Cable Sizing Guideline

8/9/2019 Cable Sizing Guideline

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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 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



Page : 2 of 27

1.0 INTRODUCTION

Start_your_text_here



Page : 3 of 27

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



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.



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.



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

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


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



Page : 7 of 27

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


iv. No. of Circuits

1 Vertical / 2 HorizontallyS=D

0.15 m touching

2

(c) MV Grouping Factor - Multicore

i. Installation

ii. Spacing


iv. No. of Circuits


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


T R A Y

Grouping

Factor

1 Vertical /

2 Horizontal A2 2f 0.81 0.80




Page : 8 of 27

5.5 Derating for Cables Laid in Ground



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



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 /


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.



Page : 9 of 27

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


iv. No. of Circuits


S=D

0.15 m touching

6

(c) LV Grouping Factor - Multicore

i. Installation

ii. Spacing


iv. No. of Circuits


S=D

0.15 m touching

6

5.8 Derating for Cables Laid in Air



T R A Y

Grouping

Factor

1 Vertical /

6 Horizontal A1, Pg 13, Tb 15 2f 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



Page : 10 of 27



T

R A Y

Grouping

Factor

1 Vertical /



* 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.



Page : 11 of 27

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.



Page : 12 of 27

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%



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;

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).



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 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



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

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.


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.]



Page : 16 of 27

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.


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]



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 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.



Page : 18 of 27

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.



Page : 19 of 27

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.



Page : 20 of 27

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


General Data

Load rating : 1500 kW


Power Factor : 1.00

Feeder Efficiency : 100 %

78.73A=

×××=

×××=

11113

1500

EEFPFkV3


Ampacity


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.



Page : 21 of 27

Voltage Drop


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


accepted.



(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


and cable is

acceptable.



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 2mm


General Data



Power Factor : 0.91


A713.76=

×××=

×××=

191.04.03

450

EEFPFkV3


A4639.44=

×= 76.7135.6currentstartingmotor phaseThree

Ampacity




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




Page : 23 of 27

Voltage Drop


(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


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.




Page : 24 of 27


(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



and cable is

acceptable.



Page : 25 of 27

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


General Data



Power Factor : 1.00


A131.22=

×××=

×××=

11113

2500

EEFPFkV3


Ampacity




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




Page : 26 of 27

Voltage Drop


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.



(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



and cable is

acceptable.



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].

Documents

Cable Sizing Guideline