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kW to amps calculator
Current is power divided by voltage, with the power factor in the divisor on alternating current and 1.732 more on three phase. A 4.5 kW water heater element at 240 V draws 18.75 A. A 75 kVA three-phase transformer carries 208.18 A at 208 V and 90.21 A at 480 V. This page converts kW, kVA or watts to amps, and amps back to kW and kVA. For the wire and the breaker, take the amps to the wire size calculator.
- Choose kW, kVA or watts to amps, or amps to kW and kVA, and enter the number from the nameplate, the label or a meter.
- Pick DC, single phase or three phase and enter the voltage. On three phase use the voltage between two lines, such as 208 or 480 V.
- Use 1 for heaters and other resistive loads, and the data sheet or a power meter reading for a motor. kVA and DC need none.
Example result for the default inputs
4.5 kW, single phase, 240 V, power factor 1.
Current
18.75 A
| Current | 4.5 kW x 1,000 / (240 V x 1) = 18.75 A |
|---|---|
| Apparent power | 4.500 kW / 1 = 4.500 kVA. Transformers and generators are rated in kVA. |
| Breaker and wire | Not from this number alone. A load that runs 3 hours or more is a continuous load, and the NEC sizes its conductors and breaker at 125 percent of it. The wire size calculator works that from the amps. |
- At a power factor of 1 the current is the smallest it can be for this power. Heaters, ovens and incandescent lamps run close to 1. A motor, a compressor or a power supply usually runs below 1, which raises the current; use the power factor from the nameplate, the data sheet or a power meter. A motor nameplate kW is the shaft output, not the electrical input.
For estimation purposes only. Consult a licensed engineer for final designs.
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How power becomes current
Current is power divided by voltage. On alternating current the power factor sits in the divisor too, because a motor or a power supply draws more amps than its watts alone would need. Three phase divides by the square root of 3 (1.732) times the line-to-line voltage, and the result is the current in each line. A kVA rating already holds the volts times the amps, so it needs no power factor.
- DC
I = P / VWatts are volts times amps. 100 W at 12 V is 8.33 A.- Single phase, kW
I = kW x 1,000 / (V x PF)A 4.5 kW water heater element at 240 V and a power factor of 1 is 18.75 A, the figure in Mike Holt's EC&M example. At a power factor of 0.8 a 10 kW load at 240 V draws 52.08 A instead of 41.67 A.- Three phase, kW
I = kW x 1,000 / (1.732 x V x PF)DOE's Equation 1 for motors, run backward. 10 kW at 480 V and a power factor of 0.85 is 14.15 A in each line. V is line to line.- kVA
I = kVA x 1,000 / V, or / (1.732 x V)The full-load current of a transformer. 75 kVA three phase is 208.18 A at 208 V and 90.21 A at 480 V; Hammond's card prints 208 and 90.2.- Amps to power
kVA = V x I / 1,000 (x 1.732), kW = kVA x PFIn DOE's example a three-phase motor at 469.7 V, 37 A and a power factor of 0.763 is 30.10 kVA and 22.97 kW, which DOE prints as 22.9 kW.
Eight loads through the same formulas
The first row is Mike Holt's EC&M example, 4,500 W at 240 V. Hammond's full-load current cards print 208, 90.2 and 104 A for the transformer rows. The motor row shows what a power factor below 1 does: 10 kW at 480 V would be 12.03 A at a power factor of 1 and is 14.15 A at 0.85.
| Load | Supply | Power factor | Current, A |
|---|---|---|---|
| 4.5 kW water heater element | 240 V single phase | 1 | 18.75 |
| 1,500 W space heater | 120 V single phase | 1 | 12.50 |
| 10 kW resistive load | 240 V single phase | 1 | 41.67 |
| 10 kW resistive load | 208 V three phase | 1 | 27.76 |
| 10 kW motor load, PF 0.85 | 480 V three phase | 0.85 | 14.15 |
| 75 kVA transformer | 208 V three phase | not needed | 208.18 |
| 75 kVA transformer | 480 V three phase | not needed | 90.21 |
| 25 kVA transformer | 240 V single phase | not needed | 104.17 |
Three-phase currents are per line, with the voltage taken line to line. A kVA rating needs no power factor; a kW rating does, and at a power factor of 1 the current is the least the load can draw.
Standards and data sources
- U.S. Department of Energy, Determining Electric Motor Load and Efficiency (Motor Challenge fact sheet DOE/GO-10097-517), Equation 1 - three-phase power from line-to-line voltage, line current and power factor, and the worked example this page is tested against: 469.7 V, 37 A and 0.763 give 22.9 kW.
- Hammond Power Solutions, Full Load Currents, single-phase and three-phase dry-type transformers - the kVA to amps formulas and the rated line current tables, 0.5 to 333 kVA single phase and 3 to 3000 kVA three phase from 120 to 4160 V. Every printed value is checked in the tests.
- Mike Holt in EC&M, Code Q&A: Sizing Circuit Conductors for Individual Appliances (2020, based on the 2017 NEC) - I = P / E for a 4,500 W, 240 V water heater, 18.75 A, the page default, and the 125 percent of a continuous load that follows it.
Limitations of this calculator
- This page converts between power, voltage, current and power factor. It does not size a breaker or a wire, and it is not a design. A licensed electrician sizes and installs circuits under the permit and the inspection of the local authority having jurisdiction (AHJ).
