How this instrument works
Amperage is what electricians call current — the same physical quantity physicists write as I, filtered through a trade that cares less about charge carriers than about what a conductor can survive. Divide the watts a device is rated to consume by the volts pushing electrons through it, and the coulombs-per-second left over is that draw: I = P ⁄ V. The relationship is inverse in voltage, which is why a device's amperage changes depending on which supply it plugs into — the same 1,500 W of heat can be delivered by a smaller current pushed harder, or a larger current pushed gently, and the wattage alone will not tell you which.
André-Marie Ampère built the underlying law in 1820, within weeks of Hans Christian Ørsted's discovery that a current-carrying wire swings a compass needle: Ampère measured the mechanical force between two parallel current-carrying wires and worked out how it scaled with the current flowing in each. For most of the twentieth century, the unit bearing his name was defined electrochemically — by how many milligrams of silver a current would plate out of solution every second, a standard agreed at an international conference in London in 1908. Only in 1948 did the definition shift to the force Ampère himself had measured. The 2019 overhaul of the SI moved the ampere onto firmer ground still, a fixed count of elementary charges, but the practical size of the unit — enough current to run a bright desk lamp — has not shifted by any measurable amount across two centuries of redefinition.
Wire, breakers, and fuses all carry an amp rating rather than a watt rating, because heat inside a conductor tracks current, not whatever the current is doing at the far end: a wire dissipates I²R regardless of whether it is warming a room or driving a saw. A 12-gauge copper conductor carries roughly the same maximum amperage whether it feeds a 120 V heater or a 240 V one, because copper itself is indifferent to voltage — it only feels the current passing through it. This instrument returns the current a resistive, single-phase load draws; a motor or ballast whose current waveform lags behind its voltage waveform draws somewhat more amperage than watts divided by volts predicts, and its nameplate figure should be trusted over this arithmetic.
- Enter Power in watts — switch the unit to kW first if that is how the nameplate or spec sheet states it.
- Enter Voltage in volts: 120 for a standard US outlet, 240 for a US dryer or range circuit, 230 across most of Europe.
- Read Current in amps; switch to mA instead if you are working with a low-power device rather than an appliance.
- Compare that Current reading against the breaker or fuse rating protecting the circuit, not just the wire gauge alone.
- For a load that stays on three hours or longer without a break, check Current against 80% of the breaker rating rather than the full number stamped on it.
Worked example — a 1,500 W space heater on a 120 V line
A portable space heater rated 1,500 W plugged into a standard 120 V US outlet draws I = 1500 ⁄ 120 = 12.5 A, exactly the arithmetic this instrument returns for Power 1500 and Voltage 120. That figure sits uncomfortably close to the ceiling of an ordinary 15 A branch circuit. The National Electrical Code requires a circuit supplying a continuous load — anything running three hours or longer — to be rated at least 1.25 times that load; 12.5 A × 1.25 = 15.625 A, already more than a 15 A breaker can supply. A dedicated 20 A circuit, not a 15 A one, is what the heater actually needs to run safely through a cold afternoon.
Move the identical heater to a 240 V circuit — common in North America for dedicated appliance lines, and the household standard across most of Europe — and the same 1,500 W now costs only 1500 ⁄ 240 = 6.25 A, half the current for identical heat output, which is why higher-voltage circuits get away with thinner wire for the same job. Double the heater's power instead, to 3,000 W on the original 120 V line, and current doubles too, to 25 A — beyond what any standard 15 A or 20 A branch circuit is permitted to carry, which is exactly why heaters much above 1,500 W are built for 240 V supplies or dedicated circuits rather than an ordinary wall outlet.
Questions
What is the difference between amperage and current?
None, physically. Amperage is simply the everyday and trade name for electric current, the same quantity physicists write as I and measure in amperes. Electricians, appliance labels, and breaker panels favor "amperage" or "amps," while textbooks favor "current," but both describe the identical flow of charge past a point each second, and typing your figures into this instrument returns one answer regardless of which word you had in mind.
Why does the same wattage draw more amps at 120 V than at 240 V?
Because current and voltage trade off for a fixed amount of power: I = P ⁄ V, so halving the voltage doubles the current needed to deliver identical watts. A 1,500 W heater on a 120 V circuit pulls 12.5 A, while the identical heater wired to 240 V settles for 6.25 A — same heat output, but the lower-voltage version pushes twice the charge through its cord and plug each second to get there.
Can I run a 1,500 W space heater on an ordinary 15 A circuit?
Briefly, yes; continuously, not comfortably. It draws 12.5 A, inside a 15 A breaker's instantaneous rating but above the 12 A ceiling — 80% of the breaker — that the code sets once a load has been drawing power for three hours straight, and it leaves almost nothing for anything else sharing that circuit. Manufacturers who spec 1,500 W heaters for a 20 A circuit are building in exactly that headroom.
Why are wires and breakers rated in amps instead of watts?
Because heat inside a conductor is generated by current, not by whatever that current is doing at the far end — a wire dissipates I²R regardless of the load's voltage or purpose. Amps describe how hard a conductor is working; watts describe how much energy the load beyond it is converting, which is a separate question the wire itself cannot answer.
Will this arithmetic still work for a motor or another AC load?
Only if that load is resistive, or the figure is being read at unity power factor. Motors, ballasts, and switching supplies draw current whose waveform is skewed relative to the voltage feeding them, so their true amperage runs higher than watts divided by volts predicts, sometimes by 20 to 40 percent. For anything built around a motor, read the nameplate current directly rather than dividing its wattage by its voltage.
What if I only know an appliance's amp rating and voltage, not its wattage?
Multiply them instead: P = V × I gives watts from volts and amps, the reverse of what this page calculates. A 12 A hair dryer running on a 120 V circuit is a 1,440 W appliance whether the box states the wattage or not — the two figures are always tied together by the same relationship, only rearranged to solve for whichever quantity is missing.