How it works
V = I × R. P = V × I = I² × R = V² / R. (Rearranged 12 ways to solve for any one quantity from any two others.)
The single relationship V = I × R combines with the power identity P = V × I to give twelve rearrangements — the calculator picks the one that matches the two values you entered and prints it in the result so you can see the work. Voltage is in volts, current in amperes, resistance in ohms and power in watts. The math is exact only for a purely resistive load (heaters, incandescent lamps, resistor banks); motors, dimmers, LED drivers and fluorescent ballasts draw reactive current, so their real power in watts is less than volts × amps by the power factor — for those, size to apparent power in volt-amps and verify the conductor against the conduit fill limits once you know the wire size.
Code references
- Continuous-load 125% / 80% rule NEC 2023, 210.20(A) — overcurrent protection ≥ noncontinuous + 125% continuous
- Standard breaker ratings NEC 2023, 240.6(A) — standard ampere ratings for fuses and breakers
FAQ
How many amps does a 1500 W heater draw on 120 V?
I = P / V = 1500 / 120 = 12.5 A. That fits a 15 A circuit for an ordinary load, but a space heater runs for hours, so it is a continuous load: NEC 210.20(A) limits a continuous load to 80% of the breaker, i.e. 12.5 / 0.8 = 15.6 A, which rounds up to a 20 A circuit. This is the calculator’s default scenario — it is already solved on the page.
What is the difference between watts and volt-amps (VA)?
Watts are real power (the heat or work actually produced); volt-amps are apparent power (volts × amps). For a purely resistive load they are equal, so Ohm’s law is exact. For a motor or any inductive load they differ by the power factor: a 1 hp motor might draw 1200 VA but only 1000 W. Size conductors and breakers to the volt-amp (current) figure, not the watt figure.
Can I use this calculator for a motor or a dimmer?
Only as a rough first pass. Motors, dimmers, LED drivers and fluorescent ballasts are reactive loads — their current is not simply V / R, and the resistance you measure cold is not the running impedance. Use the nameplate amperage or volt-amps for those, and apply the NEC motor rules (Article 430) rather than Ohm’s law.
Why does my result differ from my clamp-meter reading?
Three common reasons: the load is reactive so true power is below V × I; your meter reads RMS while a cheap meter may read average and mis-scale a non-sinusoidal waveform; or the actual supply voltage differs from the nominal value you entered (a heater on a 117 V outlet draws slightly less than on 120 V). For resistive loads at the real measured voltage, the calculator and a true-RMS meter agree.
This calculator covers resistive circuits and is provided for estimation and learning. Reactive loads, ambient derating and conductor ampacity are not modelled here. Always verify breaker and conductor sizing against the current NEC edition and local amendments with a licensed electrician or electrical engineer.