Watts to Amps Calculator

The formula, and why voltage decides the answer

Power is voltage times current, so current is power divided by voltage. That single rearrangement covers every case on this page, and the differences between DC, single phase and three phase all live in how many voltage and current terms have to be multiplied together.

The consequence is that a watt figure alone says nothing about current. The same 1,500 W is 125 A at 12 V, 12.5 A at 120 V and 6.25 A at 240 V. Wiring, breakers and switches are all rated in amps, which is why every load calculation has to start with the supply voltage rather than the wattage.

The amps to watts calculator runs the same relation the other way, and is the page to use when a nameplate gives current and the question is about power. This page starts from watts, which is the number printed on heaters, kettles, lamps and most appliance labels.

DC and resistive AC loads

A resistive load converts all the current it draws into heat or light, so the current is exactly watts divided by volts. Space heaters, incandescent lamps, kettles and toasters are close enough to resistive that the simple division is accurate to a percent or two.

DC circuits use the same division with no correction at all. A 100 W 12 V lamp draws 8.333 A, and a 500 W inverter load at 12 V draws 41.67 A, which is why low voltage systems need thick cable for modest power.

The 12 V column is the one that catches people out. A 2,000 W load on a 12 V battery bank needs 166.7 A, which is beyond what a single 4 AWG cable wants to carry and far beyond a cigarette lighter socket. The same 2,000 W at 240 V needs 8.333 A and a 14 AWG cord is enough.

AC loads with a power factor

Motors, compressors, LED drivers and anything with a switching supply draw current that is not perfectly in step with the voltage. The power factor is the cosine of that gap, and it means the current is larger than watts divided by volts would suggest. The correction is to divide by the power factor as well.

A 1,800 W load with a power factor of 0.9 on 120 V draws 16.67 A rather than 15 A. That extra 1.67 A is real current in the wires, so it heats the cable and trips the breaker, even though the meter only bills for 1,800 W. Utilities charge large customers for the reactive part, which is why power factor correction equipment exists.

Power factor is usually printed on a motor nameplate or found in the datasheet, and typical values run from 0.8 to 0.95 for motors and close to 1.0 for heaters. When it is unknown, 0.8 is the cautious assumption for a motor and 1.0 for a resistive load. A value above 1.0 is not physical, and a value of 0 makes the formula blow up, so both are worth rejecting at the input stage.

Three phase power

Three phase systems carry power on three conductors whose voltages peak at different times, and the total is the square root of 3 times the line to line voltage times the current times the power factor. Dividing watts by that product gives the current in each line for a balanced load.

The 1.732 factor is not a fudge. It comes from summing three sinusoidal power flows 120 degrees apart, and the US Department of Energy motor systems guide writes the same expression as input power equals voltage times current times power factor times the square root of 3, divided by 1,000 for kilowatts. The voltage is the line to line value, not the line to neutral value, and mixing the two is the usual source of a result that is 1.732 times too large.

The 480 V row shows why factories use higher voltages. The same 15 kW at 208 V and 0.9 power factor draws 46.26 A, while at 480 V it draws 20.05 A, so the higher voltage cuts the current by a factor of 2.3 and shrinks the cable and the losses along it.

Motors: rated output, not input watts

A motor nameplate gives horsepower, and horsepower is mechanical output at the shaft. The electrical input is larger because no motor is 100 percent efficient, so the current comes from output divided by efficiency first, then divided by voltage and power factor.

One horsepower is 746 W of output. A 1 hp motor at 85 percent efficiency draws 877.6 W at the terminals, and at 120 V with a power factor of 0.8 that is 9.14 A. Ignoring efficiency gives 7.77 A, which understates the load by 15 percent and can leave a circuit undersized.

Nameplate current, usually marked FLA for full load amps, already folds in efficiency and power factor, so it is the number to trust when it is available. The DOE guide recommends measuring voltage, current and power factor when the nameplate is missing, since a motor running at part load draws less than its rated current.

Starting current is a separate matter. A motor can pull five to eight times its running current for a fraction of a second at start, which is why a 9.14 A motor on a 15 A circuit can still trip a breaker sized for running current alone.

Breakers, the 125 percent rule and wire size

A breaker protects the wire, not the appliance, and it is sized above the running current so that normal operation does not trip it. For a load that runs for three hours or more, the NEC treats it as continuous and requires the overcurrent device to be at least 125 percent of the load current. The TxDOT electrical design manual restates the rule with a worked case: an 18 A lighting load needs 22.5 A of breaker, so the next standard size of 30 A is used.

Applying it to the numbers on this page: 12.5 A becomes 15.625 A and lands on a 20 A breaker, 16.67 A becomes 20.83 A and lands on 25 A, and 24.49 A becomes 30.62 A and lands on 35 A. The standard sizes come from NEC 240.6(A), which lists 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 and 100 A among others, as reproduced in the MSHA training copy of Article 240.

The 80 percent column is the current a continuous load may draw on that circuit. A 1,500 W space heater at 120 V draws 12.5 A, just over the 12 A allowance on a 15 A circuit, so a 20 A circuit with 12 AWG wire is the correct match. The small conductor limits above are fixed by NEC 240.4(D); larger sizes depend on insulation temperature rating and installation method, which is a job for the code book and a licensed electrician.

Nameplate, inrush and what the meter says

Three numbers can disagree on the same appliance: the nameplate rating, the measured running current and the start current. Nameplate watts are usually a maximum, measured current at part load is lower, and inrush can be several times the running figure for a few cycles.

A refrigerator is the standard example. Its nameplate may say 6 A running, and the compressor can draw three times that for under a second on each start. A breaker tolerates that because the magnetic trip curve is slow for short spikes, but an inverter or a generator may not, which is why sizing those needs the surge figure rather than the running watts.

Voltage also sags under load, and a sagging supply raises the current for the same wattage. A 1,500 W heater rated at 120 V draws 12.5 A at 120 V and 13.16 A at 114 V, which is a 5 percent rise for a 5 percent voltage drop. Long extension cords produce exactly that situation, and the voltage drop calculator covers the wire side of it.

Measuring is the tiebreaker when the label is missing. A clamp meter on the hot conductor gives running amps directly, and multiplying by volts gives volt amps, which equals watts only when the power factor is 1.

Mistakes that give the wrong current

Using line to neutral voltage in the three phase formula makes the answer 1.732 times too large. A 208 V three phase system has 120 V from line to neutral, and the formula wants the 208 V figure. The same mistake in reverse makes a single phase load look smaller than it is.

Leaving out the power factor understates current for any motor or compressor. A 1,800 W load at 0.9 draws 16.67 A, not 15 A, and the difference is enough to put a 15 A circuit over its continuous rating before the appliance even reaches full load.

Treating horsepower as watts is the third. A 1 hp motor is 746 W of shaft output, and the supply sees 877.6 W at 85 percent efficiency before the power factor is applied. Sizing from 746 W gives a current that is too low by a wide margin.

The fourth is mixing unit prefixes. Kilowatts have to be multiplied by 1,000 before dividing, and milliamps divided by 1,000 after. A 2.4 kW load at 240 V is 10 A, not 0.01 A, and a decimal slip of that size usually shows up as a breaker that trips immediately.

Finally, the result is current per line for a balanced three phase load. An unbalanced load puts different currents in each conductor, and the formula returns an average rather than a maximum. The generator size calculator follows the same current logic when the question is how much capacity a supply needs.