Convert watts to amps for DC, single-phase AC and three-phase circuits. Enter the power, the voltage and, for AC, the power factor. Free, private, and runs entirely in your browser.
Current is power divided by voltage. In AC circuits the power factor reduces the real power delivered for a given current, and three-phase systems add a factor of the square root of 3 for line-to-line voltage.
How many amps is 1000 watts? At 120 V DC, or an AC power factor of 1, it is 8.33 A. At 240 V it is 4.17 A.
What power factor should I use? Use 1 for resistive loads such as heaters and incandescent lights. Motors and electronics are often 0.8 to 0.95.
A 1,200 W heater runs on 120 V. Current = 1,200 ÷ 120 = 10 A. A motor drawing 1,200 W at 120 V with a power factor of 0.8 needs 1,200 ÷ (120 × 0.8) = 12.5 A. A three-phase load of 10,000 W on 400 V with a power factor of 0.9 draws 10,000 ÷ (1.732 × 400 × 0.9) ≈ 16.04 A.
Can I convert watts to amps without voltage? No. Watts and amps are linked by voltage, so you need it, or a known resistance.
Is this enough to choose a breaker? It gives the load current only. Breaker and wire sizing include safety margins and code rules, so consult an electrician.
The watt is named for James Watt, the Scottish engineer whose steam engine improvements made power a practical concept, and the ampere honors the French physicist André-Marie Ampère, a founder of electrodynamics in the 1820s. International electrical congresses fixed the units in the late 19th century, which is why P = V × I works the same worldwide. Three-phase power spread in the 1890s because it delivers power more smoothly and needs less copper per kilowatt.
Use this calculator to estimate the current an appliance draws, to check whether a circuit can carry a load, or to compare 120 V and 240 V options. A 1,500 W space heater on 120 V draws 12.5 A, which is most of a typical 15 A circuit, so it should not share the circuit with other heavy loads. A common rule limits continuous loads to 80% of the breaker rating.
What is the difference between watts and volt-amps? Watts are real power. Volt-amps are voltage times current, and equal watts only when the power factor is 1.
Why does three-phase use the square root of 3? The three line-to-line voltages are 120 degrees apart, so combining them gives a factor of about 1.732.