A1, Pg 13, Tb 16 1f 1.00 1.00
T R A Y
Grouping
Factor
1 Vertical /
6 Horizontal A1, Pg 13, Tb 15 2f 0.80 0.70
Derating Factor 21 f f × 0.80 0.70
(a) Maximum conductor operating temperature 40 º C
(b) Minimum design air temperature 14.5 º C(c) Air temperature 40 º C
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11kV** Criteria Parameter/ Units References Derating SINGLE MULTI
Temperature 40 º C A2, Pg 24, Tb 34 1f 0.86 0.86
T
R A Y
Grouping
Factor
1 Vertical /
2 Horizontal A2 2f 0.81 0.80
Derating Factor 21 f f × 0.70 0.69
* Assume arrangement of cables S=D / Touching
** 11kV Grouping factors is in Appendix 2 on page 26 Table 39 for single core and page 28,
table 42 for multicore.
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5.9 Other Assumptions
(a) Cable lengths are estimated.
(b) Cable shall be protected by fuse or breaker against cable short circuit current.
(c) Ground temperature is assumed.
(d) Soil resistivity is assumed.
5.10 Other Considerations
After cable order is awarded, ampacity factor shall be taken from manufacturer firm data and used
for final re-calculation.
5.11 Cable Connected in Parallel
Where cables are connected in parallel, they should be of the same type and length, and should
have conductors of the same cross section and the conductors be arranged to carry equal
current.
The current capacity ( PI ) of parallel connected cables should be based on the following
formulae (as per IEE guideline):
(i) for multi-cored cables and single core cables in trefoil, nIIP ×=
(ii) for other single-cored cables, nIIP ××= 9.0
where
I = current rating (of cable)
n = number of parallel connected conductors.
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5.12 Acceptance Criteria
The calculated cables are acceptable if the following criteria are met:
(a) Ampacity In all circumstances zI must not less than bI and nI
also must not less than bI ; (BS 433 – Protection against
overload current);
znb III ≤≤
where
zI the current – carrying capacity of a
cable for continues service, under
particular installation conditions
concerned.
bI the design current of the circuit, i.e. the
current intended to be carried by the
circuit in normal service.
nI the nominal current or current setting of
the devise protecting the circuit against
overcurrent.
(b) Voltage Drop Cable voltage drop shall be within the following range of
its nominal value:
i. For motor under starting conditions ≤ 20%
ii. For motor under running conditions ≤ 2.5%
iii. For Feeders to MCC and DB’s ≤ 2.5%
iv. For lighting ≤ 3%
v. For Instrumentation DB/Panels ≤ 2.5%
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(c) Short Circuit Check The cable size, selected by ampacity and voltage drop
verification, is subjected to the thermal stress withstand
ability check during short circuit.
As an additional check of the cable short circuit
withstand, the cable withstand energy shall be
compared with the protection device let through energy
(operating time <0.2s).
For cable protected by circuit breaker;
CABLECB tItI 22 < ( CBtI 2 can be obtained from the
manufacturer).
For cable protected by fuse;
CABLEFUSE tItI 22 < ( FUSEtI 2 can be obtained from the
manufacturer).
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6.0 ALGORITHMS
6.1 Ampaci ty Calculation
The cross section area of the cable is selected on the basis of the ambient conditions, laying methodand thermal protective device characteristics.
