Watts to Amps Calculator
Current in amps from power in watts and voltage in volts
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What this calculator computes
Watts to amps conversion is the inverse of the power formula, solving for current when the power consumption and supply voltage are known. The relationship rearranges P = V × I into I = P / V for DC and AC single-phase circuits, and into I = P / (V × PF) once power factor is included for AC loads, and into I = P / (√3 × V_LL × PF) for balanced three-phase systems. Knowing the current draw is essential whenever you need to size a circuit breaker, choose conductor gauge, plan a generator transfer load, or verify that an appliance will not exceed the rated capacity of the outlet it plugs into. A 1500 W space heater on a 120 V US circuit pulls 12.5 A and is therefore right at the practical limit of a standard 15 A receptacle, while the same 1500 W heater on a 230 V European circuit pulls only 6.5 A and leaves comfortable headroom on a 16 A breaker. This calculator supports DC, AC single-phase, and AC three-phase modes, applies the appropriate power factor for AC loads, and returns the line current in amps along with the apparent volt-amperes the wires must actually carry. The current it returns is the steady-state RMS value, which is the figure NEC ampacity tables and breaker trip curves are calibrated against. Inrush current — the brief surge that motors and switching supplies pull during start-up — can be 3–10× higher and is not modelled here.
Calculator
The formula
Formula
I (A) = P / (V × PF) (3-phase: I = P / (√3 × V × PF))
Worked example
When to use this calculator
Use this calculator whenever you need to convert a known wattage rating into the current draw needed to size circuits, breakers, conductors, generators, or transfer switches. The most common cases are residential appliance circuits (water heaters, dryers, ranges, EV chargers), generator and inverter sizing where the load mix is in watts but the generator is rated in amps and volt-amperes, solar-system DC-side wiring where the panel and battery currents must clear the fuse and combiner ratings, and three-phase commercial machinery where the wattage is on the nameplate but the supply transformer is rated in amps. The calculator is also useful for checking that a planned addition to a circuit will not push the steady-state load past 80% of breaker capacity, the standard NEC continuous-duty derating applied to long-running loads. For inrush-current sizing on motor-start scenarios, multiply the steady-state result by the motor's locked-rotor current factor (typically 5–7×) and use a time-delay breaker.
Common input mistakes
- Forgetting NEC continuous-load derating. Loads running 3+ hours continuously must be sized at 125% of the calculated current, so a calculated 16 A heater needs a 20 A breaker on 12 AWG, not a 15 A breaker on 14 AWG. The calculator returns the raw steady-state current; multiply by 1.25 yourself when the load is continuous.
- Confusing line-to-line and line-to-neutral voltage in three-phase mode. The three-phase formula uses V_LL with √3; using V_LN without removing the √3 over-states current by 73%. If your voltage is given as 277 V (the US 480/277 wye phase voltage) or 230 V (the EU 400/230 wye phase voltage), you are looking at line-to-neutral and should switch to single-phase mode or convert to V_LL = V_LN × √3.
Frequently asked questions
How do I convert watts to amps?
For DC and AC single-phase circuits, divide the power in watts by the voltage in volts (and by the power factor for AC): I = P / (V × PF). For balanced three-phase circuits, divide by √3 × V_LL × PF, where V_LL is the line-to-line voltage. The result is the steady-state RMS current that NEC ampacity tables and breaker trip curves use; inrush and starting currents are higher and require separate consideration.
How many amps does a 1500 W appliance draw?
On a 120 V US circuit a 1500 W appliance draws 12.5 A (1500 / 120) at unity power factor, which is right at the practical 80% limit of a 15 A receptacle. On a 230 V European circuit the same 1500 W appliance draws only 6.5 A. On a 240 V US split-phase circuit it draws 6.25 A. The voltage of the supply circuit dominates the current, which is why high-wattage appliances in North America are wired to 240 V split-phase whenever possible.
Should I size the breaker to the calculated current exactly?
No — NEC requires breakers to be sized at the next standard rating above the calculated current, and continuous loads (3+ hours) must be derated to 80% of breaker capacity. A calculated 12.5 A continuous load needs 12.5 / 0.8 = 15.6 A of breaker, rounded up to a 20 A breaker on 12 AWG conductor. Sizing breakers exactly to load leaves no margin for transient spikes and risks nuisance trips.
What power factor should I use for motors?
Induction motors typically run between 0.7 and 0.85 power factor at full load, with smaller motors and partially-loaded motors trending toward the low end of that range. A 1 hp single-phase motor is often around 0.75; large three-phase industrial motors run 0.85–0.9. Motor nameplates list the actual PF; if unknown, 0.8 is a safe default that errs on the side of higher current. Premium-efficiency motors achieve 0.9+ PF.
Why does my generator say it can deliver more amps in DC than AC at the same wattage?
Generator AC outputs are typically rated in volt-amperes (VA) rather than watts, and the amp output is VA divided by voltage. A 5000 VA generator at 240 V AC delivers 20.8 A regardless of load PF. The same generator at 120 V DC could theoretically deliver 41.6 A if it had a DC output, because DC ignores PF and uses the full VA capacity. In practice, AC generators are derated to 80% real-power capacity for typical load mixes, not because the windings cannot deliver the current but because the loads draw reactive components.