Appliance labels are written in watts, but wiring, fuses, and breakers are rated in amps. This calculator bridges the two for DC, single-phase, and three-phase AC, and explains why the same wattage can draw very different current depending on voltage, phase, and power factor.
Why watts, amps, and volts are not interchangeable
Voltage is the electrical "pressure," current is the flow of charge, and power is the rate of energy use. They are linked by P = V × I: power equals voltage times current. Because of that relationship, you cannot convert watts to amps without knowing the voltage. A 1,500 W load draws 12.5 A at 120 V but only 6.25 A at 240 V — same power, half the current, because the voltage doubled.
This is the core reason high-draw appliances use 240 V: halving the current means smaller conductors, smaller breakers, and lower resistive losses in the wire.
Power factor and apparent power
For DC and purely resistive AC loads (heaters, incandescent bulbs, toasters) the power factor is 1, so I = P ÷ V. But motors, transformers, and many electronics draw current that is out of phase with the voltage. Their apparent power (volt-amps) is larger than the real power (watts) they consume, and current follows apparent power. That is why the AC formula divides by V × pf: a load with pf 0.8 draws 25% more current than its wattage alone would suggest.
Why three-phase uses √3
In a balanced three-phase system the three line voltages are 120° apart. When power is expressed against the line-to-line voltage, the total real power is P = √3 × V_LL × I × pf, so the current is I = P ÷ (√3 × V_LL × pf). If instead you reference the line-to-neutral (phase) voltage, total power is simply three single-phase loads: P = 3 × V_LN × I × pf. Mixing up line-to-line and line-to-neutral voltage is the most common three-phase mistake, so always confirm which voltage your nameplate or meter reports.
From amps to a breaker size
Once you know the current, sizing the overcurrent device follows the NEC. For a continuous load (running three hours or more) the breaker and conductors must be rated at 125% of the load — equivalently, the load may use only 80% of the breaker rating. The calculator divides the current by 0.8 and rounds up to the next standard size (15, 20, 25, 30 A, …). This is a planning hint only: real circuit design must account for conductor temperature rating, ambient and bundling derates, motor and HVAC rules, and your local code amendments.