Select cable from vendor cable data (see Appendix 1.0 and 2.0) and its rated current carrying
capacity. Get ICABLE. Apply de-rating factor from Section 5.3/4:
CABLESITE If f f f I ××××= 4321
where,
ISITE = Site cable rating
ICABLE = Vendor cable current carrying rating
1f = de-rating factor based on air/ ground temperature (Appendix 1.0, Table 15-16)
2f = de-rating factor based on cable laying and grouping (Appendix 2.0, Table 34-42)
3f = de-rating factor based on cable laying depth (for buried cables) (Appendix 2.0,
Table 34-42)
4f = de-rating factor based on soil thermal resistivity (for buried cables) (Appendix 2.0,
Table 34-42)
6.2 Motor Running / Static Load Voltage Drop
( )φ φ sincos3 ×+××××=Δ LLRunning XRIU l
6.3 Motor Starting Voltage Drop
( )'' sincos3 φ φ ×+××××=Δ LLStarting XRIU l
6.4 Percentage Voltage Drop
%100U
Uu
Δ=Δ
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where,
U = Phase to phase voltage
IRunning = User phase to phase running current
IStarting = User phase to phase starting current
RL = Cable resistance Ohm/km
XL = Cable reactance Ohm/km
φ cos = Running power factor
'cos φ = Starting power factor
l = One-way length of conductor
6.5 Short Circui t Thermal Withstandabili ty
Minimum short circuit current at user terminal;
⎟⎟ ⎠
⎞⎜⎜⎝
⎛ +
=
n
ZZ
UI
cf
nSCMIN
3
where
nU = Nominal phase to phase Voltage
f Z = System Impedance
cZ = Cable Impedance
n = Number of cables
6.6 Cable Protected by Circu it Breaker
222MINCB SnKtI <
2
2
nK
tIS CB
MIN >
where
CBtI 2 = let through energy of circuit breaker (A2s) see Note 1.
MINS = Cross section area of cable.
n = Number of cables
K = Short circuit density coefficient
[K=143 for copper cable with XLPE insulation, K= 110 for copper with PVC
insulation and K=142 for copper with EPR insulation.]
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Note 1:
For circuit breaker with direct tripping, the I2tCB can be obtained from the manufacturer let through
energy curves. For circuit breaker with external relay the CBtI 2 is calculated as follows:
tItI CCMAXCB ⋅= 22
where
t = Maximum operating time of protection relay (including C.B. breaking time)
CCMAXI = Maximum short circuit value of system including d.c. decaying component (A)
6.7 Cable Protected by Fuse
222MINFUSE SnKtI <
2
2
nK
tIS FUSE
MIN >
where
FUSEtI
2
= Let through energy of circuit breaker (A
2
s)
[Typical fuse curves (25kA SC) FUSEt is in the range of 10 – 250ms]
MINS = Cross section area of cable.
n = Number of cables
K = Short circuit density coefficient
[K=143 for copper cable with XLPE insulation, K= 110 for copper with PVC
insulation and K=142 for copper with EPR insulation]
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7.0 SAMPLE HAND CALCULATION
7.1 Calculation for 400V LV Motor – Test 1
Installation : Laid buried underground
Load description : Ore Storage Building Sump Pump
Feeder Tag No : P-1P1001
Cable size : 1 Run x 3 Core x 50 2mm
Estimated Length : 50 m
General Data
Load rating : 50 kW
Voltage Rating : 400 V
Running Power Factor : 0.82
Starting Power Factor : 0.3
Motor Efficiency : 100 %
A132.02=
×××=
×××=
182.04.03
75
EEFPFkV3
kWcurrent,loadfullphaseThree FLCI
A858.1=
×= 02.1325.6currentstartingmotor phaseThree
Ampacity
Ampacity per cable (Appendix 6, Tb 1) : 200 A
Ampacity×
No. of Cables : 200 ADe - rating Factor : 0.57
Derated Rating of Cable : 114 A
Selected cable site rating ISITE of 114 A is smaller than IFLC of 132.02 A therefore cable current
carrying capacity is not acceptable.
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Voltage Drop
Total three-phase voltage drop across the 50 m cable is,
(a) Running Voltage drop
( )( )( )
V
XRIU LLRunning
56.2
82.0cossin0739.082.04940.0025.002.1323
sincos3
1
=
×+×××=
×+××××=Δ
−
φ φ l
*
%64.0
%100400
76.6
%100
=
=
Δ=Δ
U
Uu
The running voltage drop across cable of 0.64% is below criteria of 2.5%. Therefore cable is
accepted.
(b) Starting Voltage drop
( )( )( )
V
XRIU LLStarting
13.8
3.0cossin0739.03.04940.0025.01.8583
sincos3
1
''
=
×+×××=
×+××××=Δ
−
φ φ l
*
%03.2
%100400
13.8
%100
=
=
Δ=Δ
U
Uu
The running voltage drop across cable of 2.03% is below criteria of 20%. Therefore cable is
accepted.
* For resistance and reactance values, please refer Appendix 6, Table 2 and 3.
