Here are some common formulas that are frequently used in the field.
If you're mathematically challenged, using an online calculator can make these kinds of calculations super easy! You can access some great online calculators on your laptop computer when you're out in the field from the link near the bottom of this page. Just remember to check your preventive maintenance software updates often as well as your antivirus software definitions to keep your computer running smoothly at all times.
Options:
 Motor Formulas
 Transformer Formulas
E = Voltage / I = Amps /W = Watts / PF = Power Factor / Eff = Efficiency / HP = Horsepower
AC/DC Formulas  
To Find  Direct Current  AC /
1phase 115v or 120v 
AC /
1phase 208,230, or 240v 
AC 3
phase All Voltages 
Amps when Horsepower is Known 
HP x 746 E x Eff 
HP x 746 E x Eff X PF 
HP x 746 E x Eff x PF 
HP x 746 1.73 x E x Eff x PF 
Amps when Kilowatts is known 
kW x 1000 E 
kW x 1000 E x PF 
kW x 1000 E x PF 
kW x 1000 1.73 x E x PF 
Amps when kVA is known 
kVA x 1000 E 
kVA x 1000 E 
kVA x 1000 1.73 x E 

Kilowatts  I x E 1000 
I x E x PF 1000 
I x E x PF 1000 
I x E x 1.73 PF 1000 
KilovoltAmps  I x E 1000 
I x E 1000 
I x E x 1.73 1000 

Horsepower (output) 
I x E x Eff 746 
I x E x Eff x PF 746 
I x E x Eff x PF 746 
I x E x Eff x 1.73 x PF 746 
Three Phase Values 
For 208 volts x 1.732, use 360 For 230 volts x 1.732, use 398 For 240 volts x 1.732, use 416 For 440 volts x 1.732, use 762 For 460 volts x 1.732, use 797 For 480 Volts x 1.732, use 831 
E = Voltage / I = Amps /W = Watts / PF = Power Factor / Eff = Efficiency / HP = Horsepower
AC Efficiency and Power Factor Formulas  
To Find  Single Phase  Three Phase 
Efficiency  746 x HP E x I x PF 
746 x HP E x I x PF x 1.732 
Power Factor  Input Watts V x A 
Input Watts E x I x 1.732 
Power  DC Circuits 
Watts = E xI 
Amps = W / E 
Ohm's Law / Power Formulas  
P = watts I = amps R = ohms E = Volts 
Voltage Drop Formulas  
Single Phase (2 or 3 wire) 
VD =  2 x K x I x L CM 
K = ohms per mil foot (Copper = 12.9 at 75°) (Alum = 21.2 at 75°)
L = Length of conductor in feet 

CM=  2K x L x I VD 

Three Phase  VD=  1.73 x K x I x L CM 

CM=  1.73 x K x L x I VD 
Check out these Online Calculators!
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