How this calculation works
Reading it in the field
Voltage is pressure. Current is flow. Resistance is restriction. Hold any one constant and the other two trade off directly.
The version that matters most on a job is P = I²R, because it explains heat. Power lost in a connection goes up with the square of the current. Double the load on a loose lug and you get four times the heat. That is why a marginal termination that has been fine for years suddenly fails when someone adds a load, and it is why torque specs exist.
Where the straight relationship breaks
Ohm's law describes DC and purely resistive AC. Add anything inductive — a motor, a transformer, a ballast — and the current lags the voltage. Now P = V × I gives you volt-amperes, not watts, and you need power factor to get from one to the other.
That is the whole reason kVA and kW are different numbers. On a resistive load they are the same. On a motor they are not.
Useful conversions
1 horsepower is 746 watts. 1 kilowatt is 1,000 watts. The square root of 3 is 1.732. Those three, plus this calculator, cover most of what comes up.
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Frequently asked questions
Why does the calculator show horsepower?
Motor loads are stated in horsepower and everything else is in watts. 746 watts is one horsepower, so having both on screen saves a conversion when you are working between a nameplate and a load calculation.
Does Ohm's law work on AC?
On purely resistive AC loads, yes — heaters, incandescent lamps, resistive elements. Once you add inductance or capacitance the current shifts out of phase with the voltage, and V × I gives you volt-amperes rather than watts. You need power factor to bridge them.
What is the practical use of P = I²R?
It explains why connections fail. Heat at a termination rises with the square of the current, so a loose lug that runs warm at 20 amps runs four times as hot at 40. It is the reason torque matters and the reason thermal scans find problems before they become fires.
// DisclaimerThe National Electrical Code® (NEC®) and NFPA 70® are registered trademarks and copyrighted publications of the National Fire Protection Association (NFPA). This calculator is an independent educational tool and is not affiliated with, endorsed by, or sponsored by the NFPA. All code references are paraphrased for educational purposes only and do not reproduce the official text of any code edition. This tool is not a substitute for the official NFPA 70 publication. Electricians, contractors, and students are encouraged to purchase the official National Electrical Code from the NFPA at
nfpa.org for complete and authoritative code text. Always verify requirements with the code edition adopted by your jurisdiction and your Authority Having Jurisdiction (AHJ).