8/9/2019 Cable Sizing Guideline

Page : 1 of 27

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 AL GORITHMS 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

http://slidepdf.com/reader/full/cable-sizing-guideline 1/27
8/9/2019 Cable Sizing Guideline

Page : 2 of 27

1.0 INTRODUCTION

Start_your_text_here

http://slidepdf.com/reader/full/cable-sizing-guideline 2/27
8/9/2019 Cable Sizing Guideline

Page : 3 of 27

2.0 APPLICABL E 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

http://slidepdf.com/reader/full/cable-sizing-guideline 3/27
8/9/2019 Cable Sizing Guideline

Page : 4 of 27

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.

http://slidepdf.com/reader/full/cable-sizing-guideline 4/27
8/9/2019 Cable Sizing Guideline

Page : 5 of 27

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.

http://slidepdf.com/reader/full/cable-sizing-guideline 5/27
8/9/2019 Cable Sizing Guideline

Page : 6 of 27

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

(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

5.2 Conversio n Facto rs for Cables in Air

(a) Ambient Temperature in Air 40 º C

(b) LV Grouping Factor – Single core
i. Installation 1 Vertical / 6 Horizontally

ii. Spacing S=D

iii. Grouping Factor 0.15 m touching
iv. No. of Circuits 6
(c) LV Grouping Factor - Multicore
i. Installation 1 Vertical / 6 Horizontally
ii. Spacing S=D
iii. Grouping Factor 0.15 m touching
iv. No. of Circuits 6

http://slidepdf.com/reader/full/cable-sizing-guideline 6/27
8/9/2019 Cable Sizing Guideline

Page : 7 of 27

5.3 Conversio n Factors for Cables Lain in Ground

(a) Ambient Temperature in Ground 30 º C

(b) MV Grouping Factor – Single core

i. Installation 1 Vertical / 2 Horizontally

ii. Spacing S=D
iii. Grouping Factor 0.15 m touching
iv. No. of Circuits 2
(c) MV Grouping Factor - Multicore
i. Installation 1 Vertical / 2 Horizontally
ii. Spacing S=D
iii. Grouping Factor 0.15 m touching
iv. No. of Circuits 2

5.4 Deratin g for Cables Laid in Air

LV Criteria Parameter/ Units References Deratin g SINGLE MULTI

Temperature 40 º C A1, Pg 13, Tb 16 f1 1.00 1.00

Y
A
R Grouping 1 Vertical /
T A1, Pg 13, Tb 15 f2 0.80 0.70
Factor 6 Horizontal
Derating Factor f1 × f 2 0.80 0.70

11kV** Crit eria Parameter/ Unit s References Deratin g SINGLE MULTI

Temperature 40 º C A2, Pg 24, Tb 34 f1 0.86 0.86

Y
A
R Grouping 1 Vertical /
T A2 f2 0.81 0.80
Factor 2 Horizontal

Derating Factor f1 × f2 0.70 0.69

http://slidepdf.com/reader/full/cable-sizing-guideline 7/27
8/9/2019 Cable Sizing Guideline

Page : 8 of 27

5.5 Deratin g for Cables Laid in Ground

LV Criteria Parameter/ Units References Deratin g SINGLE MULTI

Temperature 30 º C A1, Pg 13, Tb 16 f1 0.96 0.96

Dept Laying 1.00m A2, Pg 24, Tb 35 f2 0.98 0.98

H
C
N
E Thermal
R 2.5 K.m/W A2, Pg 24, Tb 36 f3 0.73 0.76
T Resistivity
Grouping 1 Vertical /
A1, Pg 13, Tb 15 f4 0.85 0.80
Factor 6 Horizontal

Derating Factor f1 × f2 × f3 × f4 0.58 0.57

11kV** Crit eria Parameter/ Unit s References Deratin g SINGLE MULTI

Temperature 30 º C A2, Pg 24, Tb 34 f1 0.89 0.89

Dept Laying 1m A2, Pg 24, Tb 34 f2 0.98 0.98

H
C
N
E Thermal
R 2.5 K.m/W A2, Pg 24, Tb 34 f3 0.73 0.76
T Resistivity
Grouping 1 Vertical /
A2 f4 0.80 0.80
Factor 2 Horizontal

