Electrical Thumb Rules You MUST Follow (Part 1)
Electrical Thumb Rules You MUST Follow (Part 1)

Electrical Thumb Rules For:

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

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Current Capacity of Equipment

  • 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)

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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 Volt
  • Resistance between Neutral and Earth <= 1Ω
  • Creepage Distance18 to 22mm/KV (Moderate Polluted Air) or
  • Creepage Distance = 25 to 33mm/KV (Highly Polluted Air)

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Minimum Bending Radius

  • Minimum Bending Radius for LT Power Cable = 12 x Dia of Cable.
  • Minimum Bending Radius for HT Power Cable = 20 x Dia of Cable.
  • Minimum Bending Radius for Control Cable = 10 x Dia of Cable.

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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.

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Lighting Arrestor

Arrestor have Two Rating:

  1. MCOV=Max. Continuous Line to Ground Operating Voltage.
  2. Duty Cycle Voltage. (Duty Cycle Voltage > MCOV).

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  • Current Rating of Transformer = KVA x 1.4
  • 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.

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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 at 1 meter.
  • 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.

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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).

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Quick Electrical Calculation

1HP = 0.746KWStar Connection
1KW = 1.36HPLine Voltage = √3 Phase Voltage
1Watt = 0.846 Kla/HrLine Current = Phase Current
1Watt = 3.41 BTU/HrDelta Connection
1KWH = 3.6 MJLine Voltage = Phase Voltage
1Cal = 4.186 JLine Current = √3 Phase Current
1Tone = 3530 BTU
85 Sq.ft Floor Area = 1200 BTU
1Kcal = 4186 Joule
1KWH = 860 Kcal
1Cal = 4.183 Joule

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About Author //


Jignesh Parmar

jiguparmar - Jignesh Parmar has completed M.Tech (Power System Control) ,B.E(Electrical). He is member of Institution of Engineers (MIE),India. Membership No:M-1473586.He has more than 13 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 Deputy 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.


  1. shah faisal
    Sep 29, 2015

    how i find cable for a suitable load like a if load is 23 which cable is-perfect for this load and how i find by formula ?

  2. gurjit
    Aug 11, 2015


  3. ankesh mishra
    Aug 02, 2015

    Tell me the short cut for calculating load for a flat

  4. Deepak Kumar
    Jul 06, 2015

    Please tell me how to find current rating of the conductor

  5. mohan dass
    Jul 03, 2015

    Can we use 2 run of 25 mm sq instead of 50 mm sq cable of Cu. If yes then tell me full details and If not then tell me also full details.

  6. manish chovatia
    May 21, 2015

    How many distance between transformer and diesel genrator as per ie rules

  7. sowmya
    May 01, 2015

    I have a current transformer with 1 main meter outside the building and a control panel having 5 CT meters with black round coils inside the building. I want to know, A) when current pass through the Transformers, will the current be reduced or be the same and after that B) when current pass through the CT meters with black round coils, will the current be reduced or be the same. C) in the above process will the reading of the main meter be same or different, from the total reading of all the 5 CT meters.

  8. swapneswar pradhan
    Apr 07, 2015

    What is the simple formula for electrical, current , voltage

  9. swapneswar pradhan
    Apr 07, 2015

    Thanks sir u help for electrical problem .

  10. omar
    Feb 08, 2015

    we known low pf increase line current so does it increase kwh reading in metering…but pf varation is related also actually totally in reactive part…so what is the impact of pf in metering.

  11. Bassey Friday
    Jan 31, 2015

    Please .I have a 500KVA what % of load should I load it considering the fact that max load is about 700Amp

    • Etop Inyang
      Aug 22, 2015

      Is your generator sets for emergency supply or prime supply? For Prime you can load up to 100% and 70% for emergency generator sets.

  12. mdmukhter
    Jan 03, 2015

    I want to take part in to the technical debates and weekly technical discussion.

  13. abdelsalam mikky
    Dec 22, 2014

    How diesel engine frame should be grounded (I think shall be two earthing diagonal points ) according to NFPA 70 (NEC), could you please provide the supporting standard cod of this

    • Edvard
      Jan 03, 2015

      Every Genset has predefined points to ground engine frame. Don’t forget to ground gas reservoir if it exist.

  14. madhu
    Nov 18, 2014

    How to calculate Cable sizes for the Below in thumb rule,
    01.Single core cable above 30Sqmm
    02.Three phase cable size.
    Pls send it with a clear Example

  15. rahul
    Oct 15, 2014

    what is maximum distance between VCB panel and transformer at 11 kv voltage supply using HT cable of size 3cX95sqmm. Is it ok that transformer is installed at 150 meter away from VCB panel.

