Electrical Cable Sizing

ELECTRICAL CABLE SIZING

ELECTRCIAL CABLE SIZING

Page 2 of 21

Table of Contents

1.0

G E NE R AL C ONS IDE R AT IONS

3

1.1

S hort C irc uit C urrent R ating

3

1.2

V oltage Drop

3

1.3

C able C urrent R ating

3

1.4

Other F requenc y

6

1.5

R es is tanc e Data

6

2.0

ME T HODOL OG Y

6

2.1

F ull L oad C urrent (I F L ) C alc ulations

6

2.2

C able V olt Drop C alc ulations

7

2.3

HV C able s izing (S hort C irc uit R ating)

8

3.0

S AMP L E – OXY P R OJ E C T

9

3.1

G eneral Data

9

3.2

C as e S tudy I - HV C able S izing

9

3.3

C as e S tudy II - L V C able S izing

13

4.0

C AB L E E L E C T R IC AL DAT A

18

4.1

C able R es is tanc e (Nexans T ec hnic al Data for OXY P rojec t)

18

4.2

C able R eac tanc e (Nexans T ec hnic al Data for OXY P rojec t)

19

4.3

C able C urrent R ating (Nexans T ec hnic al Data for OXY P rojec t)

20

4.4

C able S hort C irc uit R ating (Nexans T ec hnic al Data for OXY P rojec t)

20

5.0

C ONC L US ION

21

6.0

AP P E NDIE S

21

Page 2 of 21

ELECTRCIAL CABLE SIZING 1.0

1.1

Page 3 of 21

GENERAL CONSIDERATIONS o

Thermal short circuit capacity (for H.V Cables)

o

Voltage drop

o

Current rating

o

Maximum cable loop impedance for earth faults shall not exceeding 1s under solider earth fault conditions. It is meanly for long distance cables.

Short Circuit Current Rating The short circuit breaking current of the source switchboard also takeing into account the short circuit limiting characteristics of, e.g., fuse, relays, CBs. The fault clearance time associated with operation of the primary, see section 2.2.

1.2

Voltage Drop Voltage drop under steady state and motor starting condition with standard (DEP 33.64.10.10-Gen.) and project requirements. In normally shall be follows o

Less than 5% based on continuous maximum current loading and rated voltage in A.C cables.

o

Less than 20% of the rated equipment voltage during motor starting

o

Less than 5% from min. system voltage at distribution board and the min. equipment operation voltage in DC cables.

1.3

Cable Current Rating

1.3.1

Rating factor for cable ampaciry o

Onshore •

Depth of laying (IEE wiring regulations 16th 19991)

Correction factor shall be considered if cables in enclosed trench 450mm wide by 300mm deep minimum. Correction factor shall be applied based on Table 4B3Method 18 (Refer to appendix – C) Correction factor shall be considered if cables in enclosed trench 450mm wide by 600mm deep minimum. Correction factor shall be applied based on Table 4B3Method 19 (Refer to appendix – C). Correction factor shall be considered if cables in enclosed trench 450mm wide by 760mm deep minimum. Correction factor shall be applied based on Table 4B3Method 20 (Refer to appendix – C).

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Page 4 of 21

For example: A correction factor of 0.80 shall be applied for conductor cross-section area 10mm2 cables if 6 single-core cables, 4 two-core cables or 3 three or four-core cables are laying the ground.

o

•

Soil thermal resistively

•

Ground/ambient temperature (see offshore statement)

Offshore •

Ground/ambient temperature (IEE wiring regulations 16th 19991)

This derating (correction) factor is determined by design temperature. ♦ Correction factor for ambient temperature where protection is against short-circuit. Refer to Table 4C1 (Refer to appendix - D) ♦ Correction factor for ambient temperature where the overload protective device is a semi-enclosed fuse to BS3036. Refer to Table 4C2 (Refer to appendix - D) Table 4C1 & 4C2 show correction factor is 1 if cable in air based on ambient temperature on 300C, For example a correction factor of 0.91 shall be applied for XLPE & EPR cables and 0.87 for PVC if maximum design ambient temperature is 400C required by project. The choice of the type of insulation of the cables shall also consider the ambient temperature correction factor. •

