Current Calculator

Current is the flow, not the push

The ampere is the SI unit of electric current. Since May 20, 2019 it is fixed by the elementary charge of a single electron, as the NIST ampere introduction explains. For everyday work the older picture is easier: one amp is one coulomb of charge passing a point every second.

Voltage is the push, resistance is the pinch, and power is how fast energy is delivered. Know any two of voltage, resistance and power, and the current comes out of a formula that fits on one line.

Scale matters when you read a number. A phone charger moves about 2 A at 5 V. An LED bulb on a 120 V circuit draws 0.075 A. A space heater draws 12.5 A, and the main breaker on a house is often 200 A. Reading milliamps as amps turns a dime into a twenty dollar bill, a factor of 1000.

Three formulas, and when each one applies

Every current calculation on this page is Ohm's law with a power equation folded in. The first form is I = V / R, which you use when a supply meets a known load. Push 120 V through 12 ohms and the current is 120 / 12 = 10 A, while the power is 120 x 10 = 1200 W.

The second form is I = P / V, which you use when the nameplate gives watts. A 1500 W space heater on a 120 V circuit draws 1500 / 120 = 12.5 A. The same heater on a 240 V circuit draws 1500 / 240 = 6.25 A, half the current for the same heat, which is why dryers and water heaters run on 240 V.

The third form is I = the square root of P / R, which you use when watts and ohms are the pair you have. A 100 W load on 4 ohms gives 100 / 4 = 25, and the square root of 25 is 5 A. The voltage across that resistor is 5 x 4 = 20 V.

One check ties all three together: power is also current squared times resistance, so 5 x 5 x 4 = 100 W. If a computed current fails that check, either an input is wrong or the units are mixed.

Amps, milliamps and microamps

One amp is 1000 milliamps and 1,000,000 microamps. Each prefix moves the decimal point three places, and that is the only arithmetic in this section.

So 12.5 A is 12,500 mA and 12,500,000 uA. A 75 mA draw is 0.075 A. A leakage current of 500 uA is 0.0005 A. Data sheets mix the three units freely, and that is how a 12.5 A heater ends up compared against a 12.5 mA reading from a sensor.

House wiring is sized in whole amps. Breadboards and phone circuits run in milliamps. Insulation testers and sensor leakage are quoted in microamps. A digital meter set to the wrong range is not damaged by the mistake, but the number on the screen is meaningless until you check the unit at the right edge of the display.

What household loads draw at 120 volts

A nameplate in watts converts straight to current once the voltage is known. The table uses 120 V and running watts, the figure on the label rather than the marketing number on the box.

A 1500 W heater at 12.5 A is the number to remember, because it sits right at the edge of a 15 A circuit. Run that heater for three hours or more and it counts as a continuous load, which caps the breaker at 80 percent of its rating, or 12 A on a 15 A unit. The heater needs a 20 A circuit.

Two cautions on the watts. Motors and electronics rate power in volt amps, and the current comes from VA when the power factor is below 1. A load of 1000 VA at a power factor of 0.8 delivers 800 W and still draws 1000 / 120 = 8.33 A. A fridge label may also show 200 W running and a start-up surge several times higher, which is the figure that trips a shared circuit.

Series and parallel: one current, or a split

In a series circuit the same current passes through every part. A 20 ohm and a 40 ohm resistor on a 120 V supply add to 60 ohms, so the current is 120 / 60 = 2 A through both. The voltage divides in proportion: 2 x 20 = 40 V across the first resistor and 2 x 40 = 80 V across the second, and 40 + 80 = 120 V.

In a parallel circuit the voltage is the same on every branch and the currents add. Branches of 60 ohms and 40 ohms on 120 V carry 120 / 60 = 2 A and 120 / 40 = 3 A, which is 5 A in total. The equivalent resistance is 24 ohms, because 1 / 60 + 1 / 40 = 0.041667 and 1 / 24 = 0.041667.

Add a third branch and the total climbs again. Branches of 30, 20 and 60 ohms carry 4 A, 6 A and 2 A, so 12 A in all, with an equivalent resistance of 10 ohms. That is the counterintuitive part of parallel wiring: more branches means less total resistance and more total current, not less.

House wiring is parallel by design, so every outlet sees the full 120 V and switching on a lamp does not dim the fridge. A series connection would cut the voltage at each device, and one broken link would stop all of them.

Breakers, wire size and the 80 percent rule

A breaker protects the wire, not the appliance. Section 210.20(A) of the 2023 National Electrical Code requires the breaker to be rated at least 125 percent of a continuous load plus 100 percent of a noncontinuous load, where continuous means three hours or more. Turned around, a continuous load may use no more than 80 percent of the breaker rating.

A 20 A breaker therefore carries 16 A of continuous load, which is 1920 W at 120 V. The 1500 W heater at 12.5 A uses 62.5 percent of that breaker and 83.3 percent of a 15 A breaker, which is why it belongs on the 20 A circuit.

Wire size follows the breaker: 14 AWG copper for 15 A, 12 AWG for 20 A and 10 AWG for 30 A in the 60 degree column of NEC Table 310.16, before any adjustment for high temperature or bundled cables. A 20 A breaker on 14 AWG wire is a fire waiting for a load, because the wire heats up faster than the breaker trips.

Measuring current: the meter goes in series

An ammeter has almost no resistance of its own, and it works by becoming part of the circuit. To measure current you break the circuit, put the meter in the gap, and let the current pass through it. A clamp meter does the same job without the break, by reading the magnetic field around one conductor.

Never connect a meter in current mode across a live supply. In that mode the meter is close to a short circuit, so the current jumps to whatever the supply can deliver, the fuse inside the meter blows, and the arc can burn a hand. Voltage mode is the one that goes across two points in parallel.

Two habits keep a reading honest. Clamp one conductor at a time, since a clamp around a cord that carries both hot and neutral reads near zero. Start on the highest range when the current is unknown, and remember that on most meters the 10 A terminal is separate from the milliamp terminal and has its own fuse.

Test instruments must be rated for the circuit they touch, a requirement in OSHA 1910.334(c)(3). For panel work that means a CAT III rating at the voltage you expect, not a cheap meter with a 600 V label and no category mark.

Mistakes that give the wrong current

Mixing watts and volt amps is the most common error. A load rated 1000 VA at a power factor of 0.8 draws 1000 / 120 = 8.33 A, and the 800 W of real power is a separate number. Computing 800 / 120 = 6.67 A undersizes the breaker by 20 percent.

Adding parallel resistances instead of their reciprocals is the second. Two branches of 60 and 40 ohms look like 100 ohms, which would give 1.2 A, while the real total is 5 A.

Ignoring motor inrush is the third. A motor that draws 10 A while running can pull 50 A for a fraction of a second, and a breaker that trips at start-up is usually reacting to that surge rather than to a fault.

Using peak voltage is the fourth. A 120 V AC supply peaks near 170 V, so a current computed from the peak comes out about 1.41 times too high. Nameplate voltages and meter readings are RMS values, and every formula on this page expects RMS.