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Home / Technical Articles / Electrical Thumb Rules You MUST Follow (2)
Electrical Thumb Rules You MUST Follow (Part 2)
Electrical Thumb Rules You MUST Follow (Part 2)

Continued from first part: Electrical Thumb Rules You MUST Follow (Part 1)

Useful Electrical Equations

  • For Sinusoidal Current: Form Factor = RMS Value/Average Value = 1.11
  • For Sinusoidal Current: Peak Factor = Max Value/RMS Value = 1.414
  • Average Value of Sinusoidal Current (Iav) = 0.637 x Im (Im = Max.Value)
  • RMS Value of Sinusoidal Current (Irms) = 0.707 x Im (Im = Max.Value)
  • A.C Current = D.C Current/0.636.
  • Phase Difference between Phase = 360/ No of Phase (1 Phase=230/1=360°, 2 Phase=360/2=180°)
  • Short Circuit Level of Cable in KA (Isc) =
    0.094 x Cable Dia in /√ Short Circuit Time (Sec)
  • Max.Cross Section Area of Earthing Strip (mm2) = √(Fault Current x Fault Current x Operating Time of Disconnected Device ) / K
    K = Material Factor, K for Cu = 159, K for Al = 105, K for steel = 58 , K for GI = 80
  • Most Economical Voltage at given Distance = 5.5 x √ ((km/1.6) + (kw/100))
  • Cable Voltage Drop (%) =
    1.732 x current x (RcosǾ+jsinǾ) x 1.732 x Length (km) x 100) / (Volt(L-L) x Cable Run.
  • Spacing of Conductor in Transmission Line (mm) = 500 + 18 x (P – P Volt) + (2 x (Span in Length)/50).
  • Protection radius of Lighnting Arrestor = √h x (2D-h) + (2D+L).
    Where h= height of L.A, D-distance of equipment (20, 40, 60 Meter), L=V x t (V=1m/ms, t=Discharge Time).
  • Size of Lightning Arrestor = 1.5x Phase to Earth Voltage or 1.5 x (System Voltage/1.732).
  • Maximum Voltage of the System = 1.1xRated Voltage (Ex. 66KV = 1.1 × 66 = 72.6KV)
  • Load FactorAverage Power/Peak Power
  • If Load Factor is 1 or 100% = This is best situation for System and Consumer both.
  • If Load Factor is Low (0 or 25%) = you are paying maximum amount of KWH consumption. Load Factor may be increased by switching or use of your Electrical Application.
  • Demand Factor = Maximum Demand / Total Connected Load (Demand Factor <1)
  • Demand factor should be applied for Group Load
  • Diversity Factor =
    Sum of Maximum Power Demand / Maximum Demand (Demand Factor >1)
    Diversity factor should be consider for individual Load
  • Plant Factor (Plant Capacity) = Average Load / Capacity of Plant
  • Fusing Factor = Minimum Fusing Current / Current Rating (Fusing Factor>1).
  • Voltage Variation (1 to 1.5%) = ((Average Voltage – Min Voltage) x 100)/Average Voltage
    Ex: 462V, 463V, 455V, Voltage Variation= ((460 – 455)  x 100)/455 = 1.1%.
  • Current Variation (10%) = ((Average Current – Min Current) x 100)/Average Current
    Ex: 30A,35A,30A, Current Variation = ((35-31.7) x 100)/31.7 = 10.4%
  • Fault Level at TC Secondary
    = TC (VA) x 100 / Transformer Secondary (V) x Impedance (%)
  • Motor Full Load Current = Kw /1.732 x KV x P.F x Efficiency

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More Information

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 20 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.


  1. Swapnendu Sil
    May 04, 2016

    Hi Sir,
    I want to know what cable or wire can be used for 32 amp industrial sockets

    • Ali
      Feb 07, 2018

      For single phase you can use 3core 6sqmm cable or for 3phase you can use 5core 6 sqmm cable

  2. surinderjit singh
    Aug 14, 2015

    Dear Sir
    please let me know a lightening Arrestor fixed on a building of 50 Feet height and height of lightening Arrestor is 10 feet , how much area it will cover for protection purpose.

  3. Hasmukh Patel
    Jul 06, 2015

    Really very good knowledge, that helps to know even very difficult to find solution.

  4. deshmukh V.
    Sep 12, 2014

    thanks for ‘useful’ information.

  5. Rajan.K
    Jul 28, 2014

    The thumb rules are really great. thanks a lot for providing such information in a capsule

  6. NITIN
    May 30, 2014

    Size of Lightning Arrestor = 1.5x Phase to Earth Voltage or 1.5 x (System Voltage/1.732)

    Dec 16, 2013

    will reply once sure

  8. aakifnouman
    Sep 02, 2013

    A.C Current = D.C Current/0.636

    what is 0.636??

  9. sumitfloyd
    Sep 02, 2013

    A.C Current = D.C Current/0.636.
    Please clarify this relation on how do you get it?

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