Grouping of cables

There are many methods of installation of cables: such as Open and clipped direct (Method 1), Cables embedded direct in building materials (Method 1), in conduit (Method 3 & 4), in trucking (Method 3), on trays (Method 11), in free air, on cleats, brackets or a ladder (Method 12 & 13), Correction factor for grouped cables shall be applied refer to Tables 4B1, 4B2 and 4B3. (Refer to IEE wiring regulations 16th 19991) Generally, cable installation offshore shall be laying in cable ladder/tray. Case 1: in free air – Refer to Table 52-E4 (IEC 60364-5-523) see appendix E As per IEC60092-352, the current rating values may be considered applicable, without correction factor, for cables bunched together on cable trays, unless more than 6 cables (Diagram 1), which may be expected to operate simultaneously at their full rated current, are laid close together in a cable bunch in such a way that there is an absence of free air circulation around them.

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Page 5 of 21

In this case, a correction factor of 0.79 should be applied on current rating value.

Diagram 1 – Cables bunched (6 cables bunch) and clipped direct to a nonmetallic surface ♦ Single core cables – Refer to Table 52-E5 (see Appendix F) Both HV and LV single core cables shall be arranged in the trefoil formation and shall have a distance of half cable diameter, ½De, from the side of the cable ladder or tray. Multiple runs of trefoils shall also have a space of separation of two cable diameter, 2De, as shown in the Diagram 2 below:

½De

De

2De

Diagram 2 – Single core cables installation All single core cables shall be bunched in trefoil formation, as shown in Diagram 2 above. Since there are less than six cables bunched together, the cable grouping correction factor is 1 applicable in one layer ladder here. However, correction factor 0.97/0.96 shall be applied in two (2)/three (3) layers of ladder respectively. ♦ Multicore cables- Refer to Table 52-E4 (see Appendix E) HV multicore cables shall be installed in a single layer and touching as shown in Diagram 3 below:

Diagram 3 – HV multicore cables installation The cable grouping correction factor is 1 applicable in one (1) layer of ladder for one cable only. Others grouping factor refer to table 52-E4 (see Appendix E).

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Page 6 of 21

LV multicore cables may not be installed in single layers and may grouped as bunches of 6 cables, as shown in Diagram 1. A grouping factor 0.73 shall be applied in three (3) layers and six (6) circuits requirement. •

As a result Derating Factor = ambient temperature correction factor × cable grouping correction factor =0.91 x 0.73 =0.66.

1.4

Other Frequency In our scope of work, cable for Electrical power system usually shall be 50Hz or 60Hz. High frequency will not be applicable and shall not be covered in this presentation.

1.5

Resistance Data Normally resistance of cable is provided at 200C by Vendor. The formula for actual resistance as a function of temperature is below. R=R0 (1+ α (t-200C) R0:

Resistance at t=200C

t

Conductor temperature 0C

α

0.00393 for copper

The resistances are based on the maximum cable operating temperature of 85oC (EPR) and 90oC (XLPE) for the different cable type codes selected. For example: Resistance at 200C of the 3C-4mm2 cables is 4.70 based on Draka Catalogue page 52 see Appendix A, so Resistance of the XLPE cable shall be R= 4.70 x (1+0.00393 x (900C-200C)) =4.975ohm/km 2.0

METHODOLOGY

2.1

Full Load Current (IFL) Calculations

2.1.1

Refer to vendor data or Handbook

2.1.2

Formula

o

3phase circuit for AC system:

I FL =

Page 6 of 21

kW × 1000 3 × VSYS × COSθ × E FF

ELECTRCIAL CABLE SIZING Where: kW

Equipment Load

VSYS

System voltage (V)

IFL

Full load current (A)

θ

Angle in which the current lags the voltage (degree)

Eff

Power Efficiency

Single phase circuit for AC system

o

I FL =

kW ×1000 VSYS × COSθ × EFF

DC system:

o

I FL =

kW × 1000 VSYS

2.2

Cable Volt Drop Calculations

2.2.1

3phase circuit for AC system:

