AmpSizerPower and current conversions
Watts, amps, volt-amps and horsepower are four ways of describing the same electricity. These convert between them with the voltage, the phase and the power factor that the conversion actually depends on.
Amps to watts calculator
Amps and volts in. Watts, kilowatts, VA and horsepower out.
kVA to amps calculator
kVA and voltage in, amps out — both directions, both phases.
kW to amps calculator
Kilowatts and voltage in, amps out — with power factor where it belongs.
Ohm's law calculator
Any two of volts, amps, ohms and watts in. The other two out.
Three phase power calculator
Amps, volts and power factor in. kW, kVA and the conductor out.
Watts to amps calculator
Watts and volts in, amps out — plus the breaker that current wants.
Every one of these conversions is a rearrangement of two formulas. What makes them go wrong is never the arithmetic — it is a missing voltage, a forgotten square root of three on a three-phase circuit, or a power factor of 1 applied to a motor.
So each page asks for the three things the conversion depends on and shows all the other units at the same time, rather than making you convert twice. Where a current comes out, the conductor and the device that carry it come out with it.
Frequently asked questions
Why do I need the voltage to convert watts to amps?
Because watts are volts times amps. The same 1,500 watts is 12.5 amps at 120 V and 6.25 amps at 240 V, so a converter that does not ask for voltage is guessing. Three-phase adds the square root of three, and anything inductive adds a power factor.
What is the difference between kVA and kW?
kVA is what the supply has to deliver, kW is what the load turns into work, and the ratio between them is the power factor. A transformer or a generator is rated in kVA because it does not care what your load does with the current — it only has to carry it. A resistive heater has a power factor of 1 and the two are the same number.
Which power factor should I use if I do not know it?
Use 1 for anything purely resistive — heaters, incandescent lamps, hobs. Use 0.9 for a mixed commercial load, 0.8 for motors, and read the nameplate if there is one. Guessing high understates the current, so if you are sizing something, guess low.
Does converting watts to amps give me the wire size?
It gives you the starting number. The circuit is then sized at 100% or 125% of that current depending on whether the load is continuous, the conductor has to satisfy voltage drop over your distance as well, and the device is the next standard rating up. Each conversion page carries a line for the conductor so you can see where it lands.
How we calculate this
volts × amps × power factor; three-phase is 1.732 × volts × amps × power factor. Apparent power in volt-amps drops the power factor. Horsepower uses 745.7 watts per
mechanical horsepower, which is the output of the motor rather than its input — a motor drawing
1 HP of shaft power pulls more than 745.7 watts from the supply, by its efficiency. Ohm’s law is
V = I × R with P = V × I, and any two of the four give the other two. Where a page shows a
conductor or a device for the resulting current, it comes from the same NEC tables as the rest of
the site: 310.16 for ampacity, 240.6(A) for the device, 240.4(D) for the small-conductor ceiling.Sources: