Watts to Amps Calculator
Both directions, with the power factor that trips breakers.
Work out Watts to Amps. Both directions, with the power factor that trips breakers. Free, with no account and nothing to download.
1 for heaters and lights; around 0.8 for motors.
This is a unit conversion, not a circuit design. Breaker and conductor sizing depend on continuous-load rules, ambient temperature, insulation type and the code edition in force where you are.
Current
10.00 A
120 V, single phase at 1.00 PF
At unity power factor this is the simple division. Real motors run nearer 0.8, which would raise the current by 25% for the same wattage — set the power factor to see it.
How the Watts to Amps Calculator works
For DC and purely resistive loads, amps are watts divided by volts and that is the whole story. For real AC loads it is not: motors and electronics draw current out of phase with the voltage, so the current is higher than the wattage alone implies. That ratio is the power factor, and leaving it out is why a motor trips a breaker the arithmetic said was fine.
Also known as: amps to watts calculator · amp calculator · watts to amps conversion · three phase amps calculator
The calculation itself
For DC and purely resistive loads, amps are watts divided by volts. That is the case everyone knows and it is correct as far as it goes.
For AC with a reactive load, current and voltage are not in phase, so the useful power is less than the product of their magnitudes. The ratio between them is the power factor, and the current is watts divided by volts times power factor.
Three phase adds the square root of three, because the three line voltages sit 120 degrees apart and the line-to-line voltage is √3 times the line-to-neutral value. It is geometry rather than a correction factor.
In practice
1,200 W at 120 V is exactly 10 A at unity power factor.
Give it a power factor of 0.8 — typical for a motor — and the same 1,200 W draws 12.5 A. Twenty-five percent more current for identical useful work, and that current is what the breaker sees.
Three phase: 10 kW at 480 V and 0.9 power factor is 10,000 ÷ (1.732 × 480 × 0.9) = 13.36 A. Substantially less current than single phase would need for the same power, which is much of why industrial supplies are three phase.
Why motors trip breakers the arithmetic allowed
Two effects compound. Power factor raises the running current above what the wattage implies, and the starting surge raises it much further for a second or two.
An induction motor draws several times its running current at start, while it overcomes inertia and before back-EMF builds. A breaker sized on running watts alone sees that as a fault.
Which is why motor circuits use time-delay protection and are sized on locked-rotor characteristics rather than on running load. The conversion here tells you the steady current; it does not tell you what the circuit needs.
Where the figure deceives
It is a unit conversion, not a circuit design. Continuous loads are conventionally sized at 125% of their rating, ambient temperature derates conductors, and bundled conductors derate further — none of which appears here.
Power factor also varies with load. A motor running lightly loaded has a considerably worse power factor than the same motor near its rating, so a single figure is an approximation of a moving quantity.
And nameplate wattage is a maximum. Real draw depends on what the device is doing.
Acting on it
Use unity power factor only for genuinely resistive loads — heaters, incandescent lamps, kettles.
For anything with a motor, use the nameplate power factor if given and around 0.8 if not, then treat the result as the running figure rather than the design figure.
Size circuits from the code, using this to understand the load rather than to specify the protection.
Frequently asked questions
How do I convert watts to amps?
Divide watts by volts for DC or a resistive load: 1,200 W at 120 V is 10 A. For AC with a power factor, divide by volts times the power factor. For three phase, divide by the square root of three times volts times power factor.
What is power factor?
The ratio of real power to apparent power, between 0 and 1. Heaters and incandescent bulbs are near 1; motors run around 0.8, and the same wattage then draws 25% more current.
Why does my motor draw more current than the watts suggest?
Power factor, and starting surge. A motor pulls several times its running current for a second or two at start, which is what nuisance-trips a breaker sized on running watts alone.
Why the square root of three for three phase?
Because the three line voltages are 120 degrees apart, so the line-to-line voltage is √3 times the line-to-neutral value. It falls out of the geometry rather than being a fudge factor.
Can I use this to size a breaker?
No. Breaker and conductor sizing depend on continuous-load rules, ambient temperature, insulation type and code, and this is a unit conversion. Use it to understand the load, then the code book to size the circuit.
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