%V DROP =

3 × l × I FL × ( R cos θ + X sin θ ) ×100 V SYS × n ×1000

Where: VSYS

System voltage (V)

%VDROP

Volt drop (%)

l

Length (m)

IFL

Full load current (A)

R

Cable resistance (Ω / km)

X

Cable reactance (Ω / km)

θ

Angle in which the current lags the voltage (degree)

n

Page 7 of 21

Number of cables in parallel per phase

Page 7 of 21

ELECTRCIAL CABLE SIZING 2.2.2

Single phase circuit for AC system:

%V DROP = 2.2.3

2 × l × I FL × ( R cos θ + X sin θ ) × 100 V SYS × n × 1000

DC system:

%VDROP =

2.3

Page 8 of 21

2 × l × I FL × R × 100 VSYS × n × 1000

HV Cable sizing (Short Circuit Rating) HV power cables will also be sized based on their short circuit withstand capability.

A=

I× t k

where : A

Cable conductor cross-sectional area (mm2)

I

Short circuit current (A)

t

Duration of short circuit (s). For feeders protected by circuit breakers, t = 0.4s. For feeders protected by fuses, t = 0.25s.

K

Page 8 of 21

cable conductor material, insulation and the safe temperature rise of the insulation. See-Appendix


SAMPLE – OXY PROJECT

3.1

General Data

Page 9 of 21

o

Correction factor for HV & LV single cable shall be 0.94 x 1 = 0.94

o

Correction factor for HV & LV multicore cables shall be 0.94 x 0.85 =0.80.

o

All 6.6kV motors shall have starting current not greater than 550%. The starting current of the low voltage motors shall not exceed 600% of full load current.

o

Voltage drop under steady state and motor starting conditions with the following limitations: •

4% during steady state condition from origin of supply and user terminals

•

20% during motor starting

•

2% on feeders to lighting and small power distribution boards

•

2.5% average on lighting and small power sub-circuits

•

4% for d.c. systems

The cable data refer to the Nexans cable technical data

o 3.2

Case Study I - HV Cable Sizing

3.2.1

Short Circuit Conditions 6/10kV electrical cables shall be used for the 6.6kV power distribution system. Cables for power distribution shall be sized based on the total current, steady state volt drop and short circuit capacity. Motor feeders shall be sized based on the total current, steady state and motor starting volt drops. The short circuit rating of 6.6kV Switchboard is 50kA and for feeders protected by circuit breakers, the cable short circuit duration is 0.4 second. 143 refer to table 43A – Values of K for common materials, for calculation of the effects of fault current – (Refer to appendix – B)

A=

50000 × 0.4 = 217 mm 2 146

Therefore, the minimum cable conductor area shall be > 217mm2 For feeders protected by fuses, the cable short circuit duration is 0.25 second.

A=

50000 × 0.25 = 172mm 2 146

Therefore, the minimum cable conductor area shall be > 172mm2.

Page 9 of 21

ELECTRCIAL CABLE SIZING 3.2.2

Page 10 of 21

6.6kV Cable Sizing Calculations

G ~

GT-3101A 16.034MVA

6/10kV Cable 31-E-HD40003 6.6kV Switchboard SB-3102A 3150A, 3P 50kA

6/10kV Cable

31-E-HP40225

M ~

Page 10 of 21

PM-3156kW


o

Page 11 of 21

Cable 31-E-HP40003 from Gas Turbine Generator GT-3101A to 6.6kV Switchboard SB-3102 Gas Turbine Generator data •

Rated power = 16.034MVA

•

Rated current @ 6.6kV = 1403A

•

Cable sizing current = 1403 x 1.15 = 1613A

•

Estimated cable length = 60m

•

Resistance at 90oC – Refer to Table xx

Cable sizing No. of cores

1 core per phase

Conductor cross-section area (mm2)

400 (Max.size allowed)

Rated current at 45°C (A)

