Amp Calculator
Use this amp calculator to convert watts, kilowatts, kVA or horsepower into amps, or amps back into watts, kW, kVA and HP. Pick DC, single-phase AC or three-phase AC, enter the voltage, and add power factor and motor efficiency where they apply. It answers how many amps a load draws from its power rating, which is different from the Ohm's law, voltage drop and wire size calculators linked below.
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Current
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Real power (W)
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Real power (kW)
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Apparent power (kVA)
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Output horsepower (at this efficiency)
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DC: A = W ÷ V. Single-phase AC: A = W ÷ (V × PF). Three-phase AC with line-to-line voltage: A = W ÷ (√3 × V × PF). For kVA, A = VA ÷ V (÷ √3 V for three-phase). For HP, W = HP × 745.7 ÷ efficiency, where 1 HP = 745.7 W of mechanical output. Real motors have power factors below 1 and start with higher inrush current, so use the motor nameplate and your local code for breaker, wire and motor sizing.
Typical household circuits at 120 V
| Breaker | Maximum | Continuous load (80%) |
|---|---|---|
| 15 A | 1,800 W | 12 A = 1,440 W |
| 20 A | 2,400 W | 16 A = 1,920 W |
Maximum = breaker amps × 120 V. For loads that run 3 hours or more, NEC 210.19(A)(1) and 210.20(A) limit the load to 80% of the breaker rating (the same as a 125% sizing rule), which gives 12 A on a 15 A circuit and 16 A on a 20 A circuit. Rules differ for 100%-rated assemblies, other voltages and local amendments, so check your code or ask an electrician.
Ohms Law Calculator · Voltage Drop Calculator · Wire Size Calculator · Horsepower Calculator · Torque Calculator
More in this group
- Ohms Law Calculator Find volts, amps, ohms and watts from any two values.
- Horsepower Calculator Convert HP, kW and BTU/h, and find HP from torque and RPM.
- Torque Calculator Torque from force, arm, power and RPM, with unit conversion.
- Voltage Drop Calculator Volts lost, percent drop and minimum wire size for a cable run.
How it works
- How do I convert watts to amps?
- Divide watts by volts for DC, or by volts × power factor for AC. A 1,200 W load on 120 V at a power factor of 1 draws 1,200 ÷ 120 = 10 A. For three-phase, divide by √3 × line-to-line voltage × power factor.
- How many amps are 5 kW and 10 kW?
- 5 kW on a 240 V single-phase circuit at a power factor of 1 draws 5,000 ÷ 240 = 20.83 A. 10 kW on a 480 V three-phase supply at a power factor of 0.9 draws 10,000 ÷ (1.732 × 480 × 0.9) = 13.36 A. Change the system, voltage and power factor to see your own numbers.
- How many amps is 1 HP?
- 1 HP is 745.7 W of mechanical output. At 120 V with 100% efficiency and a power factor of 1 that is 745.7 ÷ 120 = 6.21 A. Real motors are less efficient than 100% and have a power factor below 1, so they draw more current. Use the motor nameplate full-load amps when sizing a circuit.
- How many watts can a 15 A or 20 A circuit handle?
- At 120 V, a 15 A breaker is 1,800 W and a 20 A breaker is 2,400 W at most. For continuous loads of 3 hours or more, NEC 210.19(A)(1) and 210.20(A) limit the load to 80% of the rating: 12 A = 1,440 W on a 15 A circuit and 16 A = 1,920 W on a 20 A circuit. Equipment listings and local code can change this.
- How is this different from the Ohm's law, voltage drop and wire size calculators?
- This calculator starts from power in watts, kW, kVA or HP and gives amps (or the reverse). The Ohm's law calculator relates voltage, current, resistance and power. The voltage drop calculator checks losses along a cable run, and the wire size calculator picks a conductor for a given current. Use the amps from here as the input for those.