    • ali
      Nov 08, 2014

      minimum 6 mtrs

    Oct 06, 2014


    • kalpesh
      Aug 06, 2015

      Because in 3 phase 4 wire system only unbalance current pass from the nutural so current is never higher then half of the phase current,so purpose of aluminium saving its samller size then phase in power cable of higher size

  17. balu
    Oct 01, 2014

    Dear sir,
    Please send me your mobile number because I talk personally with you so I request you please give me your mobile number

    Vagh Balu R

    • Huj
      Oct 03, 2014

      Dear Jebalu, are ju crazy. There is inernet, no personalities!

  18. Arnel
    Sep 22, 2014

    what is the meaning of 2×6+4 for wire sizing?

  19. R Knight
    Sep 21, 2014

    What do you reckon on SWA 3 phase and Neutral Cable size?

    Circuit 1
    primary fuses, 16A and 20A = 36A
    However, connected load is 9kVA and 3kVA = 17.3A and average consumption is 3kVA and 2kVA respectively, giving ~ 8A

    Circuit 2
    Primary fuses, 40A, 16A, 16A and 16A = 88A
    However, nominal current is 33A, 6A, 8A and 11A, giving ~ 58A

    Circuit 2
    11kw fan with a 32amp supply

    on Cable run of 150m

  20. kushvendra tyagi
    Sep 17, 2014

    If the load requirement is 1200 kva , what kind of independent feeder should be considered , 11th or 33 kv

  21. Glenn
    Sep 03, 2014

    Hi need clarification to choose between 3 core, 3.5 core and 4 core for cable selection application. I know we choose 3.5 core and above in case of unbalanced loads any advice on making selection more clear is greatly appreciated

  22. Thuong.nguyen
    Jul 12, 2014

    1 Phase Motor draws Current = 7Amp per HP.
    Full Load Current of 1 Phase Motor = HPx6
    I think full load current of 1 phase is too high, for examples, in catalog of manufacture, current of 1 phase motor 4HP only about 12A. Moreover, please clarify why percent of draw current and full load current is small (7/6), as i know that factor is 3/1.( In my country, there used 220V for line voltage and 380V for phase voltage). The same question for 3 phase motor.

  23. Muhammad Bilal
    Apr 15, 2014

    What is the difference between line and phase?

    • Ganesh Kaneriya
      Jul 18, 2014

      line to line voltage means voltage between 2 line
      and Phase voltage means voltage between 1 line and neutral

  24. neeraj kumar
    Apr 06, 2014

    thanks 4 d info it was really
    help full we all must share
    these things so we could get best of

  25. Joe C.
    Dec 25, 2013

    Please I have some business proposals for the author.
    Please get in touch with me at jescil@hotmail.com

  26. Annu
    Oct 28, 2013

    Thanks Dear sharing Such valuable data

  27. nmr
    Oct 03, 2013

    sirs, for aluminium cables and wires current carrying was not indicated in the thumb rules.

  28. aakifnouman
    Sep 02, 2013

    Thank You For This and for those u cant understand this is only a thumb rule just to understand and remember

  29. sumitfloyd
    Sep 02, 2013

    On the section current capacity of Equipments, it is written
    # 1 Phase Motor draws Current = 7Amp per HP.
    # 3 Phase Motor draws Current = 1.25Amp per HP.

    I am not quite clear on these points. Please clarify. Does it mean if we have 3HP single phase motor then it will draw 21Amp current throughout its operation or during its starting?

  30. kriloter
    Aug 28, 2013

    These rules could be a little bit tricky unless you don’t know further connections …
    Not all of them are aplicable in a different distribution systems worldwide, especially constants like •Current Rating of Transformer = KVA x 1.4, etc.
    I suggest author should notice the conditions and input values in which are these rules aplicable and users should modify them according to a specific distribution system.

    • alam
      Mar 14, 2014

      well said mate.!

  31. shaijal100
    Aug 24, 2013

    pls tell me the difference between wire and cable.You mentioned different capacity for cu wire and a cable. You mean cable with aluminium conductor or something else..?Pls tell me..

    • kriloter
      Aug 28, 2013

      cable is bunch of wires :-)

  32. manjunatha_pm
    Aug 24, 2013

    Please give me the solution for below mentioned problem..
    Actually in our plant we having two 500 kva DG set and One dg is new one and other one is old …efficiency is different, so when switch on both and synchronizing.. some problem is occurring and i am not able to synchronizing. .. please give any solution for this problem
    my mail id is manjunathapm8@gmail.com

    • srinivas subramani
      Nov 19, 2013

      Dear friend, just find the settings configuration in PCC2100 relay for parameter settings:)

  33. ChAliGhafoor
    Aug 23, 2013

    kindly inform frm where did u get this useful information?

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