677

Derating factor

0.94

Derated ampacity (A)

0.94 x 677 = 636

Quantity of cables per phase (round up)

1613 ÷ 636 = 2.54 ≈ 3

Total cable cross-sectional area (mm2) (VCB protected)

3 × 400 = 1200 > 217

Total number of cables installed

3 cables per phase

Resistance at 90oC (Ω/km) (XLPE)

0.0636

Reactance at 50Hz (Ω/km)

0.093

Power factor, Cos θ

0.80

Sin θ

0.60

Percentage volt drop (%)

0.08<4%

Sample calculation:

∴ %VDROP − S .STATE =

Page 11 of 21

3 × 1403 × 60 × ((0.0636 × 0.80) + (0.093 × 0.60)) × 100 = 0.08% 1000 × 3 × 6600


o

Page 12 of 21

Cable 31-E-HP40225 Seawater Injection Pump Motor (PM-3156) Motor data •

Rated motor power = 3750kW

•

Full load current = 382A (Vendor data)

•

Limited starting current = 1528A (Vendor data)

•

Cable sizing for full current = 382 x 1.15 = 439A

•


Cable sizing – steady state & motor starting No. of cores

1 core per phase


300


565

Derating factor

0.94


0.94 x 565 = 531


439 ÷ 531 = 0.85 ≈ 1

Total cable cross-sectional area (mm2) (VCB protected)

1 x 300 = 300 > 217


1 cable per phase


0.0797


0.095

Steady state power factor, Cos θ (Vendor data)

0.89 (Sinθ = 0.46)

Motor starting power factor, Cos θ (Vendor data)

0.14 (Sinθ = 0.99)

Percentage volt drop (%) – steady state

0.09<4%

Percentage volt drop (%) – motor starting

0.33<20%

Calculations:


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3 × 382 × 80 × ((0.0797 × 0.87) + (0.095 × 0.49)) × 100 = 0.09% 1000 × 1 × 6600


Page 13 of 21

∴ %VDROP − M .START =

3 × 1528 × 80 × ((0.0797 × 0.14) + (0.095 × 0.99)) × 100 = 0.33% 1000 × 1 × 6600

3.3

Case Study II - LV Cable Sizing

3.3.1

Short Circuit Conditions The short circuit conditions for LV systems are normally not calculated as there are many cables at the downstream to disperse the fault current in the event of a fault. Therefore, the short circuit rating for cables will not be used here.

3.3.2

415V Cable Sizing Calculations

TR-3105 1.6MVA LNAN

0.6/1kV Cable 31-E-LD40101 415V Switchboard SB-3103A 0.6/1kV Cable 31-E-LP40137 0.6/1kV Cable 31-E-LP40320

CP

ECP-1118A

0.6/1kV Cable

31-E-LP40480

M-3154

Page 13 of 21

Crane

EH-1118A


o

Page 14 of 21

Cable 31-E-LP40137 from 6.6/0.44kV 1.6MVA Transformer (TR-3105A) to 415V Switchboard (SB-3103) Transformer data •

Rated power = 1.6MVA LNAN

•

Full load current @ 440V= 2099A

•


•

Estimated Cable length = 45m


1 core


630


899

Derating factor

0.94


0.94 x 899 = 845


2414 ÷845 = 2.86 ≈ 3


3 cables per phase = 9


0.0580


0.097

Steady state power factor, Cos θ

0.80 (Sinθ = 0.60)


1.29 < 4%

Calculations:


Page 14 of 21

3 × 2099 × 45 × ((0.0580 × 0.8) + (0.097 × 0.6)) × 100 = 1.29% 1000 × 3 × 440


o

Page 15 of 21

Cable(s) 31-E-LP40320 from 415V Switchboard (SB-3103) to Pedestal Crane Package (M-3154) Motor data •

Motor rating = 200kW

•


•


•

Starting current = 6 x 342A = 2052A

•



1 core


240


492

Derating factor

0.94


0.94 x 492 = 462


393 ÷ 462 = 0.85 ≈ 1


1 cable per phase = 3


0.108


0.101


0.86 (Sinθ = 0.51)

