Electrical Thumb Rules-(Part 1).

  • Cable Capacity:

  • For Cu Wire Current Capacity (Up to 30 Sq.mm) = 6X Size of Wire in Sq.mm
  • Ex. For 2.5 Sq.mm=6×2.5=15 Amp, For 1 Sq.mm=6×1=6 Amp, For 1.5 Sq.mm=6×1.5=9 Amp
  • For Cable Current Capacity = 4X Size of Cable in Sq.mm ,Ex. For 2.5 Sq.mm=4×2.5=9 Amp.
  • Nomenclature for cable Rating = Uo/U
  • where Uo=Phase-Ground Voltage, U=Phase-Phase Voltage, Um=Highest Permissible Voltage
  • Current Capacity of Equipments:

  • 1 Phase Motor draws Current=7Amp per HP.
  • 3 Phase Motor draws Current=1.25Amp per HP.
  • Full Load Current of 3 Phase Motor=HPx1.5
  • Full Load Current of 1 Phase Motor=HPx6
  • No Load Current of 3 Phase Motor =30% of FLC
  • KW Rating of Motor=HPx0.75
  • Full Load Current of equipment =1.39xKVA (for 3 Phase 415Volt)
  • Full Load Current of equipment =1.74xKw (for 3 Phase 415Volt)
  • Earthing Resistance:

  • Earthing Resistance for Single Pit=5Ω ,Earthing Grid=0.5Ω
  • As per NEC 1985 Earthing Resistance should be <5Ω.
  • Voltage between Neutral and Earth <=2 Volts
  • Resistance between Neutral and Earth <=1Ω
  • Creepage Distance=18 to 22mm/KV (Moderate Polluted Air) or
  • Creepage Distance=25 to 33mm/KV (Highly Polluted Air)
  • Minimum Bending Radius:

  • Minimum Bending Radius for LT Power Cable=12xDia of Cable.
  • Minimum Bending Radius for HT Power Cable=20xDia of Cable.
  • Minimum Bending Radius for Control Cable=10xDia of Cable.
  • Insulation Resistance:

  • Insulation Resistance Value for Rotating Machine= (KV+1) MΩ.
  • Insulation Resistance Value for Motor (IS 732) = ((20xVoltage (L-L)) / (1000+ (2xKW)).
  • Insulation Resistance Value for Equipment (<1KV) = Minimum 1 MΩ.
  • Insulation Resistance Value for Equipment (>1KV) = KV 1 MΩ per 1KV.
  • Insulation Resistance Value for Panel = 2 x KV rating of the panel.
  • Min Insulation Resistance Value (Domestic) = 50 MΩ / No of Points. (All Electrical Points with Electrical fitting & Plugs). Should be less than 0.5 MΩ
  • Min Insulation Resistance Value (Commercial) = 100 MΩ / No of Points. (All Electrical Points without fitting & Plugs).Should be less than 0.5 MΩ.
  • Test Voltage (A.C) for Meggering = (2X Name Plate Voltage) +1000
  • Test Voltage (D.C) for Meggering = (2X Name Plate Voltage).
  • Submersible Pump Take 0.4 KWH of extra Energy at 1 meter drop of Water.
  • Lighting Arrestor:

  • Arrestor have Two Rating=
  • (1) MCOV=Max. Continuous Line to Ground Operating Voltage.
  • (2) Duty Cycle Voltage. (Duty Cycle Voltage>MCOV).
  • Transformer:

  • Current Rating of Transformer=KVAx1.4
  • Short Circuit Current of T.C /Generator= Current Rating / % Impedance
  • No Load Current of Transformer=<2% of Transformer Rated current
  • Capacitor Current (Ic)=KVAR / 1.732xVolt (Phase-Phase)
  • Typically the local utility provides transformers rated up to 500kVA For maximum connected load of 99kW,
  • Typically the local utility provides transformers rated up to 1250kVA For maximum connected load of 150kW.
  • The diversity they would apply to apartments is around 60%
  • Maximum HT (11kV) connected load will be around 4.5MVA per circuit.
  • 4No. earth pits per transformer (2No. for body and 2No. for neutral earthing),
  • Clearances, approx.1000mm around TC allow for transformer movement for replacement. 
  • Diesel Generator:

