Watts to Amps Calculator — DC, 1φ & 3φ
Nameplates give you watts. Conductors, breakers, and code tables all want amps. The conversion is one division — amps = watts ÷ volts — but the divisor changes with the supply type: multiply by power factor for AC, and by √3 as well for three-phase. This calculator handles all three, then carries the answer forward into the breaker and conductor it requires.
Convert watts to amps
1.0 for resistive · 0.85 typical for motors
Current
Single-phase: amps = watts ÷ (volts × power factor).
Minimum breaker
15 A
Conductor
14 AWG Cu
See the breakdown
Conductor shown is the general-purpose minimum at the 75 °C column before any derating. Motor circuits must use the NEC Table 430.248/430.250 full-load currents instead.
The formula, explained in plain English
One division. What changes between supply types is only what goes in the denominator.
Voltage is the lever
Double the voltage and the current halves for the same watts. That is why dryers, ranges, and EV chargers run at 240 V and why commercial distribution is 480 V.
Power factor only cuts one way
A power factor below 1.0 always increases current for the same real power. Omitting it on a motor load undersizes the conductor.
Where 1.732 comes from
√3, the geometric consequence of three phases 120° apart. It means three-phase carries the same power with about 42% less line current than single-phase.
Amps aren't the breaker
The calculated current is the load. The breaker is the next standard rating above it — 125% of it if the load is continuous.
Worked examples
The 1,500 W heater everyone asks about, a motor where power factor matters, and a three-phase load.
1,500 W space heater at 120 V
Resistive load, power factor 1.0. The defaults above.
breaker: next standard above 12.5 A = 15 A · conductor 14 AWG Cu
as a continuous load: 12.5 × 1.25 = 15.63 A → 20 A breaker on 12 AWG
Result: 12.5 A is exactly why 1,500 W is the ceiling for portable heaters — it sits at 83% of a 15 A circuit. Run it for three hours or more and it becomes a continuous load needing a 20 A circuit.
2,000 W motor at 240 V, PF 0.85
Single-phase inductive load.
ignoring PF: I = 2,000 ÷ 240 = 8.33 A ← 15% understated
Result: leaving power factor out of an inductive load understates current by 15% here. On a real motor circuit you would ignore both figures and use the Table 430.248 full-load current instead — but the arithmetic shows why power factor is never optional on AC.
30,000 W at 480 V three-phase, PF 0.9
A commercial rooftop unit or a bank of process heaters.
single-phase at 480 V would be: 30,000 ÷ (480 × 0.9) = 69.44 A
same power, 42% less line current
Result: 40 A of three-phase lands on 8 AWG copper; 69 A of single-phase would need 4 AWG. The conductor saving across a whole building is the commercial case for three-phase service.
Watts to amps quick chart
Common wattages at unity power factor. Notice how the same load moves down two columns simply by changing the supply voltage.
| Power | 120 V 1φ | 240 V 1φ | 480 V 3φ | Typical load |
|---|---|---|---|---|
| 100 W | 0.83 A | 0.42 A | 0.12 A | LED floodlight / small fan |
| 500 W | 4.17 A | 2.08 A | 0.60 A | Small tool, blender |
| 1,000 W | 8.33 A | 4.17 A | 1.20 A | Microwave, space heater on low |
| 1,500 W | 12.50 A | 6.25 A | 1.80 A | Portable heater, hair dryer — the 15 A limit |
| 2,400 W | 20.00 A | 10.00 A | 2.89 A | Full 20 A circuit at 120 V |
| 4,500 W | 37.50 A | 18.75 A | 5.41 A | Electric water heater element |
| 5,000 W | 41.67 A | 20.83 A | 6.01 A | Clothes dryer |
| 9,600 W | 80.00 A | 40.00 A | 11.55 A | 40 A EV charger at 240 V |
| 12,000 W | 100.00 A | 50.00 A | 14.43 A | Electric range |
Sources & standards: √3 = 1.7321. NEC (NFPA 70) 2023 — 210.20(A) continuous load, 240.6(A) standard overcurrent ratings, Table 310.16 conductor ampacities, 430.6(A)(1) and Tables 430.248 and 430.250 for motor full-load currents. Local amendments override the model code, and a licensed electrician plus the AHJ have final say on anything installed.
Frequently asked questions
Common questions about converting watts to amps and sizing the circuit.
How do I convert watts to amps?
Divide watts by volts — then adjust for the supply type. DC: I = P ÷ V. Single-phase AC: I = P ÷ (V × PF). Three-phase AC: I = P ÷ (√3 × V × PF). So 1,500 W at 120 V single-phase with unity power factor is 1,500 ÷ 120 = 12.5 amps.
How many amps is 1,500 watts?
At 120 V, 1,500 W draws 12.5 A — which is why 1,500 W is the standard ceiling for portable heaters and hair dryers on a 15 A circuit. At 240 V the same 1,500 W draws only 6.25 A. Doubling the voltage halves the current for identical power, which is the entire reason 240 V circuits exist for large appliances.
Do I need to include power factor?
For resistive loads — heaters, incandescent lamps, electric water heaters, resistance elements — power factor is 1.0 and you can ignore it. For motors, transformers, LED drivers, and switch-mode power supplies it is typically 0.7 to 0.95, and leaving it out will understate the current. A motor nameplate usually lists power factor; if it doesn't, 0.85 is a reasonable planning assumption.
Why does three-phase draw less current for the same watts?
Because the three phase currents are 120° apart, so the same real power is delivered across three conductors instead of two. The √3 factor works out to roughly 42% less line current than single-phase at the same voltage. Less current means smaller conductors, less voltage drop, and cheaper distribution — which is why commercial and industrial buildings are three-phase.
Is the breaker the same as the calculated amps?
No, and this is where people get caught. The calculated current is what the load draws; the breaker has to be the next standard rating above it, and for a continuous load (three hours or more) the requirement is 125% of the current per NEC 210.20(A). A 12.5 A continuous load needs 15.6 A of capacity, so a 20 A circuit. Toggle the continuous option above, or use the Breaker Size Calculator.
What about VA instead of watts?
Volt-amperes are apparent power and watts are real power; they differ by the power factor. Conductors and breakers are sized from VA, because the conductor carries the full current regardless of how much of it does useful work. If your nameplate gives VA, enter it here with power factor set to 1.0 — that returns the true current. The kVA Calculator handles the relationship in detail.
Does this work for motor circuits?
Use it for a rough check only. NEC 430.6(A)(1) requires motor conductors to be sized from the full-load current values in Tables 430.248 and 430.250 — not from the motor nameplate and not from a watts calculation. The Code tables are deliberately conservative. This converter is right for resistive loads, appliances, and general power estimating.
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