Watts to amps converter
Converting watts to amps means dividing the real power of a load by its supply voltage, and for AC loads also by the power factor, to find the current it draws. The converter covers DC, single-phase and three-phase supplies at the common US voltages and shows each step of the arithmetic.
Convert watts to amps
Updates as the values change. The page address keeps the values for sharing.
Conductors, breakers, receptacles and switches are all rated in amperes, while many appliance labels give only watts. Turning that wattage into current is the first step before a circuit can be compared with the ampacity of its wire or the rating of its overcurrent device.
Formulas
DC
Batteries, solar strings and other DC sources have no power factor, so the current is simply the wattage divided by the voltage at the load.
Single phase AC
Most US household loads are single phase at 120 V or 240 V. Heaters and incandescent lamps have a power factor of 1; motors and electronic power supplies run below it.
Three phase AC
For a balanced load at 208 V or 480 V the answer is the current in each line conductor. It is not split three ways and is not the sum of the three lines.
P is real power in watts, V the voltage in volts, PF the power factor (0 to 1) and √3 ≈ 1.732. On a three-phase system entered as line-to-neutral voltage, the converter first multiplies by √3 to get the line-to-line value.
Worked examples
A 1,500 W portable heater on a 120 V receptacle
A resistive heater has a power factor of 1, so only the voltage matters.
I = P ÷ (V × PF)I = 1,500 W ÷ (120 V × 1) = 12.5 A
The heater draws 12.50 A. The same 1,500 W element wired for 240 V would draw 6.25 A, because doubling the voltage halves the current for the same power.
A 4,500 W water heater element at 240 V
Electric water heater elements are resistive and are commonly connected across both legs of a 120/240 V service.
I = P ÷ (V × PF)I = 4,500 W ÷ (240 V × 1) = 18.75 A
The element draws 18.75 A on each ungrounded conductor. Under NEC 422.13 a storage water heater of 120 gallons or less is treated as a continuous load in branch-circuit sizing.
A 3,600 W rooftop unit at 208 V three phase, PF 0.8
The compressor and fan motors give the unit a power factor below 1; 0.8 is used here as an example value, not a property of every rooftop unit.
I = P ÷ (√3 × V(L-L) × PF)I = 3,600 W ÷ (1.732 × 208 V × 0.8) = 12.49 A
Each line carries 12.49 A. Leaving the power factor at 1 would give 9.99 A, an underestimate of about 20%.
Conversion table
| Watts (W) | 120 V single phase (A) | 240 V single phase (A) | 208 V three phase (A) | 480 V three phase (A) |
|---|---|---|---|---|
| 100 | 0.83 | 0.42 | 0.28 | 0.12 |
| 250 | 2.08 | 1.04 | 0.69 | 0.30 |
| 500 | 4.17 | 2.08 | 1.39 | 0.60 |
| 750 | 6.25 | 3.13 | 2.08 | 0.90 |
| 1,000 | 8.33 | 4.17 | 2.78 | 1.20 |
| 1,200 | 10.00 | 5.00 | 3.33 | 1.44 |
| 1,500 | 12.50 | 6.25 | 4.16 | 1.80 |
| 1,800 | 15.00 | 7.50 | 5.00 | 2.17 |
| 2,000 | 16.67 | 8.33 | 5.55 | 2.41 |
| 2,400 | 20.00 | 10.00 | 6.66 | 2.89 |
| 3,000 | 25.00 | 12.50 | 8.33 | 3.61 |
| 3,600 | 30.00 | 15.00 | 9.99 | 4.33 |
| 4,000 | 33.33 | 16.67 | 11.10 | 4.81 |
| 4,800 | 40.00 | 20.00 | 13.32 | 5.77 |
| 5,000 | 41.67 | 20.83 | 13.88 | 6.01 |
| 6,000 | 50.00 | 25.00 | 16.65 | 7.22 |
| 7,200 | 60.00 | 30.00 | 19.99 | 8.66 |
| 9,600 | 80.00 | 40.00 | 26.65 | 11.55 |
Values are generated by the same engine as the converter. For motor, compressor or LED driver loads the current is higher than shown by the factor 1 ÷ PF.
Common mistakes
Leaving power factor at 1 for motor loads
A well pump, compressor or fan rated in watts draws more current than watts ÷ volts suggests, because part of the current does no real work. Using PF 1 for such loads understates the current; the nameplate power factor or amperage is the better source.
Using a chart made for 230 V systems
Wattage charts written for 230 V supplies show about 4% more current than the same load draws at a US 240 V nominal voltage, and far less than at 120 V. The voltage of the actual circuit has to be used.
Dividing a whole 120/240 V panel by 120 V
Dividing the total wattage of a 120/240 V service by 120 V overstates the current on each leg roughly twofold when loads are spread across both legs. For a balanced split-phase load the per-leg current is the total wattage divided by 240 V.
Splitting three-phase current by three
The three-phase formula already returns the current in each line conductor. Dividing the result by three, or by √3 a second time, gives a value far too low for checking any one conductor.
Frequently asked questions
How many amps is 1,500 watts?
At power factor 1, 1,500 W is 12.50 A at 120 V and 6.25 A at 240 V. A motor load of the same wattage at PF 0.8 would draw 15.63 A at 120 V.
Can watts be converted to amps without the voltage?
No. Watts and amps measure different things, and the voltage links them: the same wattage draws twice the current at 120 V as at 240 V. Without a voltage there is no single answer.
What power factor should be entered?
The power factor printed on the equipment nameplate or data sheet. Resistive heating and incandescent lighting are 1. Motor-driven and electronic loads are lower; 0.8 appears on these pages only as an illustration.
Does the answer give the breaker size for the circuit?
No. The converter gives the load current only. Breaker selection also depends on whether the load is continuous, the conductor ampacity and the equipment instructions; the breaker size calculator works through those steps.
Why does a 240 V appliance draw less current than a 120 V one of the same wattage?
Power is voltage times current, so at twice the voltage the same power needs half the current. That is why ranges, dryers and water heaters are usually supplied at 240 V.