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

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About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 14 years experience in Power Transmission-Power Distribution-Electrical energy theft detection-Electrical Maintenance-Electrical Projects(Planning-Designing-coordination-Execution). He is Presently associate with one of the leading business group as a Deputy Manager at Ahmedabad,India. He is Freelancer Programmer of Advance Excel and design useful Excel Sheets of Electrical Engineering as per IS,NEC,IEC,IEEE codes. He is technical Author for "Electrical Mirror" and "Electrical India" Magazines. He is Technical Blogger and Familiar with English, Hindi, Gujarati, French languages. He wants to Share his experience & knowledge and help technical enthusiasts to find suitable solutions and updating themselves on various Engineering Topics.

10 Responses to Electrical Thumb Rules-(Part-6)

  1. siva muthusamy says:

    Dear sir/madam l had received electrical thump rules 4,5,6. so i want electrical thump rules 1,2,3. so plase kindly send me. thankyou M.SIVA

  2. Alex says:

    Thank you for the valuable notes.

  3. Amit Khari says:

    Dear Sir,
    Thanks for valuable info.Kindly send me SLD of PCC in LT panel…….

  4. KRISHNA R says:

    Sir .. Thank u for the Valuable information.. Can u pls upload details of Panel fabrication.

  5. farooq ahmed rather says:

    dear sir .
    the thumb rule of electrical note is very help full

  6. abdul khadeer says:

    thankx so much RESPECTED SIR
    to share ur lot of experience with us

  7. angelito Cortez says:

    Thank you so much for the very interesting information.More power and God Bless.Can you give me information on how to calculate cables size current capacity in meters.

    Regards, Angelito

  8. Yasir Liaquat says:

    Sir Thanks alot for Shairing Such a Very very helpful Informative… ALLAH bless you… and keep on sir… Once again Thanks

  9. syed shahbaz hussain says:

    Hi!Sir.Hope you are doing good.I’m very much thankful to you for this kind information regarding Electrical system. My question to you is my transformer is 315KVA so, what should be the size of the HT Cable.And also please let me know that how to select the HT cable?

    Thanks & Regards
    Shahbaz Hussain.

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