Current Calculator

Current Calculator finds the amps in DC, single-phase and three-phase circuits from power and voltage, voltage and resistance or power and resistance, with the breaker size needed.

Current Calculator Formula for Amps

The Current Calculator finds the amps in a circuit from any two values you know. Under Calculate From there are three pairs to pick from, and each one uses its own current formula. Together they are the three formulas of electricity most people learn first.

Power and Voltage is the pair for nameplates. Current I in amps equals power P in watts divided by voltage V, which is how you calculate the current for most appliances and fixed equipment.

$$I=\frac{P}{V}$$

The tool opens on a 1,500 W heater at 120 V with a power factor of 1, a typical plug-in load. The result is 12.5 A, and every card under it starts from that current.

$$I=\frac{1500}{120\times 1}=12.5\text{ A}$$

Voltage and Resistance is Ohm's law, the choice when you know what the load measures in ohms and not its wattage. Current equals the voltage across the load divided by its resistance.

$$I=\frac{V}{R}$$

A 12 V supply across a 4 Ω load drives 3 A, and the second card then shows the 36 W of power that current produces. Power and Resistance covers the case where voltage is the unknown.

$$I=\sqrt{\frac{P}{R}}$$

A 1,000 W element with 8 Ω of resistance carries 11.18 A, and the card reports 89.44 V across the load. To solve for voltage or resistance as the main answer, the Ohm's Law Calculator is the tool to open.

Power can be typed from milliwatts to megawatts or in VA, kVA and MVA, and voltage from millivolts to kilovolts. The line under the result prints the formula with your numbers in it, and Step-by-Step Working lists each stage.

AC, DC and Three-Phase Current

Circuit Type sets which version of the current calculator formula runs. As a current calculator for DC, it divides watts by volts and nothing else. A 60 W load on a 12 volt DC supply draws 5 A, and a 1,500 W load would pull 125 A from the same battery.

To calculate DC to AC through an inverter, divide the AC watts by the inverter efficiency first. A 1,500 W AC load through an inverter entered at 90% draws 138.9 A from a 12 V battery, which is the figure the Battery Size Calculator builds on.

As a current calculator for AC single-phase loads, the tool adds power factor. A 240 volt supply in a US home is AC, so 240 V appliances belong under this setting.

$$I=\frac{P}{V\times PF}$$

Power factor is 1 for heaters and other resistive loads. At 0.8 the same 1,500 W draws 15.63 A instead of 12.5 A. The fourth card then switches to 1.875 kVA of apparent power, 1.125 kvar of reactive power and a phase angle of 36.87°.

For a current calculator 3 phase setup, choose AC Three-Phase and say whether the voltage is line-to-line or line-to-neutral. Power is the total for all three phases.

$$I=\frac{P}{\sqrt{3}\times V_{LL}\times PF}$$

A 15 kW load at 480 V line-to-line and 0.85 power factor draws 21.23 A per line. That is the current in each of the three conductors, so it is not multiplied by three again. With line-to-neutral voltage the square root of 3 is replaced by 3. The Three-Phase Power Calculator works the same relationship from the power side.

If the nameplate gives kVA instead of watts, pick VA, kVA or MVA as the power unit. The current is then apparent power over voltage, and power factor only decides how many of those volt-amps are real watts.

Amps From Watts and kW

Used as a current calculator from watts, the tool shows one figure that makes mental checks quick. The Amps per Kilowatt row on the second card is the current each kilowatt costs at your voltage. It is 8.333 A at 120 V and falls as the voltage goes up.

The table is worked at a power factor of 1. So to convert 1 kW to amps, read the second column. What 20 amps is in watts is 2,400 W at 120 V, and 200 amp in kW is 48 kW on a 240 V single-phase service.

The third card in the Current Calculator repeats the load at three other common voltages. The 1,500 W heater that draws 12.5 A at 120 V draws 6.25 A at 240 V, 7.212 A at 208 V and 5.415 A at 277 V. With a fixed resistance the trend reverses, and the resistance modes show current rising with voltage.

kVA and kW differ only by power factor. One kW is 1 kVA at a power factor of 1 and 1.25 kVA at 0.8. Going the other way, 20 kVA at 0.8 is 16 kW. The Electrical Power Calculator handles those conversions when power is the unknown.

Breaker Size for the Current

The first card in the Current Calculator turns the amps into a breaker. It picks the next standard ampere rating at or above the load from the NEC 240.6(A) list, which Mike Holt sets out as 15, 20, 25, 30, 35, 40, 45, 50, 60 A and upward.

A continuous load, one expected to run for 3 hours or more, is counted at 125% of its current. The 12.5 A heater becomes 15.63 A, so it needs a 20 A breaker and not a 15 A one. Switch Load Duration to Non-Continuous and the card shows the share of the breaker in use.

