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 anti-virus 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
Kilovolt-Amps   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)
Note: K value changes with temperature. See Code chapter 9, Table 8

L = Length of conductor in feet

I  = Current in conductor (amperes)

CM
= Circular mil area of conductor

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!


If there is anything you would like to add or if you have any comments please feel free to email E.T.E. at http://www.elec-toolbox.com/email.htm.

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