3 Phase Current Calculator
Sizing a breaker, a motor starter, or a feeder cable for three-phase equipment starts with one number: the line current in amps. Get it wrong and the breaker nuisance-trips or the cable runs hot. Get it right and the rest of the design falls into place. This 3 Phase Current Calculator solves the four directions electricians actually need: kilowatts to amps, kVA to amps, motor horsepower to full-load amps with efficiency, and amps back to kilowatts and kVA.
Most online converters handle one of those jobs, usually kW to amps, and leave motors, apparent power, and the reverse calculation to separate pages. This page is the three-phase specialist: every mode uses line-to-line voltage with standard presets (208, 240, 400, 415, and 480 V) plus a custom option, the arithmetic is exact with no intermediate rounding, and you can copy the result straight into a panel schedule. If you need DC or single-phase instead, the general kw to amps calculator covers those; everything below is three-phase only.
Planning aid only. Final conductor, breaker, and overload sizing must follow the NEC and local code: have a qualified electrician verify.
Quick Answer
Key takeaways
- From kW: I = kW × 1000 ÷ (√3 × V × PF). From kVA: drop the PF, I = kVA × 1000 ÷ (√3 × V).
- Motor HP is shaft output, not input. Full-load amps divide by efficiency and power factor, so motor current is always higher than a naive HP-to-kW conversion suggests.
- Voltage is line-to-line in every mode. On a 480Y/277 V system, the value to enter is 480, not 277.
- This calculator keeps the arithmetic exact. Published examples elsewhere round mid-calculation and drift by half an amp on a standard motor example.
- Quick Answer
- The formula, and where the √3 comes from
- Worked examples, checked against this exact tool
- Quick reference: kW to amps at 400 V and 480 V
- Motor mode: why efficiency divides, never multiplies
- Choosing a power factor
- Line-to-line vs line-to-neutral voltage
- What competing calculators miss
- Frequently asked questions
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The formula, and where the √3 comes from
Every mode on this page is the same equation rearranged: total power in a balanced three-phase system equals √3 times line-to-line voltage times line current times power factor. Solve it for whichever variable you are missing and you get the four tabs.
The √3 (about 1.732) is geometry, not a fudge factor. The three phases sit 120 electrical degrees apart, and when two sinusoidal voltages displaced by 120 degrees combine, the line-to-line result is √3 times the individual phase voltage. That is why a 480 V wye system has 277 V from line to neutral (480 ÷ √3), and why √3 appears in every three-phase formula using line-to-line values.
Worked examples, checked against this exact tool
Each tab starts pre-filled with the example in its row, so you can reproduce every figure with one click. All four were verified against the calculator code itself, not rounded from a textbook:
| Mode | Inputs | Result |
|---|---|---|
| kW to amps | 50 kW, 400 V, PF 0.85 | 84.90 A (58.82 kVA, 30.99 kvar) |
| kVA to amps | 50 kVA, 400 V | 72.17 A |
| Motor FLA | 75 HP, 460 V, PF 0.88, 93% efficient | 85.81 A (input power 60.16 kW) |
| Amps to power | 30 A, 480 V, PF 0.9 | 22.45 kW (24.94 kVA, 10.87 kvar) |
The motor row deserves a second look. A published competitor example for this same 75 HP motor reports 86.4 A, because it rounds intermediate values mid-calculation. The exact figure is 85.81 A. Half an amp rarely changes a breaker choice, but across a panel schedule of motors the drift adds up, and exact arithmetic costs nothing.
Quick reference: kW to amps at 400 V and 480 V
For a fast panel-schedule estimate at the standard 0.85 planning power factor, use this table. For any other voltage or power factor, the calculator is exact; the table is the shortcut.
| Power (kW) | Amps at 400 V, PF 0.85 | Amps at 480 V, PF 0.85 | Typical load |
|---|---|---|---|
| 10 | 16.98 | 14.15 | Small machinery |
| 25 | 42.45 | 35.38 | Compressors, pumps |
| 50 | 84.90 | 70.75 | Large motors |
| 75 | 127.36 | 106.13 | Heavy machinery |
| 100 | 169.81 | 141.51 | Industrial systems |
Motor mode: why efficiency divides, never multiplies
A motor's horsepower rating is mechanical output at the shaft, and confusing it with electrical input is the most common three-phase sizing error. The panel does not supply 75 HP worth of electricity to a 75 HP motor; it supplies 75 HP divided by the efficiency, because the motor wastes the difference as heat. At 93% efficiency the input is 60.16 kW against 55.95 kW at the shaft, and the full-load current formula divides by both efficiency and power factor.
The strongest competitor on this keyword makes the same input-versus-shaft distinction, and it is the right call: a multiplied-instead-of-divided efficiency is how feeders end up undersized. Compressor motors on large air conditioning equipment are the everyday case: size the cooling load with an air conditioner tonnage calculator first, then bring the motor figures here for the current.
Choosing a power factor
Power factor is the ratio of real power (kW) to apparent power (kVA), and it moves the current more than most people expect: the same 50 kW load draws 72.17 A at unity and 84.90 A at 0.85. When the nameplate does not give one, pick by load type:
Power factor falls as a motor unloads, which is why an oversized motor loafing at half load draws more current per useful kilowatt than a correctly sized one. The US Department of Energy fact sheet on motor load and efficiency makes the same point: motors are most efficient near 75% of rated load.
