Watts to Amps Calculator

A wattage nameplate states real power: the energy the equipment converts into heat, light or motion every second. Current is a different quantity, and the two are only interchangeable when the load is a pure resistance. That gap is what this page is about, and it is why a converter that asks only for watts and volts is right about a toaster and wrong about a compressor.

In an alternating-current circuit, voltage and current are both sine waves. In a heating element they rise and fall together, so every ampere flowing is doing work and real power equals the voltage times the current. Put an inductive load on the same circuit — a motor winding, a transformer, a fluorescent ballast — and the current wave lags behind the voltage wave. Some of the current is now shuttling energy into the magnetic field and back out again twice per cycle without ever doing anything useful. The conductor still carries it. The breaker still sees it. The wattmeter does not.

Power factor is the ratio between the two: the real power divided by the product of voltage and current. It runs from 0 to 1, and it appears in the denominator here because a lower power factor means more current for the same watts:

Single phase: I = W ÷ (V × PF)
Three phase: I = W ÷ (V × 1.732 × PF)

The 1.732 on three phase is the square root of three. It is there because the three line-to-line voltages are 120 degrees apart in time, so the total power arrives more evenly and each conductor carries less current than a single-phase supply delivering the same watts would need. Use the line-to-line voltage with it — 208, 480 — not the line-to-neutral figure.

One thing the formula cannot tell you is which voltage to enter. That comes from the equipment, not from the building: a dryer plugged into a 240 V outlet is a 240 V load even in a house whose lighting circuits are 120 V.

Two loads with almost the same headline wattage, on two different circuits, ending up in two different places. Running them side by side is the fastest way to see what the power factor field is actually doing.

Case 1 — a 1,500 W space heater on a 120 V circuit. A heating element is a coil of resistance wire and nothing else, so the power factor is 1 and the sum is as simple as it looks: 1,500 ÷ (120 × 1) = 12.5 A. That figure is worth remembering, because 12.5 A is also the practical ceiling for a plug-in 120 V appliance. A 15 A circuit may carry 12 A continuously under the 125% rule, so a heater run all afternoon is already past what its circuit is meant to hold indefinitely — which is why 1,500 W appears on so many nameplates and 1,800 W on so few.

Case 2 — a 3,000 W motor on a 240 V circuit. Twice the wattage, twice the voltage. If the motor were a resistor the two would cancel and it would draw the same 12.5 A. It is not, and it does not. At a power factor of 0.85 the sum is 3,000 ÷ (240 × 0.85) = 14.71 A.

Put the two results together. Doubling the wattage bought nothing, because doubling the voltage cancelled it out exactly — and then the power factor added18% on top, entirely from current that does no work. Had you sized this circuit from the naive 12.5 A, you would be 2.2 A light before applying any code factor at all, and the error grows with every point the power factor drops: the same motor at 0.8 draws 15.63 A, and at 0.7 it draws 17.86 A.

Neither figure is a wire size yet. Case 2 is a motor, and motor circuits are sized from the Article 430 full-load current tables rather than from a nameplate wattage — the conversion tells you what to expect, and the tables tell you what to install.

The table below is for recognising a figure rather than deriving one. Most of the time you are not converting a wattage you have never seen — you are checking whether the number on a nameplate is plausible, or working out roughly what is already on a circuit before adding to it.

Read the character column first. Every resistive row is exact: the current shown is the wattage divided by the voltage with no adjustment, and if you measured it with a clamp meter you would get that number. Every motor and electronic row has had a typical power factor applied, and those rows are estimates. A specific unit can sit well either side of them.

Note the voltage column. Four of these appliances are 240 V loads, and they sit in the same list on purpose: the dryer draws 5,000 W and less current than the 1,875 W hair dryer beside it. Anyone converting from a memorised "amps at 120 V" habit gets those four badly wrong.

What the table is not is a load calculation. Article 220 sets its own required figures and demand factors for dwelling loads, and a service sized by adding up typical wattages will be both non-compliant and, usually, far too large.