Electrical Thumb Rules (Part 2)

Electrical Thumb Rules

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 (I_av) = 0.637 x Im (Im = Max.Value)
• RMS Value of Sinusoidal Current (I_rms) = 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 (I_sc) =
  (0.094 x Cable Dia in Sq.mm) /√ 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 = 5.5x√((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 Factor = Average 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 li="">
• 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