Power Equations

Formulas for Determining Amperes, hp, kW and kVA

Common Electrical Terms

Ampere (I)             = unit of current or rate of flow of electricity 
Volt (E)               = unit of electromotive force
Ohm (R)                = unit of resistance 
                         Ohms Law: I  =  E / R (DC or 100% pf)
                       
% eff                  = motor efficiency   (Note: If unknown assume %eff x pf = 0.7 25 hp or less)
                                            (                                    0.8 above 25 hp  )
Megohm                 = 1,000,000 ohms 
Volt Amperes (VA)      = unit of apparent power 
                       = E x I (single-phase) 
                       = E x I x 1.73 (three-phase)
Kilovolt amperes (kVA) = 1000 volt - amperes 
Power Factor (pf)      = ratio of true to apparent power 
                       = W/VA or kW/kVA = COSØ(Ø = angle current leads or lags voltage)
Watthour (Wh)          = unit of electrical work 
                       = one watt for one hour 
                       = 3.413 Btu 
Kilowatt hour (kWh)    = 1000 watt hours 
Horsepower (hp)        = measure of time rate of doing work 
                       = equivalent of raising 33,000 pounds 1 foot in 1 minute 
                       = 746 watts

Motor Application Formulas

 

Single-phase Voltage Drop = Amps x (length of circuit in ft) x 2* x (ohms per 1000 ft) / 1000

*Change to 1.73 for three-phase
 Wire Size (AWG)(Stranded)
 Ohms / 1000 ft (at 77°F)
22
20
18
16
14
12
 16.46
10.35
6.512
4.095
2.576
1.620

Typical Single-phase And Polyphase Circuits


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