Derating Factor f1 × f2 × f3 × f 4 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.

http://slidepdf.com/reader/full/cable-sizing-guideline 8/27
8/9/2019 Cable Sizing Guideline

Page : 9 of 27

SAMPLE OFFSHORE ASSUMPTIONS AND CRITERIA (Delete this section i f not applicable)

5.6 General

(a) Maximum conductor operating temperature 40 º C

(b) Minimum design air temperature 14.5 º C

(c) Air temperature 40 º C

5.7 Conversio n Facto rs for Cables in Air

(a) Ambient Temperature in Air 40 º C

(b) LV Grouping Factor – Single core
i. Installation 1 Vertical / 6 Horizontally
ii. Spacing S=D

iii. Grouping Factor 0.15 m touching

iv. No. of Circuits 6
(c) LV Grouping Factor - Multicore
i. Installation 1 Vertical / 6 Horizontally
ii. Spacing S=D
iii. Grouping Factor 0.15 m touching
iv. No. of Circuits 6

5.8 Deratin g for Cables Laid in Air

LV Criteria Parameter/ Units References Deratin g SINGLE MULTI

Temperature 40 º C A1, Pg 13, Tb 16 f1 1.00 1.00

Y
A
R Grouping 1 Vertical /
T A1, Pg 13, Tb 15 f2 0.80 0.70
Factor 6 Horizontal

Derating Factor f1 × f 2 0.80 0.70

http://slidepdf.com/reader/full/cable-sizing-guideline 9/27
8/9/2019 Cable Sizing Guideline

Page : 10 of 27

11kV** Criteria Parameter/ Unit s References Deratin g SINGLE MULTI

Temperature 40 º C A2, Pg 24, Tb 34 f1 0.86 0.86

Y
A
R Grouping 1 Vertical /
T A2 f2 0.81 0.80
Factor 2 Horizontal

Derating Factor f1 × f2 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.

http://slidepdf.com/reader/full/cable-sizing-guideline 10/27
8/9/2019 Cable Sizing Guideline

Page : 11 of 27

5.9 Other Assumpti ons

(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 Consid erations

After cable order is awarded, ampacity factor shall be taken from manufacturer firm data and used
for final re-calculation.

5.11 Cable Conn ected 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 ( IP ) 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, IP = I × n

(ii) for other single-cored cables, I P = 0.9 × I × n

where
I = current rating (of cable)
n = number of parallel connected conductors.

http://slidepdf.com/reader/full/cable-sizing-guideline 11/27
8/9/2019 Cable Sizing Guideline

Page : 12 of 27

5.12 Acc eptance Criteria

The calculated cables are acceptable if the following criteria are met:

(a) Ampacity In all circumstances I z must not less than I b and I n

also must not less than I b ; (BS 433 – Protection against

overload current);
Ib ≤ In ≤ Iz

where
Iz the current – carrying capacity of a

cable for continues service, under

particular installation conditions
concerned.
Ib the design current of the circuit, i.e. the

current intended to be carried by the

circuit in normal service.
In 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%

http://slidepdf.com/reader/full/cable-sizing-guideline 12/27
8/9/2019 Cable Sizing Guideline

Page : 13 of 27

(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;

I 2 tCB < I 2 t CABLE ( I 2 t CB can be obtained from the

manufacturer).