Motor starting power factor, Cos θ

0.30 (Sinθ = 0.95)

Steady state volt drop (%)

2.57 < 4%

Motor starting volt drop (%)

12.09 < 20%

Calculations:

Page 15 of 21


3 × 387 × 110 × ((0.108 × 0.86) + (0.101 × 0.51)) × 100 = 2.57% 1000 × 1 × 415

∴ %VDROP − M .START =

3 × 2052 × 110 × ((0.108 × 0.30) + (0.101 × 0.95)) × 100 = 12.09% 1000 × 1 × 415

ELECTRCIAL CABLE SIZING o

Page 16 of 21

Cable 31-E-LP40137 from 415V Switchboard (SB-3103) to Fuel Gas Superheater A Thyristor Control Panel (ECP-1118A) •

The voltage drop will be calculated in two stages – from the MCC to the Fuel Gas Superheater Thyristor Panel (35m) and from the Thyristor Panel to the Fuel Gas Superheater A (125m).

•

Rated power for Fuel Gas Superheater A (E-1118A) = 385kW

•


•

The heater is protected by a breaker whose amp trip rating is 800A. Therefore the cable is sized to withstand 800A.


1 core


300


565

Derating factor

0.94


0.94 x 565 = 531


800 ÷ 531 = 1.51 ≈ 2




0.0904

Reactance at 50Hz (Ω/km) (HOLD)

0.100


1.00 (∴sin θ = 0)


0.35

Calculations:


Page 16 of 21

3 × 535 × 35 × ((0.0904 × 1.00) + (0.100 × 0)) × 100 = 0.35% 1000 × 2 × 415


Page 17 of 21

Cable 31-E-LP40480 from Fuel Gas Superheater A Thyristor Control Panel (ECP-1118A) to Fuel Gas Superheater A (E-1118A) Cable sizing No. of cores

1 core


400


677

Derating factor

0.94


0.94 x 677 = 636


615 ÷ 636 = 0.97 ≈ 1




0.0763

Reactance at 50Hz (Ω/km) (HOLD)

0.100


1.00 (∴sin θ = 0)

Total percentage volt drop (%) – steady state

2.13

Calculations:


3 × 535 × 125 × ((0.0763 × 1.00) + (0.100 × 0)) × 100 = 2.13% 1000 × 1 × 415

Therefore, the final volt drop from SB-3103 to E-1118A = 0.35 + 2.13 = 2.48% < 4%

Page 17 of 21


Page 18 of 21

4.0

CABLE ELECTRICAL DATA

4.1

Cable Resistance (Nexans Technical Data for OXY Project) The resistances are based on the maximum cable operating temperature of 85oC (EPR) and 90oC (XLPE) for the different cable type codes selected. Table 1 – Cable Resistance @ 85oC (EPR) or 90oC (XLPE) Cable Resistance (Ω/km)

Conductor area

Cable Type Codes

2

(mm )

H1

H3

H4

H5

H8

L1

L2

L4

L5

2.5

-

-

-

-

-

9.6

9.64

9.49

-

4

-

-

-

-

-

5.99

5.99

5.90

-

6

-

-

-

-

-

3.97

3.97

3.91

-

10

2.36

-

-

-

-

2.35

2.35

-

-

16

1.49

1.48

1.48

-

-

1.48

1.48

1.46

-

25

0.944

0.936

0.936

-

0.922

0.939

0.937

-

-

35

0.683

0.675

0.675

-

0.664

0.677

0.676

0.664

-

50

0.508

0.499

0.499

-

0.491

0.502

0.500

-

-

70

0.354

0.345

0.345

-

0.339

0.349

0.346

-

-

95

0.260

0.249

0.249

-

0.245

0.254

0.251

0.245

0.245

120

0.208

0.197

0.197

-

0.194

0.203

0.200

0.193

-

150

0.173

0.162

0.162

-

0.159

0.168

0.165

-

-

185

0.142

0.129

0.129

-

0.127

0.137

0.133

-

-

240

0.113

0.0990

0.0996

-

0.0979

0.108

0.103

-

-

300

0.0971

0.0797

-

0.0779

-

0.0904

-

-

-

400

-

0.0636

-

-

-

0.0763

-

-

-

500

-

0.0510

-

-

-

0.0660

-

-

-

630

0.0635

0.0416

-

-

-

0.0580

-

-

-

Page 18 of 21


Page 19 of 21

Cable Reactance (Nexans Technical Data for OXY Project) Table 2 – Cable Reactance @ 50Hz Cable Reactance (Ω/km)

