Electrical Formulas

Here i discuss some of important electrical formulas.All this formulas are useful for basic calculation in Electrical Engineering including Voltage,Ampere,Power, efficiency,power factor and many more .I hope it can make your basic understanding about electrical calculation is clear.

Glossary :-

I = Amperes

E = Volts

kW = Kilowatts

kVA = Kilo volt-Amperes

HP = Horsepower

% eff. = Percent Efficiency

pf = Power Factor


Single-Phase

TO FIND :-

  • Amperes when kVA is known –>   I = kVA x 1000 / E
  • Amperes when horsepower is known –>  ( HP x 746) / ( E  x  % eff.  x pf )
  • Amperes when kilowatts are known –>  ( kW x 1000 ) / ( E x pf )
  • Kilowatts  –>  ( I x E x pf ) /1000
  • Kilovolt-Amperes  –>  ( I x E ) / 1000
  • Horsepower  –>  ( I x E x % eff. x pf  ) / 746
  • Watts   –>  E x I x pf
  • Energy Efficiency  –>  Load Horsepower x 746 / Load Input kVA x 1000
  • Power Factor  @ cos θ –>  Power Consumed /Apparent Power ( W / VA ) @ ( kW / kVA )

Two-Phase

TO FIND :-

  • Amperes when kVA is known –>   I = ( kVA x 1000 )  / ( E x 2 )
  • Amperes when horsepower is known –>  ( HP x 746) / ( E  x  2  x % eff.  x pf )
  • Amperes when kilowatts are known –>  ( kW x 1000 ) / ( E x 2 x pf )
  • Kilowatts  –>  ( I x E x 2 x pf ) /1000
  • Kilovolt-Amperes  –>  ( I x E x 2 ) / 1000
  • Horsepower  –>  ( I x E x 2 x % eff. x pf  ) / 746
  • Watts   –>  E x I x 2 x pf
  • Energy Efficiency  –>  Load Horsepower x 746 / Load Input kVA x 1000
  • Power Factor  @ cos θ –>  Power Consumed /Apparent Power ( W / VA ) @ ( kW / kVA )

Three-Phase

TO FIND :-

  • Amperes when kVA is known –>   I = ( kVA x 1000 )  / ( E x 1.73 )
  • Amperes when horsepower is known –>  ( HP x 746) / ( E  x  1.73  x % eff.  x pf )
  • Amperes when kilowatts are known –>  ( kW x 1000 ) / ( E x 1.73 x pf )
  • Kilowatts  –>  ( I x E x 1.73 x pf ) /1000
  • Kilovolt-Amperes  –>  ( I x E x 1.73 ) / 1000
  • Horsepower  –>  ( I x E x 1.73 x % eff. x pf  ) / 746
  • Watts   –>  E x I x 1.73 x pf
  • Energy Efficiency  –>  Load Horsepower x 746 / Load Input kVA x 1000
  • Power Factor  @ cos θ –>  Power Consumed /Apparent Power ( W / VA ) @ ( kW / kVA )


Others Formula

  • kW = hp x .746
  • Torque in lb-ft = hp x 5250 / rpm
  • Motor synchronous speed in rpm = 120 x Hz / number of poles
  • Three-phase full-load amp= hp x .746 / 1.73 x kV x effi ciency x power factor
  • Rated motor kVA = hp (.746) / efficiency x power factor
  • kW loss = hp (.746) (1.0 – effi ciency) / efficiency
  • kVA in-rush = percent in-rush x rated kVA
  • Approximate voltage drop (%) = motor kVA in-rush x transformer impedance / transformer kVA
  • Stored kinetic energy in kW-sec = 2.31 x (total Wk2) x rpm2 x 107
  • Inertia constant (H) in seconds = stored kinetic energy in kW-seconds / hp (.746)
  • Conversion factors: CV = (metric hp) = 735.5 watts = 75 kg-m/sec Wk2 (lb-ft) = 5.93 x GD2 (kg-m2)
  • Ventilating-air requirements: 100-125 cfm of 400C air at 1/2-in. water pressure for each kW of loss
  • Degrees C = (Degrees F-32) x 5/9
  • Degrees F = [(Degrees C) x 9/5 ] + 32