  • Diesel Generator Set Produces=3.87 Units (KWH) in 1 Litter of Diesel.
  • Requirement Area of Diesel Generator = for 25KW to 48KW=56 Sq.meter, 100KW=65 Sq.meter.
  • DG less than or equal to 1000kVA must be in a canopy.
  • DG greater 1000kVA can either be in a canopy or skid mounted in an acoustically treated room
  • DG noise levels to be less than 75dBA @ 1meter.
  • DG fuel storage tanks should be a maximum of 990 Litter per unit Storage tanks above this level will trigger more stringent explosion protection provision.
  • Current Transformer:

  • Nomenclature of CT:
  • Ratio: input / output current ratio
  • Burden (VA): total burden including pilot wires. (2.5, 5, 10, 15 and 30VA.)
  • Class: Accuracy required for operation (Metering: 0.2, 0.5, 1 or 3, Protection: 5, 10, 15, 20, 30).
  • Accuracy Limit Factor:
  • Nomenclature of CT: Ratio, VA Burden, Accuracy Class, Accuracy Limit Factor.Example: 1600/5, 15VA 5P10  (Ratio: 1600/5, Burden: 15VA, Accuracy Class: 5P, ALF: 10)
  • As per IEEE Metering CT: 0.3B0.1 rated Metering CT is accu­rate to 0.3 percent if the connected secondary burden if imped­ance does not exceed 0.1 ohms.
  • As per IEEE Relaying (Protection) CT: 2.5C100 Relaying CT is accurate within 2.5 percent if the secondary burden is less than 1.0 ohm (100 volts/100A).

Quick Electrical Calculation


Star Connection

1KW=1.36HP Line Voltage=√3 Phase Voltage
1Watt=0.846 Kla/Hr Line Current=Phase Current
1Watt=3.41 BTU/Hr

Delta Connection

1KWH=3.6 MJ Line Voltage=Phase Voltage
1Cal=4.186 J Line Current=√3 Phase Current
1Tone= 3530 BTU  
85 Sq.ft Floor Area=1200 BTU  
1Kcal=4186 Joule  
1KWH=860 Kcal  
1Cal=4.183 Joule  

About Jignesh.Parmar
Jignesh Parmar has completed M.Tech (Power System Control), B.E(Electrical) from Gujarat University. He has more than 13 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.

60 Responses to Electrical Thumb Rules-(Part 1).

  1. Mohammadasif Thakor says:

    Very useful information

  2. MASILAMANI.V says:

    excellent for this words

  3. Chisi says:

    Man, you are the best..I never knew most of these rules.
    Long live Jignesh!

  4. HEENA says:

    3 PHASE MOTOR DRAWS CURRENTS – 2.5 AMP/hp.if it is 230 v
    and 1.5A/HP If it is 440 volt .

    Can you also please add formulas too..

  5. SUBRAMANIAN.P says:

    Really the topics is very useful to me

  6. arvind says:


  7. arvind says:

    very imp info

  8. shaileshkumar.patel@Linde-LE.com says:

    Dear Jignesh
    I am regularly getting updateds from this websites. It is very useful.
    Can you please reply me for following query.

    As per Indian standrad : Emergency standby duty generator ( for
    example : 1250 KVA,415 V) can be loaded for full load (i.e 1250 KVA-
    constant or vairiable load ) in case of
    main power source outgae for 3 days. If any supporting IS number is
    available -please advise.
    In case Alternate relevant international standard is clarifying this
    query -please advise.

    With best regards

    Shailesh G Patel

  9. siddu.biradar says:

    it is very useful for electrical engineer and thank you very much Mr jignesh

  10. Suresh says:

    Very useful in short. Thanks

  11. boy sandra says:

    great refferences, i joined your page on facebook and followed your blog by email

  12. Praveen Malviya says:

    Sir please help me… My question is, what is the suitable stater and breaker for 400kw,1440rpm induction motor…

  13. engelect says:

    Thanks for this rule of thumbs. It is really usefull for short assesment.

  14. Murali.k says:

    Thanks yery useful for all electrical engineers in the world

  15. malebogo moupi says:

    hie sir help me here’a single phase load consists of 1. 12kw of lighting & heating at unity pf.2. 8kw of motors at 0.8pf lagging & 3. 10kva of motors at 0.7 pf lagging.calculate (a)kwt (b)kvart (c)kvat (d)overall pf and the total supply current at 240v.my main wory is how to come with 30.3total kva.13.1total kvar &overall pf pliz help!

  16. vamsi says:

    as a site engineer this info is very helpfull to me ..