For cable protected by fuse;

I 2 t FUSE < I 2 t CABLE ( I 2 t FUSE can be obtained from the

manufacturer).

http://slidepdf.com/reader/full/cable-sizing-guideline 13/27
8/9/2019 Cable Sizing Guideline

Page : 14 of 27

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 method
and 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:

I SITE = f1 × f2 × f3 × f 4 × ICABLE

where,
ISITE = Site cable rating
ICABLE = Vendor cable current carrying rating
f1 = de-rating factor based on air/ ground temperature (Appendix 1.0, Table 15-16)

f2 = de-rating factor based on cable laying and grouping (Appendix 2.0, Table 34-42)

f3 = de-rating factor based on cable laying depth (for buried cables) (Appendix 2.0,

Table 34-42)
f4 = 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

(
ΔU = 3 × I Running × l × RL × cos φ + X L × sin φ )

6.3 Motor Starting Voltage Drop

(
ΔU = 3 × IStarting × l × RL × cos φ ' + X L × sin φ ' )

6.4 Percentage Voltage Drop

ΔU
Δu = 100 %
U

http://slidepdf.com/reader/full/cable-sizing-guideline 14/27
8/9/2019 Cable Sizing Guideline

Page : 15 of 27
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
cos φ = Running power factor

cos φ ' = Starting power factor

l = One-way length of conductor

6.5 Short Circui t Thermal Withs tandabili ty

Minimum short circuit current at user terminal;

Un
I SCMIN =
⎛ Z ⎞
3 ⎜⎜ Z f + c ⎟⎟
⎝ n ⎠
where
Un = Nominal phase to phase Voltage

Zf = System Impedance

Zc = Cable Impedance

n = Number of cables

6.6 Cable Protected by Circu it Breaker

I 2 tCB < nK 2SMIN

2

I 2 tCB
SMIN >
nK 2

where
2
I 2 tCB = let through energy of circuit breaker (A s) see Note 1.

SMIN = 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.]

http://slidepdf.com/reader/full/cable-sizing-guideline 15/27
8/9/2019 Cable Sizing Guideline

Page : 16 of 27
Note 1:
2
For circuit breaker with direct tripping, the I tCB can be obtained from the manufacturer let through

energy curves. For circuit breaker with external relay the I 2 tCB is calculated as follows:

I 2 tCB = I 2CCMAX ⋅ t

where
t = Maximum operating time of protection relay (including C.B. breaking time)
ICCMAX = Maximum short circuit value of system including d.c. decaying component (A)

6.7 Cable Protected by Fuse

I 2 t FUSE < nK 2 S MIN

2

I 2 t FUSE
S MIN > 2
nK

where
2 2

I t FUSE = Let through energy of circuit breaker (A s)

[Typical fuse curves (25kA SC) t FUSE is in the range of 10 – 250ms]

SMIN = 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]

http://slidepdf.com/reader/full/cable-sizing-guideline 16/27
8/9/2019 Cable Sizing Guideline

Page : 17 of 27

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 mm 2

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 %

kW
Three phase full load current, I FLC =
3 × kV × PF × EEF
75
=
3 × 0.4 × 0.82 × 1
= 132.02 A

Three phase motor starting current = 6.5 × 132.02

= 858.1 A

Ampacit y
Ampacity per cable (Appendix 6, Tb 1) : 200 A
×
Ampacity No. of Cables : 200 A
De - 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 no t acceptable.

http://slidepdf.com/reader/full/cable-sizing-guideline 17/27
8/9/2019 Cable Sizing Guideline

Page : 18 of 27

Voltage Drop

Total three-phase voltage drop across the 50 m cable is,

(a) Running Voltage drop

(
ΔU = 3 × I Running × l × R L × cos φ + X L × sin φ )
(
= 3 × 132.02 × 0.025 0.4940 × 0.82 + 0.0739 × sin cos −1 0.82 * ( ))
= 2.56V

ΔU
Δu = 100%
U
6.76
= 100%
400
= 0.64%

The running voltage drop across cable of 0.64% is below criteria of 2.5%. Therefore cable is
accepted.

