Electrical Useful Equations


  • 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.
  • Short Circuit Level of Cable in KA (Isc)=(0.094xCable Dia in Sq.mm)/√ Short Circuit Time (Sec)
  • Cable Voltage Drop(%)=(1.732xcurrentx(RcosǾ+jsinǾ)x1.732xLength (km)x100)/(Volt(L-L)x Cable Run.
  •  Size of Cable according to Short circuit (for 11kV,3.3kV only)

  • Short circuit verification is performed by using following formula:
  • Cross Section area of Cable (mm2)S = x√t / K
  • Where:
  • t = fault duration (S)
  • I = effective short circuit current (kA)
  • K = 0.094 for aluminum conductor insulated with XLPE
  • Example: Fault duration(t)= 0.25sec,Fault Current (I) = 26.24 kA
  • Cross Section area of Cable = 26.24 x √ (0.25) / 0.094= 139.6 sq. mm
  • The selected cross sectional area is 185 sq. mm.
  • 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)
  • 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).
  • Protection radius of Lighting Arrestor = √hx (2D-h) + (2D+L). Where h= height of L.A, D-distance of equipment (20, 40, 60 Meter), L=Vxt (V=1m/ms, t=Discharge Time).
  • Size of Lighting Arrestor= 1.5x Phase to Earth Voltage or 1.5x (System Voltage/1.732).
  • 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. 
  • Fault Level at TC Secondary=TC (VA) x100 / Transformer Secondary (V) x Impedance (%)
  • 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).
  • Others:

  • 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)=0.637xIm (Im= Max.Value)
  • RMS Value of Sinusoidal Current(Irms)=0.707xIm (Im= Max.Value)
  • A.C Current=D.C Current/0.636.
  • Phase Difference between Phase= 360/ No of Phase (1 Phase=230/1=360°,2Phase=360/2=180°)
  • Most Economical Voltage at given Distance=5.5x√ ((km/1.6)+(kw/100))
  • Maximum Voltage of the System= 1.1xRated Voltage (Ex. 66KV=1.1×66=72.6KV)
  • Spacing of Conductor in Transmission Line (mm) = 500 + 18x (P-P Volt) + (2x (Span in Length)/50).
  • 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)
  • 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)x100)/Average Voltage
  • Ex: 462V, 463V, 455V, Voltage Variation= ((460-455) x100)/455=1.1%.
  • Current Variation(10%)= ((Average Current-Min Current)x100)/Average Current
  • Ex:30A,35A,30A, Current Variation=((35-31.7)x100)/31.7=10.4%
  • Motor Full Load Current= Kw /1.732xKVxP.FxEfficiency

Quick Electrical Calculation


Star Connection

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

Delta 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 

Cable Coding (IS 1554):( A2XFY / FRLS / FRPVC / FRLA / PILC)






Flat Armoured


Wire Armoured


Outer PVC Insulation Sheath


Steel Round Wire


Steel double round wire Armoured


Steel double Strip Armoured


Fire Retardation


Low Smoke


Low Acid Gas Emission


Non Magnetic round wire Armoured


Non Magnetic Flat wire Armoured


Double Steel Round Wire Armoured