  17. Rasika says:

    what are electrodynamic stresses?

  18. Almoutaz Safwan says:

    It’s useful info. thanks

  19. Dharmendra Kumar says:


  20. jayesh says:

    so greatful job you had done.

    Jay jay garvi gujarat

  21. chandresh says:

    thank u sir…………………………….

  22. Nilesh Prajapati says:

    Thank you sir

  23. Steve says:

    thank you for all your hard work in putting this together….Greetings from West Texas

  24. surender says:

    in what case voltage reading between ground and neutral up to 230 at distribution transformer L T side .The transformer ratio is 33 kv / .415 kv .

  25. Ravindra Kumar says:

    thank you very much sir………

  26. yadvendra sharma says:

    All the three phase are generate in the power plant…Sir plz tell me about the neutral generation…
    Plz reply soon.

  27. Sumit Sharma says:

    Dear Sir
    i am working in residential building my one flat load is 15 kw so how to calculate diversity factor.


    Sumit Sharma

  28. Bhavin Patel says:

    thanks for sharing such a information

  29. K.Gopinath says:

    Thank you sir, i am never know about this thumb rules.

  30. Alwin Paul says:

    Dear Mr. Jignesh

    ■1 Phase Motor draws Current=7Amp per HP.
    ■3 Phase Motor draws Current=1.25Amp per HP

    Can you explain the above information, please?

  31. Paramasivan Tsp says:

    Dear Mr. Jignesh,

    Kindly use reference and standards. This ‘ll help us to improve the knowledge about standards and it ‘ll also induce to search about the standards.

    -Paramasivan Tsp (My FB id)

  32. karan says:

    what about alluminium cable?

  33. prakash parmar says:

    Dear sir,
    I m gating regular mail Its a very use full information from for me

    Thank you

  34. sazzad.hossain89@gmail.com says:

    Which rule have you followed for cable current carrying capacity?

  35. ikoh says:

    thank you for all the information!!!

  36. kashyap raval says:

    Dear Sir,
    Thanks a lot for such technical information which will help to solve day to day workings.
    It is reallya most valuable and sound as well as perfect calculations ..
    Please be in touch by mail,
    I shall be grateful if send me such an electrical related informations,calculations etvc.

  37. Dear sir,

    Thanks for your lot of Technical information.

    One our customer in UK.Their asking for Below ground earthing calculation used BS (british standard).But I have IEEE80 format Ex sheet. Their not accepting IEEE80. Hence.Hence Kindly send the BS format for below ground earthing.

    This is very useful for Us.

    V.vasantha kumar.

  38. Jose says:

    Dear Sir,

    can you prepare thumb rule based on DEWA regulations.

  39. Mohanty says:

    Gud information for public knowladge.. Thanks lot.

  40. In domestic wiring, if neutral line is shotred with housing earth line, What problem we can face? Whether it will effect on KWH meter reading? Is there any chances of getting my electronic equipments damage?

  41. GK says:

    Excellent information. LIVE LONG DEAR ENGR JIGNESH. GK

  42. parag says:


  43. Aditya sharma says:

    its wonderful knowledge is here
    i m very thankful to u . Jagnesh ji

  44. Sourabh Singh says:

    Sir please suggest
    can we use elcb in horizontal mounting?

  45. taufik says:

    Thank you sir..!

    What is the minimum cable size that I can use for AL cable in building distribution. Please suggest me the reference standard also.

  46. Sanjay says:

    Thumb rule for line loss for 300 square mm cable

  47. ronak dave says:

    Cu wire current carrying capacity is wrong. If you take 25 sq.mm cable then according to you sir it should be 25*6 = 150 amp. But in the catlogue of polycab its showing 108 amp. How there is so much of difference?

  48. dipak says:

    What is this 6

  49. laxman maske says:

    very very important knowledge for us ,thank you sir.

  50. sivakumar A says:

    Hello sir….

    How to make the this kind thumb rule based on the reference document???
    sir i want know about detail manner each thumb rule or otherwise you mention standard number…sir… example u use copper current rating calculation=6* size of sq.mm…why it’s used 6….???

  51. Swapnendu Sil says:

    32 amp industrial socket what cable or wire we need? what about cu or aluminium cable ?

  52. Chakravarthi says:

    For cable current capacity example calculation is wrong

  53. rughnath prajapat says:

    very useful information thanks

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