(b) Starting Voltage drop

(
ΔU = 3 × I Starting × l × R ' L × cos φ + X ' L × sin φ )
(
= 3 × 858.1× 0.025 0.4940 × 0.3 + 0.0739 × sin cos −1 0.3 * ( ))
= 8.13V

ΔU
Δu = 100%
U
8.13
= 100%
400
= 2.03%

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.

http://slidepdf.com/reader/full/cable-sizing-guideline 18/27
8/9/2019 Cable Sizing Guideline

Page : 19 of 27

Cable short circui t rating

(a)
V R2 + X 2
System Impedance, Z = Impedance of cable, Z c = × Length of cable
f 3 × kA 1000
400 0.494 2 + 0.0739 2
= = × 50
3 × 50k 1000
= 0.0046Ω = 0.0025Ω

(b) Un
Maximum short circuit current, I CCMAX =
⎛ Zc ⎞
3 ⎜⎜ Z f + ⎟
⎝ n ⎟⎠
400
=
0.0025
3 (0.0046 + )
1
= 7.8kA
(c) This feeder is protected by moulded case circuit breaker, the minimum cross section area of
cable is

I 2 t CB
S MIN > 2
= 7.71 mm
2

nK
where
I = ICCMAX

t CB = breaker opening time (Appendix 4.0) + delay (estimated )

= 0.0025 + 0.0175
= 0.02sec
K =143
n =1

2 2
The cable cross section of 1 x 50 mm > minimum cable cross-section of 7.71 mm and cable is
acceptable.

http://slidepdf.com/reader/full/cable-sizing-guideline 19/27
8/9/2019 Cable Sizing Guideline

Page : 20 of 27

7.2 Calculation for Cable from 11kV MV Switc hgear to 20 MVA Transform er – 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 mm 2
Estimated Length : 50 m

General Data
Load rating : 1500 kW
Voltage Rating : 11000 V
Power Factor : 1.00
Feeder Efficiency : 100 %

kW
Three phase full load current, I FLC =
3 × kV × PF × EEF
1500
=
3 × 11× 1× 1
= 78.73A

Ampacit y

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.

http://slidepdf.com/reader/full/cable-sizing-guideline 20/27
8/9/2019 Cable Sizing Guideline

Page : 21 of 27

Voltage Drop

Total three-phase voltage drop across the 50 m cable is,

Running Voltage drop

(
ΔU = 3 × I Running × l × RL × cos φ + X L × sin φ )
( (
= 3 × 78.73 × 0.05 0.196 × 1 + 0.195 × sin cos −1 1 *))
= 1.34V

ΔU
Δu = 100 %
U
1.34
= 100%
11000
= 0.00012 %

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) Maximum short circuit current, ICCMAX = 25kA

(b) This feeder is protected by vacuum circuit breaker and O/C protection relay , the minimum
cross section area of cable is

I 2 t CB
S MIN > 2
= 45.13 mm
2

nK
where
I = ICCMAX = 25kA

t CB = breaker opening time (Appendix 5.0) + protection time + delay (estimated )

= 0.06 + 0.05 + 0.09

= 0.2sec
K =143
n=3

2 2
The cable cross section of 120 mm > minimum cable cross-section of 45.13 mm and cable is
acceptable.

http://slidepdf.com/reader/full/cable-sizing-guideline 21/27
8/9/2019 Cable Sizing Guideline

Page : 22 of 27

7.3 Calculation for 400V LV Motor Feeder – Test 3

Installation :
Load description :

Feeder Tag No :
Cable size : 2 x 3 core x 300 mm 2
Estimated Length : 100 m

General Data
Load rating : 450 kW
Voltage Rating : 400 V
Power Factor : 0.91
Feeder Efficiency : 100 %

kW
Three phase full load current, I FLC =
3 × kV × PF × EEF
450
=
3 × 0.4 × 0.91× 1
= 713.76 A

Three phase motor starting current = 6.5 × 713.76

= 4639.44 A

Ampacit y
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.

http://slidepdf.com/reader/full/cable-sizing-guideline 22/27
8/9/2019 Cable Sizing Guideline

Page : 23 of 27

Voltage Drop

Total three-phase voltage drop across the 100 m cable is,

(a) Running Voltage drop

( )
ΔU = 3 × I Running × l × RL × cos φ + X L × sin φ / No.of phase
= 3 × 713.76 × 0.1(0.0799 × 0.91 + 0.0697 × sin(cos − 0.91))/ 2 *
1

