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Home / Technical Articles / Electrical Thumb Rules You MUST Follow (6)
Electrical Thumb Rules You MUST Follow, Part 6
Electrical Thumb Rules You MUST Follow, Part 6 (On photo: View in the Transformer Room in the underground area during White House renovation on March 26, 1952 – via trumanlibrary.org)

Continued from: Electrical Thumb Rules You MUST Follow (Part 5)


12 electrical thumb rules to follow:

  1. Transformer earthing wire / Strip size
  2. Motor earthing wire / Strip size
  3. Panel earthing wire / Strip size
  4. Electrical equipment earthing
  5. Earthing wire (As per BS 7671)
  6. Area for transformer room: (As per NBC-2005)
  7. Span of transmission line (Central electricity authority)
  8. Max. lock rotor amp for 1-phase 230V motor (NEMA)
  9. Three phase motor code (NEMA)
  10. Service factor of motor
  11. Type of contactor
  12. Contactor coil

1. 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

Go back to Rules ↑


2. 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

Go back to Rules ↑


3. 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

Go back to Rules ↑


4. 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

Go back to Rules ↑


5. 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

Go back to Rules ↑


6. 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)
1 x 160 14 90 9
2 x 160 28 118 13.5
1 x 250 15 91 9
2 x 250 30 121 13.5
1 x 400 16.5 93 9
2 x 400 33 125 13.5
3 x 400 49.5 167 18
2 x 500 36 130 14.5
3 x 500 54 172 19
2 x 630 36 132 14.5
3 x 630 54 176 19
2 x 800 39 135 14.5
3 x 800 58 181 14
2 x 1000 39 149 14.5
3 x 1000 58 197 19

– The capacitor bank should be automatically switched type for substation of 5MVA and higher.
– Transformer up to 25 KVA can be mounted direct on pole.
– Transformer from 25 KVA to 250KVA can be mounted either on “H” frame of plinth.
– Transformer above 250 KVA can be mounted plinth only.
– Transformer above 100 MVA shall be protected by drop out fuse or circuit breaker.

Go back to Rules ↑


7. 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

Go back to Rules ↑


8. Max. lock rotor amp for 1-phase 230V 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

Go back to Rules ↑


9. 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 HP H
>15 HP G

Go back to Rules ↑


10. 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

Go back to Rules ↑


11. 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)

Go back to Rules ↑


12. 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

Go back to Rules ↑

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author-pic

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 13 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

6 Comments


  1. Madhu
    Sep 17, 2019

    Hi sir iam working for railways, What are steps to follow to reduce energy consumption, we converted 100%LED energy efficient motors, pumps, rather than this what more can we do for save energy, we are using solar also in our station


  2. Rajesh Kaley
    Apr 28, 2017

    Hi Jignesh,
    We have to do motors as well electrical panels body earthing as per standard electricity rules.Can you please guide which is the applicable rule number so that we can refer to that.

    Thanks
    Rajesh


  3. Sikandar
    Nov 05, 2015

    You are working very well and your website article is very helpful for electrical technology. i love this type of site and save this link for further visiting.
    you can see by site by link
    http://www.electricalonline4u.com/2015/10/wiring-of-distribution-board.html


  4. Muhamma Younad Rana
    Oct 09, 2014

    Hi,
    Every body i want to know about energy conservation in detail,so i am working as a power and electric chief engineer in Pakistan with the largest group incorporate in Pakistan House of Habib(HOH) Pakistan and also i am working on energy saving project.


  5. Narbheshankar Modha
    Dec 23, 2013

    Pl. arrange to give full technical detail about protection relay setting and co ordination


  6. Engr Khan
    Dec 09, 2013

    Superb..I was expect the same from you people… Also, Can i write these type articles for Electrical engineering portal.com…as i have wrote many articles like this for Electrical Technology
    As you can see as a ref
    Three Phase Electrical Wiring Installation in Home
    http://www.electricaltechnology.org/2013/12/three-phase-electrical-wiring.html

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