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
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|
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.