= 6.28V
ΔU
Δu = 100%
U
6.28
= 100 %
400
= 1.57%

The running voltage drop across cable of 1.57% is below criteria of 2.5%. Therefore cable is
accepted.
(b) Starting Voltage drop

( )
ΔU = 3 × I Starting × l × R ' L × cos φ + X ' L × sin φ / No of phase
( (
= 3 × 4639.44 × 0.1 0.0799 × 0.3 + 0.0697 × sin cos −1 0.3 / 2 * ))
= 36.25V

ΔU
Δu = 100%
U .25
36
= 100%
400
= 9.06%

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.

http://slidepdf.com/reader/full/cable-sizing-guideline 23/27
8/9/2019 Cable Sizing Guideline

Page : 24 of 27

Cable short circui t rating

(a)
V R2 + X 2
System Impedance, Z f = Impedance of cable, Z c = × Length of cable
3 × kA 1000
400 0.0799 2 + 0.0697 2
= = ×
3 × 80 1000 100
= 0.0029Ω = 0.0106Ω

(b) Un
Maximum short circuit current, I CCMAX =
⎛ Zc ⎞
3 ⎜⎜ Z f + ⎟
⎝ n ⎟⎠
400
=
0.0106
3 (0.0029 + )
2
= 28.16kA
(c) This feeder is protected by moulded case circuit breaker, the minimum cross section area of
cable is

I 2 t CB
S MIN > 2
= 19.69 mm
2

nK
where
I = ICCMAX

t CB = breaker opening time (Appendix 4.0) + delay (estimated )

= 0.0025 + 0.0175
= 0.02sec
K =143
n=2

2 2
The cable cross section of 2 x 300 mm > minimum cable cross-section of 19.69 mm and cable is
acceptable.

http://slidepdf.com/reader/full/cable-sizing-guideline 24/27
8/9/2019 Cable Sizing Guideline

Page : 25 of 27

7.4 Calculation for Cable Substati on Incomer 11kV Transform er – Test 4

Installation :
Load description :

Feeder Tag No :
Cable size : 1 x 3 core x 150 mm 2
Estimated Length : 750 m

General Data
Load rating : 2500 kW
Voltage Rating : 11000 V
Power Factor : 1.00
Feeder Efficiency : 100 %

kW
Three phase full load current, I FLC =
3 × kV × PF × EEF
2500
=
3 × 11× 1× 1
= 131.22 A

Ampacit y
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.

http://slidepdf.com/reader/full/cable-sizing-guideline 25/27
8/9/2019 Cable Sizing Guideline

Page : 26 of 27

Voltage Drop

Total three-phase voltage drop across the 750 m cable is,

Running Voltage drop

(
ΔU = 3 × I Running × l × RL × cos φ + X L × sin φ )
( (
= 3 × 131.22 × 0.75 0.159 × 1.0 + 0.09 × sin cos −1 1.0 ))
= 27.10V
ΔU
Δu = 100%
U
27.10
= 100%
11000
= 0.25%

The running voltage drop across cable of 0.25% 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) Maximum short circuit current, ICCMAX = 25kA

(b) This feeder is protected by vacuum circuit breaker and O/C protection relay, the minimum
cross section area of cable is

2
S MIN > I t CB = 78.18 mm 2
nK 2
where
I = ICCMAX = 25kA

t CB = breaker opening time (Appendix 5.0) + protection time + delay (estimated )

= 0.060 + 0.05 + 0.09
= 0.2sec
K =143
n=1

2 2
The cable cross section of 2 x 300 mm > minimum cable cross-section of 78.18 mm and cable is
acceptable.

http://slidepdf.com/reader/full/cable-sizing-guideline 26/27
8/9/2019 Cable Sizing Guideline

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
2
than the specific let through energy of the cables [A s].

http://slidepdf.com/reader/full/cable-sizing-guideline 27/27