Electrical Thumb Rules-(Part-7)


Overhead Conductor /Cable Size:

Voltage

Overhead Conductor

Cable Size

33 KV ACSR-Panther/Wolf/Dog , AAAC 150,185,300,400,240 mm2 Cable
11 KV ACSR-Dog/Recon/Rabbit , AAAC 120, 150,185,300 mm2 Cable
LT ACSR-Dog/Recon/Rabbit , AAC,AAAC 95,120, 150,185,300 mm2 Cable

 Transmission / Distribution Line:

Span

Height of Tower

400KV=400 Meter

400KV=30Meter (Base 8.8 Meter)

220KV=350 Meter

220KV=23Meter (Base 5.2 Meter)

132KV=335 Meter

220KV Double Circuit=28 Meter

66KV=210 Meter

66KV=13Meter

Conductor Ampere

Voltage wise Conductor

Dog=300Amp

400KV=Moose ACSR=500MVA Load

Panther=514Amp

220KV=Zebra ACSR=200MVA Load

Zebra=720Amp

132KV=Panther ACSR=75MVA Load

Rabbit=208Amp

66KV=Dog ACSR=50MVA Load

Moose=218Amp

 

 Type of Tower:

Type

Used

Angle/Deviation

A

Suspension Tower Up to 2°

B

Small Angle Tower 2° to 15°

C

Medium Angle Tower 15° to 30°

D

Large Angle / Dead End Tower 30° to 60° & Dead End

 Tower Swing Angle Clearance (Metal Part to Live Part):

Swing Angle

Live Part to Metal Part Clearance (mm)

66KV

132KV

220KV

400KV

915mm

1530mm

2130mm

3050mm

15°

915mm

1530mm

2130mm

22°

3050mm

30°

760mm

1370mm

1830mm

44°

1860mm

44°

610mm

1220mm

1675mm

 Cable Coding (IS 1554) 😦 A2XFY / FRLS / FRPVC / FRLA / PILC)

A

Aluminium

2X

XLPE

F

Flat Armoured

W

Wire Armoured

Y

Outer PVC Insulation Sheath

W

Steel Round Wire

WW

Steel double round wire Armoured

YY

Steel double Strip Armoured

FR

Fire Retardation

LS

Low Smoke

LA

Low Acid Gas Emission

WA

Non Magnetic round wire Armoured

FA

Non Magnetic Flat wire Armoured

FF

Double Steel Round Wire Armoured

 Corona Ring Size:

Voltage

Size

<170 KV 160mm Ring put at HV end
>170 KV 350mm Ring put at HV end
>275 KV 450mm Ring put at HV end & 350 mm Ring put at Earth end

 Load as per Sq.Ft:

Type of Load

Load/Sq.Ft

Diversity Factor

Industrial

1000 Watt/Sq.Ft

0.5

Commercial

30 Watt/Sq.Ft

0.8

Domestic

15 Watt/Sq.Ft

0.4

Lighting

15 Watt/Sq.Ft

0.8

  Size of Ventilation Shaft:

Height of Building in meter Size of ventilation shaft in sq meter Minimum size of shaft in meter

9.0

1.5

1.0

12.5

3.0

1.2

15 and above

4.0

1.5

 

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Electrical Thumb Rules-(Part-6)


Transformer Earthing Wire / Strip Size:

Size of T.C or DG

Body Earthing

Neutral Earthing

<315 KVA 25×3 mm Cu / 40×6 mm GI Strip 25×3 mm Cu Strip
315 KVA to 500 KVA 25×3 mm Cu / 40×6 mm GI Strip 25×3 mm Cu Strip
500 KVA to 750 KVA 25×3 mm Cu / 40×6 mm GI Strip 40×3 mm Cu Strip
750 KVA to 1000 KVA 25×3 mm Cu / 40×6 mm GI Strip 50×3 mm Cu Strip

 Motor Earthing Wire / Strip Size:

Size of Motor

Body Earthing

< 5.5 KW

85 SWG GI Wire

5.5 KW to 22 KW

25×6 mm GI Strip

22 KW to 55 KW

40×6 mm GI Strip

>55 KW

50×6 mm GI Strip

 Panel Earthing Wire / Strip Size:

