SizingKitPower & current
Watts to Amps Calculator
Amperes are watts divided by volts on DC, by volts times power factor on single-phase AC, and by 1.732 times volts times power factor on three-phase — this free calculator runs all three from one form, with no signup. A 1,500 W resistive heater on 120 V draws 12.5 A; the same 1,500 W on a compressor at 0.80 power factor draws 15.6 A, and the conductor is sized on the second figure.
- 100% free
- No signup
- DC, 1-phase and 3-phase
- Editable power factor
- Reverse direction
Three-phase divides by √3 = 1.7321 as well as by the voltage, because the line-to-line voltage you measured is not the voltage across any one winding.
120 V and 240 V are the two halves of a US residential service. 208 V is what a single phase of a 120/208 V panel measures against another phase, not against neutral.
Watts unless you say otherwise — kW, hp and BTU/h are all accepted in this field.
Default 1.00 — Definitional. Power factor is real power over apparent power, and where the load is pure resistance — a heating element, an incandescent filament, a soldering iron — current and voltage stay in step, the two powers are the same figure and their ratio is exactly one.
Current drawn
12.5 A
Apparent power on the conductor
1.5 kVA
| Current | 12.5 A |
|---|---|
| Real power (what the meter bills) | 1,500 W |
| Real power in kilowatts | 1.5 kW |
| Mechanical equivalent | 2.01 hp |
| Heat equivalent | 5,118 BTU/h |
| Apparent power (what the wire carries) | 1,500 VA |
| Apparent power in kilovolt-amperes | 1.5 kVA |
Nothing is derated at unity: volt-amperes and watts are the same number here, so the identity collapses to 120 V and the figure you typed. Switch the load kind to a motor and watch the current climb without the wattage moving.
Nameplate watts to amperes, single-phase, unity power factor
Single-phase, computed rather than transcribed. Multiply a row by 1 ÷ PF for a load that is not resistive: at 0.80 the current is a quarter higher than the column shows.
| Load | 120 V | 208 V | 240 V | 277 V | 480 V |
|---|---|---|---|---|---|
| 100 W | 0.83 A | 0.48 A | 0.42 A | 0.36 A | 0.21 A |
| 250 W | 2.08 A | 1.2 A | 1.04 A | 0.9 A | 0.52 A |
| 500 W | 4.17 A | 2.4 A | 2.08 A | 1.81 A | 1.04 A |
| 750 W | 6.25 A | 3.61 A | 3.13 A | 2.71 A | 1.56 A |
| 1,000 W | 8.33 A | 4.81 A | 4.17 A | 3.61 A | 2.08 A |
| 1,200 W | 10 A | 5.77 A | 5 A | 4.33 A | 2.5 A |
| 1,440 W | 12 A | 6.92 A | 6 A | 5.2 A | 3 A |
| 1,500 W | 12.5 A | 7.21 A | 6.25 A | 5.42 A | 3.13 A |
| 1,800 W | 15 A | 8.65 A | 7.5 A | 6.5 A | 3.75 A |
| 2,000 W | 16.7 A | 9.62 A | 8.33 A | 7.22 A | 4.17 A |
| 2,400 W | 20 A | 11.5 A | 10 A | 8.66 A | 5 A |
| 3,000 W | 25 A | 14.4 A | 12.5 A | 10.8 A | 6.25 A |
| 3,600 W | 30 A | 17.3 A | 15 A | 13 A | 7.5 A |
| 4,500 W | 37.5 A | 21.6 A | 18.8 A | 16.2 A | 9.38 A |
| 5,500 W | 45.8 A | 26.4 A | 22.9 A | 19.9 A | 11.5 A |
| 7,200 W | 60 A | 34.6 A | 30 A | 26 A | 15 A |
277 V and 480 V are line-to-neutral and line-to-line on the same 277/480 V system; a 277 V column entry is one lighting circuit, a 480 V entry is a two-wire load across two phases of it.
How to turn a nameplate wattage into amperes
Three fields decide the answer, and the third is the one every other converter leaves out.
Pick the supply the device is connected to
DC removes the power factor field entirely, because a direct-current circuit has no phase angle for one to describe. Single-phase divides by the voltage alone; three-phase divides again by 1.7320508, since the line-to-line voltage on your meter is not the voltage across any one winding.
Enter the voltage the device actually sees
Use the nameplate voltage, not the panel label: a 240 V dryer element on a 120/240 V service sees the full 240 V, while its drum motor and timer sit at 120 V. A unit you type beats the field's assumption, so 4.16 kV is read as kilovolts.
Set the power factor from the load, not from habit
Leave it at 1.00 for anything that is only a resistance — elements, filaments, glue pots. Choose the motor prefill of 0.80 for a compressor, pump or fan, then replace it with the marked figure from the motor's own nameplate as soon as you can read one, because the same machine at part load falls well below the prefill.
