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 DGBody EarthingNeutral Earthing
<315 KVA25×3 mm Cu / 40×6 mm GI Strip25×3 mm Cu Strip
315 KVA to 500 KVA25×3 mm Cu / 40×6 mm GI Strip25×3 mm Cu Strip
500 KVA to 750 KVA25×3 mm Cu / 40×6 mm GI Strip40×3 mm Cu Strip
750 KVA to 1000 KVA25×3 mm Cu / 40×6 mm GI Strip50×3 mm Cu Strip

Go back to Rules ↑


2. Motor earthing wire / Strip size

Size of MotorBody Earthing
< 5.5 KW85 SWG GI Wire
5.5 KW to 22 KW25×6 mm GI Strip
22 KW to 55 KW40×6 mm GI Strip
>55 KW50×6 mm GI Strip

Go back to Rules ↑


3. Panel earthing wire / Strip size

Type of PanelBody Earthing
Lighting & Local Panel25×6 mm GI Strip
Control & Relay Panel25×6 mm GI Strip
D.G & Exciter Panel50×6 mm GI Strip
D.G & T/C Neutral50×6 mm Cu Strip

Go back to Rules ↑


4. Electrical equipment earthing

EquipmentBody Earthing
LA (5KA,9KA)25×3 mm Cu Strip
HT Switchgear50×6 mm GI Strip
Structure50×6 mm GI Strip
Cable Tray50×6 mm GI Strip
Fence / Rail Gate50×6 mm GI Strip

Go back to Rules ↑


5. Earthing wire (As per BS 7671)

Cross Section Area of Phase, Neutral Conductor(S) mm2Minimum Cross Section area of Earthing Conductor (mm2)
S<=16S (Not less than 2.5 mm2)
16<S<=3516
S>35S/2

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6. Area for transformer room: (As per NBC-2005)

Transformer SizeMin. Transformer Room Area (M2)Min. Total Sub Station Area( Incoming HV,LV Panel, T.C Roof) (M2)Min. Space Width (Meter)
1 x 16014909
2 x 1602811813.5
1 x 25015919
2 x 2503012113.5
1 x 40016.5939
2 x 4003312513.5
3 x 40049.516718
2 x 5003613014.5
3 x 5005417219
2 x 6303613214.5
3 x 6305417619
2 x 8003913514.5
3 x 8005818114
2 x 10003914914.5
3 x 10005819719

- 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)

VoltageNormal Span
765 KV400 to 450 Meter
400 KV400 Meter
220 KV335,350,375 Meter
132 KV315,325,335 Meter
66 KV240,250,275 Meter

Go back to Rules ↑


8. Max. lock rotor amp for 1-phase 230V motor (NEMA)

HPAmp
1 HP45 Amp
1.5 HP50 Amp
2 HP65 Amp
3 HP90 Amp
5 HP135 Amp
7.5 HP200 Amp
10 HP260 Amp

Go back to Rules ↑


9. Three phase motor code (NEMA)

HPCode
<1 HPL
1.5 to 2.0 HPL,M
3 HPK
5 HPJ
7 to 10 HPH
>15 HPG

Go back to Rules ↑


10. Service factor of motor

HPSynchronous Speed (RPM)
3600 RPM1800 RPM1200 RPM900 RPM720 RPM600 RPM514 RPM
1 HP1.251.151.151.15111
1.5 to 1.25 HP1.151.151.151.151.151.15>1.15
150 HP1.151.151.151.151.151.151
200 HP1.151.151.151.151.1511
> 200 HP11.1511111

Go back to Rules ↑


11. Type of contactor

TypeApplication
AC1Non Inductive Load or Slightly Inductive Load
AC2Slip Ring Motor, Starting, Switching OFF
AC3Squirrel Cage Motor
AC4,AC5,AC5a, AC5b,AC6aRapid Start & Rapid Stop
AC 5aAuxiliary Control circuit
AC 5bElectrical discharge Lamp
AC 6aElectrical Incandescent Lamp
AC 6bTransformer Switching
AC 7aSwitching of Capacitor Bank
AC 7bSlightly Inductive Load in Household
AC 5aMotor Load in Household
AC 8aHermetic refrigerant compressor motor with Manual Reset O/L  Relay
AC 8bHermetic refrigerant compressor motor with Automatic Reset O/L  Relay
AC 12Control of Resistive Load & Solid State Load
AC 13Control of Resistive Load & Solid State Load with Transformer Isolation
AC 14Control of small Electro Magnetic Load (<72 VA)
AC 15Control of Electro Magnetic Load (>72 VA)

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12. Contactor coil

Coil VoltageSuffix
24 VoltT
48 VoltW
110 to 127 VoltA
220 to 240 VoltB
277 VoltH
380 to 415 VoltL

Go back to Rules ↑


author-pic

jiguparmar - Jignesh Parmar has completed his B.E(Electrical) from Gujarat University. He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 12 years experience in Transmission -Distribution-Electrical Energy theft detection-Electrical Maintenance-Electrical Projects (Planning-Designing-Technical Review-coordination -Execution). He is Presently associate with one of the leading business group as a Assistant Manager at Ahmedabad,India. He has published numbers of Technical Articles in "Electrical Mirror", "Electrical India", "Lighting India", "Industrial Electrix"(Australian Power Publications) Magazines. He is Freelancer Programmer of Advance Excel and design useful Excel base Electrical Programs as per IS, NEC, IEC,IEEE codes. 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.



2 Comments


  1. Narbheshankar Modha
    Dec 23, 2013

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


  2. 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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