Type of Panel

Body Earthing

Lighting & Local Panel

25×6 mm GI Strip

Control & Relay Panel

25×6 mm GI Strip

D.G & Exciter Panel

50×6 mm GI Strip

D.G & T/C Neutral

50×6 mm Cu Strip

 Electrical Equipment Earthing:

Equipment

Body Earthing

LA (5KA,9KA)

25×3 mm Cu Strip

HT Switchgear

50×6 mm GI Strip

Structure

50×6 mm GI Strip

Cable Tray

50×6 mm GI Strip

Fence / Rail Gate

50×6 mm GI Strip

 Earthing Wire (As per BS 7671)

Cross Section Area of Phase, Neutral Conductor(S) mm2

Minimum Cross Section area of Earthing Conductor (mm2)

S<=16

S (Not less than 2.5 mm2)

16<S<=35

16

S>35

S/2

 Area for Transformer Room: (As per NBC-2005):

Transformer Size Min. Transformer Room Area (M2) Min. Total Sub Station Area( Incoming HV,LV Panel, T.C Roof) (M2) Min. Space Width (Meter)
1X160

14

90

9

2X160

28

118

13.5

1X250

15

91

9

2X250

30

121

13.5

1X400

16.5

93

9

2X400

33

125

13.5

3X400

49.5

167

18

2X500

36

130

14.5

3X500

54

172

19

2X630

36

132

14.5

3X630

54

176

19

2X800

39

135

14.5

3X800

58

181

14

2X1000

39

149

14.5

3X1000

58

197

19

  •  The Capacitor Bank should be automatic Switched type for Sub Station of 5MVA and Higher.
  • Transformer up to 25KVA can be mounted direct on Pole.
  • Transformer from 25KVA to 250KVA can be mounted either on “H” Frame of Plinth.
  • Transformer above 250KVA can be mounted Plinth only.
  • Transformer above 100MVA shall be protected by Drop out Fuse or Circuit Breaker.

 Span of Transmission Line (Central Electricity Authority):

Voltage

Normal Span

765 KV

400 to 450 Meter

400 KV

400 Meter

220 KV

335,350,375 Meter

132 KV

315,325,335 Meter

66 KV

240,250,275 Meter

 Max. Lock Rotor Amp for 1 Phase 230 V Motor (NEMA)

HP

Amp

1 HP

45 Amp

1.5 HP

50 Amp

2 HP

65 Amp

3 HP

90 Amp

5 HP

135 Amp

7.5 HP

200 Amp

10 HP

260 Amp

 Three Phase Motor Code (NEMA)

HP

Code

<1 HP

L

1.5 to 2.0 HP

L,M

3 HP

K

5 HP

J

7 to 10 HPPHPHPHHHHH

H

>15 HP

G

 Service Factor of Motor:

HP

Synchronous Speed (RPM)

3600 RPM

1800 RPM

1200 RPM

900 RPM

720 RPM

600 RPM

514 RPM

1 HP

1.25

1.15

1.15

1.15

1

1

1

1.5 to 1.25  HP

1.15

1.15

1.15

1.15

1.15

1.15

1.15

150 HP

1.15

1.15

1.15

1.15

1.15

1.15

1

200 HP

1.15

1.15

1.15

1.15

1.15

1

1

> 200 HP

1

1.15

1

1

1

1

1

 Type of Contactor:

Type

Application

AC1

Non Inductive Load or Slightly Inductive Load

AC2

Slip Ring Motor, Starting, Switching OFF

AC3

Squirrel Cage Motor

AC4,AC5,AC5a, AC5b,AC6a

Rapid Start & Rapid Stop

AC 5a

Auxiliary Control circuit

AC 5b

Electrical discharge Lamp

AC 6a

Electrical Incandescent Lamp

AC 6b

Transformer Switching

AC 7a

Switching of Capacitor Bank

AC 7b

Slightly Inductive Load in Household

AC 5a

Motor Load in Household

AC 8a

Hermetic refrigerant compressor motor with Manual Reset O/L  Relay

AC 8b

Hermetic refrigerant compressor motor with Automatic Reset O/L  Relay

AC 12

Control of Resistive Load & Solid State Load

AC 13

Control of Resistive Load & Solid State Load with Transformer Isolation

AC 14

Control of small Electro Magnetic Load (<72 VA)