Electricians on the Mike Holt forum work a 4,000 W, 240 V heater the same way. It draws 16.67 A, which is 20.83 A at 125%, so the breaker moves past 20 A to the next size.

Standard Breaker Sizes under the cards lists every rating up to 6,000 A with the continuous current it can carry, which is 80% of the rating, and the power at your voltage. A 20 A breaker at 120 V carries 16 A continuously and 2.4 kW at its full rating.

These standard sizes apply up to 1,000 V AC or 1,500 V DC. Higher voltages come under NEC Article 245, which Electrical Contractor Magazine describes as the 2023 home for medium-voltage overcurrent protection, so the card stops naming a breaker there.

Once the current is known, the Wire Size Calculator picks the conductor for the run length and the Wire Ampacity Calculator checks a size you already have.

Motor Current From Horsepower

A motor is rated by the power at its shaft, so a current calculator for motor loads needs one extra step. Multiply horsepower by 746 to get output watts, then divide by the motor efficiency to get the watts it takes from the supply.

A 5 hp motor delivers 3,730 W. At 88% efficiency it takes 4,239 W, and on 460 V three-phase at 0.85 power factor that is 6.26 A per line. Enter 4,239 W, 460 V and 0.85 to see it, using the efficiency and power factor from the motor nameplate.

For sizing motor conductors and breakers the NEC does not use that figure. Section 430.6(A)(1) sends you to the full-load current tables in Article 430, and Mike Holt's guide explains that the nameplate current is kept for overload protection only.

The fourth card helps in the other direction. For a resistive load it shows the horsepower equivalent of the watts, 2.011 hp for 1,500 W, and the heat output, 5,118 BTU per hour, which is the number to match against a room heating load.

11 kV and 33 kV Line Current

How to calculate 33kV current?

Use the three-phase formula with the voltage in volts. Set the voltage unit to kV, the power unit to MVA or MW, and Circuit Type to AC Three-Phase. Each MVA on a 33 kV line is 17.5 A, so a 10 MVA load draws 175 A per line. Above 1,000 V the first card reads Medium-Voltage Circuit in place of a breaker size. For this load it shows 218.7 A at 125% and 12.03 kA for the same load at 480 V. The Standard Breaker Sizes table stays as a low-voltage reference, with no row highlighted and its power column worked at 480 V.

To calculate current in an 11 kV line the method is the same, and each MVA is 52.49 A. A 5 MW load at 0.9 power factor draws 291.6 A at 11 kV and 97.2 A at 33 kV. How much current is in a 33kV line therefore depends on the load connected, not on the voltage alone.

What is 33kV in volts?

One kV is equal to 1,000 volts, so 33 kV is 33,000 V and 1 volt is 0.001 kV. The voltage in an 11 kV system is 11,000 V between lines and about 6,351 V from each line to neutral, which is the line voltage divided by the square root of 3.

How to convert 33kV to 11kV?

A step-down transformer does it. Power stays nearly the same on both sides, so current rises as voltage falls. A load of 100 A on the 33 kV side becomes about 300 A on the 11 kV side.

Converting 11 kV to 440 V works the same way with a ratio of 25. A 1 MVA transformer carries 52.49 A at 11 kV and about 1,312 A at 440 V. Enter the same power at each voltage in the Current Calculator to get both currents.

Why is 11kV converted to 132kV?

Stepping up the voltage cuts the current for the same power. At 132 kV the current is one twelfth of what it is at 11 kV, 4.37 A per MVA against 52.49 A. Heat lost in the line goes with the square of the current, so the loss in the same conductor falls by a factor of 144.

Amp, Cost and Balance Questions

What is 1 amp of current?

One amp is one coulomb of charge passing a point each second. NIST puts that at about 6.24 × 10¹⁸ electrons per second. In household terms it is the current a 120 W load draws at 120 V.

How to calculate current price?

Electricity is billed by energy, not by amps. The Current Calculator shows Energy per Day and Energy per 30 Days for a load that runs around the clock, 36 kWh and 1,080 kWh for the 1,500 W heater. Multiply by your rate per kWh, so at 15 cents that is $5.40 a day. The Energy Consumption Calculator handles loads that run only part of the day.

What is the formula for current balance?

Current balances at every junction, which is Kirchhoff's current law. The currents flowing in add up to the currents flowing out. In a balanced three-phase load the three line currents are equal and no current returns on the neutral, which is the case the three-phase result assumes.

On a phone the tool behaves like a current calculator app. Copy Results and Share Results under the cards send the current, the breaker and the working to a message or a note.