Conductor area

Cable Type Codes

2

(mm )

H1

H3

H4

H5

H8

L1

L2

L4

L5

2.5

-

-

-

-

-

0.158

0.093

0.0892

-

4

-

-

-

-

-

0.148

0.086

0.0833

-

6

-

-

-

-

-

0.143

0.084

0.0783

-

10

0.138

-

-

-

-

0.134

0.079

-

-

16

0.130

0.136

0.113

-

-

0.125

0.0753 0.0683

-

25

0.123

0.128

0.106

-

0.179

0.119

0.0754

-

35

0.118

0.123

0.102

-

0.174

0.117

0.0734 0.0650

-

50

0.114

0.119

0.099

-

0.170

0.114

0.0731

-

-

70

0.109

0.113

0.095

-

0.164

0.111

0.0709

-

-

95

0.105

0.109

0.091

-

0.159

0.107

0.0694 0.0625 0.0817

120

0.102

0.105

0.088

-

0.155

0.105

0.0691 0.0608

-

150

0.0987

0.102

0.086

-

0.153

0.104

0.0703

-

-

185

0.0967

0.101

0.084

-

0.151

0.104

0.0707

-

-

240

0.0935

0.096

0.081

-

0.147

0.101

0.0697

-

-

300

0.0934

0.095

-

0.118

-

0.100

-

-

-

400

-

0.093

-

-

-

0.100

-

-

-

500

-

0.090

-

-

-

0.099

-

-

-

630

0.0879

0.088

-

-

-

0.097

-

-

-

Page 19 of 21

-


Page 20 of 21

Cable Current Rating (Nexans Technical Data for OXY Project) Table 3 – Cable Current Ratings / Ampacity @ 45oC Ambient

4.4

3 Core

Conductor area (mm2)

1 Core

3 Core

28

20

95

276

193

4

38

27

120

319

223

6

49

34

150

367

257

10

68

47

185

418

293

16

91

63

240

492

344

25

120

84

300

565

-

35

148

103

400

677

-

50

184

129

500

778

-

70

228

159

630

899

-


1 Core

2.5

Current Rating (A)

Current Rating (A)

Cable Short Circuit Rating (Nexans Technical Data for OXY Project) Table 4 – Cable Short Circuit Ratings @ t = 1 Second

Page 20 of 21


Short Circuit Rating (kA)


Short Circuit Rating (kA)

2.5

0.365

95

13.9

4

0.583

120

17.5

6

0.875

150

21.9

10

1.46

185

27.0

16

2.33

240

35.0

25

3.65

300

43.7

35

5.10

400

58.3

50

7.29

500

72.9

70

10.2

630

91.9


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CONCLUSION For cable sizing, the following point shall be considered: o

Current Rating Correction of the Cable current ampacity shall be more than full current

o

Voltage drop Calculated voltage drop value shall be less than project requirement

o

Thermal short circuit capacity Calculated short circuit value shall be more than system’s short circuit rating

The largest cable size shall be used based on the above point. 6.0

APPENDIES Appendix A - Draka Cable Data Appendix B – Table 43A (IEE Wiring Regulation) Appendix C – Table 4B3 (IEE Wiring Regulation) Appendix D – Table 4C1 & 4C2 (IEE Wiring Regulation) Appendix E – Table 52-E4 (IEC 60364-5-523) Appendix F - Table 52-E5 (IEC 60364-5-523) Appendix G – Tables of cable sizing

Page 21 of 21

Electrical Cable Sizing

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