What this converter computes, and from what
| Single-phase identity | I = W ÷ (V × PF). At 120 V and unity power factor, 1,500 W returns exactly 12.5 A. |
|---|---|
| Three-phase identity | I = W ÷ (√3 × V × PF), with √3 carried as 1.7320508. 15 kW at 480 V and PF 0.85 returns 21.2 A. |
| DC identity | I = W ÷ V, with no power-factor term offered. 100 W at 12 V returns 8.33 A. |
| Resistive prefill | Power factor 1.00, exact by definition — power factor is real power over apparent power, and those are one number in a pure resistance. |
| Motor prefill | Power factor 0.80, taken from the ISO 8528-1 generating-set rating convention, marked as varying and editable in place. |
| Horsepower reported alongside | 1 hp = 550 ft·lbf/s = 745.6999 W, computed from the foot, the pound-force and the second rather than typed in as 745.7. |
| Heat equivalent reported alongside | 1 W = 3.412142 BTU/h through the International Table BTU of 1,055.05585262 J, so 1,500 W of resistance heat is 5,118 BTU/h. |
| Printed chart | 80 computed cells: sixteen loads from 100 W to 7,200 W against 120, 208, 240, 277 and 480 V, sized for one sheet of paper. |
| Where it runs | Entirely in this browser tab. No nameplate figure you type is sent anywhere. |
Frequently asked questions
Why does my 1,500 W heater draw 12.5 A on 120 V but only 6.25 A on 240 V?
Because the wattage is fixed and the current is what adjusts. Power is volts times amps, so doubling the voltage across the same load halves the current it needs to deliver the same 1,500 W. This is the entire reason a shop puts a large heater or a welder on a 240 V circuit: the same heat travels down a smaller conductor.
Does power factor apply to a DC circuit?
No, and this page removes the field rather than setting it to one. Power factor describes how far the current waveform lags or leads the voltage waveform, and direct current has no waveform to shift. A converter that leaves a power factor box on a DC form is telling you something false about what it is doing.
My motor plate says 1,200 W and 12 A at 120 V, but the math gives 10 A. Which is right?
Both, and the gap is the power factor: 1,200 ÷ (120 × 12) = 0.83, so the plate has quietly told you its power factor. Type 0.83 into the field and the arithmetic lands back on the marked 12 A. When a nameplate gives you both the watts and the amps, you never need to assume a power factor at all.
Can I pick a breaker straight from the amperes this page returns?
Not on its own — this is the draw of one device, and a circuit is sized on everything connected to it plus the continuous-load rule. A load running three hours or more is taken at 125 percent under NEC 210.20(A), which turns a 16 A continuous draw into a 20 A minimum device. Build the schedule on the amperage calculator instead of adding the numbers by hand.
Which voltage do I use for a 120/240 V appliance like a range or a dryer?
Use 240 V for the heating elements and 120 V for whatever is fed from a leg to neutral. A dual-voltage appliance is really two circuits sharing one cord: the elements sit across both ungrounded conductors while the clock, lamp and control transformer sit between one of them and the neutral, which is why the cord has four wires and not three.
Why is the three-phase divisor 1.732 rather than 3?
Because the voltage you measure between two lines is not the voltage across one winding of the load. Line-to-line voltage is √3 times line-to-neutral in a balanced system, and the total of three phases works out to √3 × V(line) × I(line) rather than three times a single-phase result. Using 3 understates the current by about 42 percent, which is one wire size or two.
Can I get amps from watts without knowing the voltage?
No, and any page that offers to do it is guessing 120 V for you. The watt is a rate of energy and the ampere is a rate of charge; the voltage is the exchange rate between them, and without it the question has no numerical answer. If the nameplate genuinely has no voltage, take it from the circuit the device plugs into.
About watts, amps and the power factor between them
A watt measures energy per second and an ampere measures charge per second, so nothing converts one into the other without the voltage standing between them. That is why every form of this calculation — DC, single-phase, three-phase — is the same identity with a different constant in front of it, and why the honest presentation puts the identity on screen next to the number rather than hiding it behind a button. If you would rather solve the same triangle from a different pair of knowns, the volts amps watts calculator takes any two of the four and returns the rest.
The trap in alternating current is that a nameplate wattage is real power — the part that becomes heat, torque or light and that the utility meter bills — while the conductor, the breaker and the transformer all carry the apparent power, which is larger whenever the current does not stay in step with the voltage. Motors, welders, fluorescent ballasts and older switch-mode supplies all draw more current than their watt rating suggests, and the ratio between the two is the power factor. On a purely resistive load the two are the same figure and the distinction disappears, which is exactly why converters that quietly assume 1.0 look correct on a space heater and get a compressor wrong by a quarter.
Nameplates in kilowatts belong to a different world from nameplates in watts: a kW plate usually sits on industrial equipment, where the number is shaft output and an efficiency term has to be unwound before any current falls out, so that case is handled on the kW to amps calculator. And if the load you are converting will be running off a battery rather than a service, watts is only half the story — the battery runtime calculator takes the same figure and asks how long the bank holds it up.
Where these nameplate figures are worked out
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.