- The power factor is your entry. The page does not supply a typical value for a motor, a compressor or a drive, because those vary with the machine and its load. At a power factor of 1 the current is the least it can be for the power entered.
- Three phase assumes a balanced load and takes the voltage line to line. For an unbalanced load, work each line from the power on it.
- A motor nameplate gives the shaft output, not the electrical input, and NEC sizes motor circuit conductors and protection from Tables 430.248 and 430.250 rather than from a nameplate kW. Use the motor's full-load current table or its data sheet for a motor circuit.
- The voltage entered is the one the current divides by. A resistive heater or element changes its power with the voltage, so its rating at 240 V does not hold at 208 V; a load that holds its power, such as a drive or a power supply, draws more current at a lower voltage.
- kVA to amps gives the rated full-load current of a transformer or a generator. Inrush, harmonics, transformer protection (NEC 450.3) and the feeder are not worked.
- The NEC sizes the conductors and the breaker of a continuous load, one that runs 3 hours or more, at 125 percent of its current. That step is on the wire size calculator, not here.
- On DC there is no power factor and kVA does not apply; the page works watts and kW only.
Common mistakes converting kW to amps
- Using 277 V for a three-phase load. 277 V is the line-to-neutral voltage of a 480Y/277 V system. A 10 kW three-phase load worked at 277 V comes out at 20.84 A, while at the 480 V between lines it is 12.03 A. The same holds for 120 V on a 208Y/120 V system.
- Leaving the power factor at 1 for a motor. At a power factor of 1, 10 kW at 240 V single phase is 41.67 A. At 0.8 it is 52.08 A, a quarter more. Heaters run close to 1; motors and compressors usually do not.
- Dropping the 1.732 on three phase, or dividing by 3. 10 kW three phase at 208 V and a power factor of 1 is 27.76 A in each line. Dividing by 208 V alone gives 48.08 A, and dividing by three times 208 V gives 16.03 A. Both are wrong.
- Dividing a 240 V rating by 208 V. A 4,500 W element rated at 240 V is a fixed resistance. On 208 V it makes about 3,380 W and draws 16.25 A, not the 21.63 A that 4,500 W divided by 208 V suggests. Use the rating at the voltage the load sees, or the maker's 208 V figure.
- Taking a motor's nameplate kW as its input. A motor's kW or horsepower rating is the shaft output. The electrical input is that divided by the motor efficiency, so the current is higher than the output power gives. For a motor circuit, NEC Tables 430.248 and 430.250 give the full-load current to size from.
- Sizing the wire from the running current. A 4.5 kW water heater draws 18.75 A, but it is a continuous load, so the conductors are sized for 125 percent of that, 23.44 A. Holt's answer is 10 AWG. The wire size calculator does that step.
Common questions about kW, kVA and amps
How many amps is 1 kW?
It depends on the voltage and the phase. At a power factor of 1, 1 kW is 8.33 A at 120 V and 4.17 A at 240 V single phase, and 2.78 A at 208 V and 1.20 A at 480 V three phase. A lower power factor raises each figure.
How do you convert kW to amps on three phase?
Multiply the kW by 1,000 and divide by 1.732 times the line-to-line voltage times the power factor. 10 kW at 480 V and a power factor of 0.85 is 10,000 / (1.732 x 480 x 0.85) = 14.15 A in each line.
How many amps is a 75 kVA transformer?
On three phase, 208.18 A at 208 V and 90.21 A at 480 V: kVA x 1,000 / (1.732 x V). Hammond's full-load current card prints 208 A and 90.2 A. A 25 kVA single-phase transformer at 240 V is 104.17 A.
What is the difference between kW and kVA?
kW is the real power that does work. kVA is volts times amps, the apparent power. They are equal at a power factor of 1, and kVA = kW / PF otherwise. A 10 kW motor load at a power factor of 0.85 is 11.76 kVA. Transformers and generators are rated in kVA because their heating follows the current, whatever the power factor.
What power factor should I use?
1 for heaters, ovens, water heater elements and incandescent lamps. For a motor, a compressor or a drive, use the figure on its data sheet or one read with a power meter on the running load. This page does not guess a typical value, and at a power factor of 1 the current it gives is the least the load can draw.
How many amps does a 4,500 watt water heater draw?
18.75 A at 240 V: 4,500 W / 240 V. That is the figure in Mike Holt's EC&M example. On 208 V the same 240 V element makes about 3,380 W and draws 16.25 A, because its resistance is fixed.
How do you convert amps to kW?
Multiply the volts by the amps (and by 1.732 on three phase) for the kVA, then by the power factor for the kW. DOE's example motor at 469.7 V, 37 A and a power factor of 0.763 is 30.10 kVA and 22.97 kW.
Is three-phase current per line or the total?
Per line. The three-phase formula gives the current in each of the three line conductors when the load is balanced. It is not a total to split three ways.
How many amps is 100 watts at 12 volts DC?
8.33 A. On DC, watts are volts times amps, with no power factor.
Does this tell me the breaker or wire size?
No. A load that runs 3 hours or more is a continuous load, and the NEC sizes its conductors and breaker at 125 percent of the current: 18.75 A becomes 23.44 A. The wire size calculator works that, with the temperature and the number of conductors.