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Cable short circui t rating
(a)
Ω=
×=
×=
0046.0
503
400
kA3
VImpedance,System
k
Zf
Ω=
×+
=
×+
=
0025.0
501000
0739.0494.0
cableof Length
1000
cable,of Impedance
22
22 XRZc
(b)
kA
n
ZZ
UI
cf
nCCMAX
8.7
)1
0025.00046.0(3
400
3
current,circuitshortMaximum
=
+
=
⎟⎟ ⎠
⎞⎜⎜⎝
⎛ +
=
(c) This feeder is protected by moulded case circuit breaker, the minimum cross section area of
cable is
2
2
nK
tIS CB
MIN > = 7.71 mm2
where
CCMAXII =
0.02sec
0.01750.0025
)(estimateddelay4.0) Appendix(timeopeningbreaker
=+=
+=CBt
K =143
n =1
The cable cross section of 1 x 50 mm2
> minimum cable cross-section of 7.71 mm2
and cable is
acceptable.
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7.2 Calculation for Cable from 11kV MV Switchgear to 20 MVA Transformer – Test 2
Installation : Laid on trays in open air
Load description : Ore storage Building Dust Collector & Fan
Feeder Tag No : P-4TR8510
Cable size : 3 Run x 1 Core x 120 2mm
Estimated Length : 50 m
General Data
Load rating : 1500 kW
Voltage Rating : 11000 V
Power Factor : 1.00
Feeder Efficiency : 100 %
78.73A=
×××=
×××=
11113
1500
EEFPFkV3
kWcurrent,loadfullphaseThree FLCI
Ampacity
Ampacity per cable (Appendix 6, Tb 1) : 520 A
Ampacity × No. of Cables : 1560 A
De - rating Factor : 0.7
Derated Rating of Cable : 1092 A
Selected cable site rating ISITE of 1092 A is larger than IFLC of 78.73 A therefore cable current
carrying capacity is acceptable.
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Voltage Drop
Total three-phase voltage drop across the 50 m cable is,
Running Voltage drop
( )( )( )
V
XRIU LLRunning
34.1
1cossin195.01196.005.073.783
sincos3
1
=
×+×××=
×+××××=Δ−
φ φ l
*
%00012.0
%10011000
34.1
%100
=
=
Δ=Δ
U
Uu
The running voltage drop across cable of 0.00012% is below criteria of 2.5%. Therefore cable is
accepted.
* For resistance and reactance values, please refer Appendix 6, Table 2 and 3.
Cable short circui t rating
(a) kAICCMAX 25current,circuitshortMaximum =
(b) This feeder is protected by vacuum circuit breaker and O/C protection relay , the minimum
cross section area of cable is
2
2
nK
tIS CB
MIN > = 45.13 mm2
where
kAII CCMAX 25==
0.2sec0.090.050.06
)(estimateddelaytimeprotection5.0) Appendix(timeopeningbreaker
= ++=
++=CBt
K =143
n = 3
The cable cross section of 120 mm2
> minimum cable cross-section of 45.13 mm2
and cable is
acceptable.
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7.3 Calculation for 400V LV Motor Feeder – Test 3
Installation :
Load description :
Feeder Tag No :
Cable size : 2 x 3 core x 300 2mm
Estimated Length : 100 m
General Data
Load rating : 450 kW
Voltage Rating : 400 V
Power Factor : 0.91
Feeder Efficiency : 100 %
A713.76=
×××=
×××=
191.04.03
450
EEFPFkV3
kWcurrent,loadfullphaseThree FLCI
A4639.44=
×= 76.7135.6currentstartingmotor phaseThree
Ampacity
Ampacity per cable (Appendix 6, Tb 1) : 574 A
Ampacity × No. of Cables : 1148 A
De - rating Factor : 0.8
Derated Rating of Cable : 918.4 A
Selected cable site rating ISITE of 918.4 A is larger than IFLC of 713.76 A therefore cable current
carrying capacity is acceptable.
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Voltage Drop
Total three-phase voltage drop across the 100 m cable is,
(a) Running Voltage drop
( )( )( )
V
XRIU LLRunning
28.6
2/91.0cossin0697.091.00799.01.076.7133
phaseNo.of /sincos3
1
=
×+×××=
×+××××=Δ
−
φ φ l
*
%57.1
%100400
28.6
%100
=
=
Δ=Δ
U
Uu
The running voltage drop across cable of 1.57% is below criteria of 2.5%. Therefore cable is
accepted.
