AmpSizerWatts to amps
Watts to amps
12.5 amps
from 1500 watts at 120 V, power factor 1.00
| Also equal to | Value |
|---|---|
| Current | 12.5 A |
| Real power | 1,500 W = 1.50 kW |
| Apparent power | 1,500 VA = 1.50 kVA |
| Mechanical equivalent | 2.01 HP |
| In milliamps | 12,500 mA |
- Breaker for this as a continuous load
- 20 A
- Copper conductor that carries it
- 12 AWG
Leave the power factor at 1 for anything that just gets hot. Use 0.8 for a motor and 0.9 for a mixed commercial load.
A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.
One circuit check a month
MonthlyOne thing a month worth measuring, testing or recalculating on a home electrical system — the part of sizing that happens after the install. No product pitches, and nothing that needs a live panel.
Try also
How do you convert watts to amps?
The AmpSizer watts-to-amps result for 1,500 watts on a 120 volt single-phase circuit is 12.5 amps — watts divided by volts, and nothing else when the load is resistive.
Voltage is what a converter cannot do without. The same 1,500 watts is 12.5 A at 120 V and 6.25 A at 240 V, which is the entire argument for 240 volt appliances.
Two things change the formula. A three-phase circuit divides by 1.732 × volts, because the three legs share the work. And anything inductive — a motor, a transformer, a fluorescent ballast — divides by the power factor as well, so the current is higher than watts over volts alone would suggest.
The conversions people look up most, in one place. Everything here is at unity power factor, which is correct for resistive loads.
| Watts | 120 V | 208 V | 240 V | 480 V, 3-phase |
|---|---|---|---|---|
| 500 W | 4.2 A | 2.4 A | 2.1 A | 0.6 A |
| 1,000 W | 8.3 A | 4.8 A | 4.2 A | 1.2 A |
| 1,500 W | 12.5 A | 7.2 A | 6.3 A | 1.8 A |
| 2,000 W | 16.7 A | 9.6 A | 8.3 A | 2.4 A |
| 3,000 W | 25.0 A | 14.4 A | 12.5 A | 3.6 A |
| 5,000 W | 41.7 A | 24.0 A | 20.8 A | 6.0 A |
| 10,000 W | 83.3 A | 48.1 A | 41.7 A | 12.0 A |
The three-phase column is the reason industrial equipment is 480 V: the same power moves at a seventh of the current, which means a seventh of the copper and a fortieth of the voltage-drop problem.
Why 240 volts costs less to wire
Halving the current does two things at once. It halves the conductor requirement on ampacity, and it quarters the voltage drop problem in percentage terms — because the volts lost halve while the number they are divided by doubles.
That compounds over distance. A 5,000 W load 100 ft away needs 4 AWG copper at 120 V and 10 AWG at 240 V. The wire is the expensive part of a long circuit, which is why detached buildings get a 240 V feed and step down at the far end rather than a long 120 V run.
Frequently asked questions
How many amps is 1500 watts?
12.5 A at 120 V, 7.2 A at 208 V, 6.25 A at 240 V. That is why a 1,500 W heater is comfortable on a 15 A circuit at 120 V but three of them are not, and why the same 1,500 W of heating on a 240 V circuit barely registers.
Do I use watts or volt-amps to size a circuit?
Volt-amps, if the two differ. The conductor and the breaker carry current, and current comes from apparent power — so for a load with a power factor below 1 you size on VA and not on W. For a resistive load they are the same number and the distinction never arises.
Why does my appliance draw more amps than the label says?
Usually power factor, sometimes start-up. A 1,000 W motor at 0.8 power factor draws 10.4 A at 120 V, not 8.3. And almost anything with a motor pulls several times its running current for the first second, which is what trips a breaker without ever showing on a meter.
Does the answer change for DC?
It gets simpler: DC has no power factor and no phase, so amps are just watts over volts. That is the one case where the naive formula is exactly right, and it is why 12 V and 48 V systems are easy to convert and hard to wire.
What power factor should I assume if there is no nameplate?
1 for heaters, kettles, hobs and incandescent lamps. 0.8 for motors and compressors. 0.9 for a mixed load such as a workshop or a small commercial unit. Assuming too high understates the current, so for sizing, guess low.
How we calculate this
watts ÷ (volts × power factor). Three-phase current is
watts ÷ (1.732 × volts × power factor), where 1.732 is the square root of three. Apparent power in
volt-amps is current × volts × the same root-three factor, without the power factor. Horsepower uses
745.7 watts per mechanical horsepower, which is shaft output rather than electrical input. Where a
breaker and conductor are shown for the resulting current, the current is taken at 125% as a continuous
load, the device is the next standard rating in NEC 240.6(A), and the conductor satisfies Table 310.16
ampacity with the small-conductor ceiling of 240.4(D) applied.Worked example
With Amount 1500, Measured in watts, Convert to amps, Voltage 120 V, Supply Single phase, Power factor 1.00, this page works out 12.5 amps. from 1500 watts at 120 V, power factor 1.00
- Breaker for this as a continuous load20 A
- Copper conductor that carries it12 AWG
Sources:
Reading the number off the appliance
Input watts, not output watts. A 700 W microwave cooks with 700 W and draws about 1,100 to 1,200. An amplifier rated 500 W output draws far less than that on average and far more on peaks. The plate on the back is the number that matters.
Heating appliances are honest. A 1,500 W heater draws 1,500 W whenever it is on, and its only two states are on and off — which is why they are the appliances that overload circuits.
Nameplates in amps are already done for you. If a plate gives amps, use them and skip the conversion; the manufacturer has already accounted for power factor and efficiency.