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Electrical Calculation Formulas

At Chapman Electric Supply, we’re committed to helping you get your job done right. Below you will find various charts to aid you in calculating your needs for a wide array of electrical applications. They are useful for both professional and hobby electricians. You can also find more articles in our resources section.

If you still have questions, contact us today!  We’re ready to help you tackle any problem. We have the know-how and many years of quality service and supply behind us.

AC/DC Formulas

Use these formulas to calculate amps, kilowatts, kilovolts, and horsepower for different voltages and phases

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

 

Key:
E = Voltage I = Amps W = Watts
PF = Power Factor Eff = Efficiency HP = Horsepower

 

Ohm's Law

Ohm’s Law consists of the mathematical relationship between Voltage, Current and Resistance. Ohm's Law states that for an electrical circuit, the current is directly proportional to the voltage and inversely proportional to the resistance.

Ohm’s Law Pie Chart

 Ohm's Law Chart

P = watts
I = amps
R = ohms
E = Volts

AC Efficiency & Power Factor Formulas

AC Efficiency

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

 

Key:
E = Voltage I = Amps W = Watts
PF = Power Factor Eff = Efficiency HP = Horsepower

Voltage Drop Formulas Chart

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

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

Note: K value changes with temperature.