(b) Starting Voltage drop
( )( )( )
V
XRIU LLStarting
25.36
2/3.0cossin0697.03.00799.01.044.46393
phaseof No/sincos3
1
''
=
×+×××=
×+××××=Δ
−
φ φ l
*
%06.9
%100400
25.36
%100
=
=
Δ=Δ
U
Uu
The running voltage drop across cable of 9.06% is below criteria of 20%. Therefore cable is
accepted.
* For resistance and reactance values, please refer Appendix 6, Table 2 and 3.
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Cable short circui t rating
(a)
Ω=×
=
×=
0029.0
803
400
kA3
VImpedance,System f Z
Ω=
×+
=
×+
=
0106.01001000
0697.00799.0
cableof Length1000
cable,of Impedance
22
22 XRZc
(b)
kA
n
ZZ
UI
cf
nCCMAX
16.28
)2
0106.00029.0(3
400
3
current,circuitshortMaximum
=
+
=
⎟⎟ ⎠
⎞⎜⎜⎝
⎛ +
=
(c) This feeder is protected by moulded case circuit breaker, the minimum cross section area of
cable is
2
2
nK
tIS CB
MIN > = 19.69 mm2
where
CCMAXII =
0.02sec
0.01750.0025
)(estimateddelay4.0) Appendix(timeopeningbreaker
=
+=
+=CBt
K =143
n = 2
The cable cross section of 2 x 300 mm2
> minimum cable cross-section of 19.69 mm2
and cable is
acceptable.
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7.4 Calculation for Cable Substation Incomer 11kV Transformer – Test 4
Installation :
Load description :
Feeder Tag No :
Cable size : 1 x 3 core x 150 2mm
Estimated Length : 750 m
General Data
Load rating : 2500 kW
Voltage Rating : 11000 V
Power Factor : 1.00
Feeder Efficiency : 100 %
A131.22=
×××=
×××=
11113
2500
EEFPFkV3
kWcurrent,loadfullphaseThree FLCI
Ampacity
Ampacity per cable (Appendix 6, Tb 1) : 361 A
Ampacity × No. of Cables : 361 A
De - rating Factor : 0.53
Derated Rating of Cable : 191.33 A
Selected cable site rating ISITE of 191.33 A is larger than IFLC of 131.22 A therefore cable current
carrying capacity is acceptable.
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Voltage Drop
Total three-phase voltage drop across the 750 m cable is,
Running Voltage drop
( )( )( )
V
XRIU LLRunning
10.27
0.1cossin09.00.1159.075.022.1313
sincos3
1
=
×+×××=
×+××××=Δ−
φ φ l
%25.0
%10011000
10.27
%100
=
=
Δ=Δ
U
Uu
The running voltage drop across cable of 0.25% is below criteria of 2.5%. Therefore cable isaccepted.
* For resistance and reactance values, please refer Appendix 6, Table 2 and 3.
Cable short circui t rating
(a) kAICCMAX 25current,circuitshortMaximum =
(b) This feeder is protected by vacuum circuit breaker and O/C protection relay, the minimum
cross section area of cable is
2
2
nKtIS CB
MIN > = 78.18 mm2
where
kAII CCMAX 25==
0.2sec
0.090.050.060
)(estimateddelaytimeprotection5.0) Appendix(timeopeningbreaker
=
++=
++=CBt
K =143
n = 1
The cable cross section of 2 x 300 mm2
> minimum cable cross-section of 78.18 mm2
and cable is
acceptable.
8/9/2019 Cable Sizing Guideline
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Page : 27 of 27
8.0 CONCLUSION
Cable sizing in the spreadsheet is verified by the hand calculations.
The cable sizes recommended are based on the assumptions in Section 5.0 and the attached
appendices, the data in Section 5.0 shall be verified prior to purchase/installation of cables. Any
changes shall be updated into this report.
All the MV and LV cables shall be protected by fuses or protection relays (50/51) against short circuit
faults. The specific let through energy of the fuses or direct tripping protection shall not be higher
than the specific let through energy of the cables [A2s].