AC 15

Control of Electro Magnetic Load (>72 VA)

 Contactor Coil:

Coil Voltage

Suffix

24 Volt

T

48 Volt

W

110 to 127 Volt

A

220 to 240 Volt

B

277 Volt

H

380 to 415 Volt

L

Electrical Thumb Rules-(Part-5)


Standard Size of Transformer (IEEE/ANSI 57.120):

Single Phase Transformer

Three Phase Transformer

5KVA,10 KVA,15 KVA,25 KVA,37.5 KVA,50 KVA,75 KVA,100 KVA,167 KVA,250 KVA,

333 KVA,500 KVA,833 KVA,1.25 KVA,1.66 KVA,2.5 KVA,3.33 KVA,5.0 KVA,6.6 KVA,8.3 KVA,10.0 KVA,12.5 KVA,16.6 KVA,20.8 KVA,25.0 KVA,33.33 KVA

3 KVA,5 KVA,9 KVA,15 KVA,30 KVA,45 KVA,75 KVA,112.5 KVA,150 KVA,225 KVA,300 KVA,500 KVA,750 KVA,1MVA,1.5 MVA,2 MVA,2.5 MVA,3.7 MVA,5 MVA,7.5MVA, 10MVA ,12MVA,15MVA,20MVA ,25MVA, 30MVA,37.5MVA ,50MVA ,60MVA,75MVA,100MVA

 Standard Size of Motor (HP):

Electrical Motor (HP)

1,1.5,2,3,5,7.5,10,15,20,30,40,50,60,75,100,125,150,200,250,300,400,450,500,600,700,

800,900,1000,1250,1250,1500,1750,2000,2250,3000,3500,4000

 Approximate RPM of Motor

HP

RPM

< 10 HP

750 RPM

10 HP to 30 HP

600 RPM

30 HP to 125 HP

500 RPM

125 HP to 300 HP

375 RPM

 Standard Size of Motor (HP):

Electrical Motor (HP)

1,1.5,2,3,5,7.5,10,15,20,30,40,50,60,75,100,125,150,200,250,300,400,450,500,600,700,

800,900,1000,1250,1250,1500,1750,2000,2250,3000,3500,4000

 Motor Line Voltage:

Motor (KW)

Line Voltage

< 250 KW

440 V (LV)

150 KW to 3000KW

2.5 KV to 4.1 KV (HV)

200 KW to 3000KW 3.3 KV to 7.2 KV (HV)
1000 KW to 1500KW 6.6 KV to 13.8 KV (HV)

 Motor Starting Current:

Supply

Size of Motor

Max. Starting Current

1 Phase < 1 HP 6 X Motor Full Load Current
1 Phase 1 HP to 10 HP 3 X Motor Full Load Current
3 Phase 10 HP 2 X Motor Full Load Current
3 Phase 10 HP to 15 HP 2 X Motor Full Load Current
3 Phase > 15 HP 1.5 X Motor Full Load Current

Motor Starter:

Starter

HP or KW

Starting Current

Torque

DOL <13 HP(11KW) 7 X Full Load Current Good
Star-Delta 13 HP to 48 HP 3 X Full Load Current Poor
Auto TC > 48 HP (37 KW) 4 X Full Load Current Good/ Average
VSD   0.5 to 1.5 X Full Load Current Excellent
Motor > 2.2KW Should not connect direct to supply voltage if it is in Delta winding

 Impedance of Transformer (As per IS 2026):

MVA

% Impedance

< 1 MVA

5%

1 MVA to 2.5 MVA

6%

2.5 MVA to 5 MVA

7%

5 MVA to 7 MVA

8%

7 MVA to 12 MVA

9%

12 MVA to 30 MVA

10%

> 30 MVA

12.5%

 Standard Size of Transformer:

Standard Size of Transformer

KVA

Power Transformer (Urban)

3,6,8,10,16

Power Transformer (Rural)

1,1.6,3.15,5

Distribution Transformer

25,50,63,100,250,315,400,500,630

 

 

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