Line-to-line vs line-to-neutral voltage
All four modes expect line-to-line voltage: 208, 240, 400, 415, or 480 V, or a custom value like 460. If your source gives line-to-neutral instead (277 V on a 480 V wye system, 120 V on a 208 V system), either multiply it by √3 to get the line-to-line value, or use the line-to-neutral form of the equation directly:
Once the reverse mode gives you kilowatts for a running load, a utilities cost per month calculator turns that figure into a monthly running cost, usually the number the building owner actually asks for. And with the current known, the next design step is the cable: the electrical cable size calculator picks a conductor size for the amps you found here.
What competing calculators miss
We reviewed the dedicated three-phase tools ranking for this keyword. The field is stronger than most calculator niches, and the gaps are specific:
CalculatorsConversion is the strongest rival, and it still stops short. Its three-phase page correctly separates electrical input kW from shaft output kW and carries honest "not a code table" caveats, but the tool has no voltage presets, no copy or share for panel schedules, and no reverse amps-to-power mode. This page has all four modes, presets with a custom fallback, and one-click copy.
ElectricianCalc is deep theory with a power-first tool. Its solve-for tables and wye/delta content are excellent, but the tool runs from voltage and current toward power, backwards for the common job of sizing from a kW rating, and there is no motor-efficiency mode. The Motor FLA tab here is built for exactly that job.
Electrical-World rounds mid-calculation and drifts. Its 75 HP worked example reports 86.4 A against the exact 85.81 A, a small error that compounds across a panel schedule. This calculator carries full precision to the final display and shows its formula with every result.
AreaCalculators covers one direction only. Its amps-per-phase page is a single mode (power, voltage, and power factor in, amps per phase out), and its sister page states a single-phase formula with no power factor at all before contradicting itself further down. Four modes, one page, no contradictions is the bar this page sets.
Frequently asked questions
Use I = kW × 1000 ÷ (√3 × V × PF), where V is the line-to-line voltage. A 50 kW load at 400 V with a 0.85 power factor draws 84.90 A. The kW in this formula is electrical input power; if your figure is motor shaft output, use the Motor FLA mode so efficiency is handled correctly.
I = kVA × 1000 ÷ (√3 × V). Power factor drops out because kVA is already apparent power: it counts the full current the conductors carry, not just the useful part. A 50 kVA transformer load at 400 V draws 72.17 A.
FLA = HP × 746 ÷ (√3 × V × efficiency × PF). A 75 HP motor at 460 V, 93% efficiency, and 0.88 power factor draws 85.81 A, with an electrical input of 60.16 kW. For code sizing, NEC Table 430.250 values and the nameplate govern; this formula is the engineering check behind them.
Because horsepower is what the motor delivers at the shaft, and the supply must provide that plus the losses. Input = output ÷ efficiency, so a 93% efficient motor needs about 7.5% more electrical power than its shaft rating. Multiplying would understate the current and undersize the circuit.
Use 0.85 as the planning default. Loaded induction motors typically run 0.8 to 0.9, resistive heaters are 1.0, and the Engineering ToolBox power factor table lists a fully loaded induction motor at exactly 0.85. An unloaded motor can fall to 0.17, which is why lightly loaded motors draw surprising current.
Not directly: the tool expects line-to-line voltage. Multiply the line-to-neutral value by √3 first (277 × 1.732 = 480 V), or use the line-to-neutral form I = kW × 1000 ÷ (3 × V × PF), which gives the same result on a balanced system.
No, and forcing it would give wrong answers: the √3 factor belongs to three-phase only. Single-phase current is I = kW × 1000 ÷ (V × PF), and DC is I = W ÷ V. The general kw to amps calculator on this site handles DC, single-phase, and three-phase with equipment presets; this page is the three-phase specialist with the extra modes.
The calculated current is the starting point, not the breaker size. NEC Article 430 sizes motor conductors at 125% of the table full-load current and sets overcurrent protection by motor type and starting method, and other loads have their own rules. Use the figure here for load and panel planning, then have a qualified electrician confirm the final breaker and conductor sizes.
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References
- The Engineering ToolBox, "Three-Phase Power: Equations" - real, apparent, and reactive power equations in line-to-line and line-to-neutral forms, with a typical power factor table (loaded induction motor 0.85). Engineering ToolBox: Three-Phase Power Equations. Accessed October 2026.
- All About Circuits, "Three-phase Y and Delta Configurations" - how wye and delta connections relate line and phase voltages and currents. All About Circuits: Three-phase Y and Delta Configurations. Accessed October 2026.
- All About Circuits, "Phase Rotation" - the 120-degree phase displacement between the three phases that produces the √3 relationship. All About Circuits: Phase Rotation. Accessed October 2026.
- EC&M, "Motors and the NEC" - applying Article 430 allowances for motor overcurrent protection, and when Table 430.250 full-load currents versus nameplate values govern. EC&M: Motors and the NEC. Accessed October 2026.
- Electrical Contractor Magazine, "Branch-Circuit, Feeder and Service Calculations, Part XVI" - worked feeder calculations using NEC Table 430.250 motor full-load currents and the 125% rule in 430.24. EC Magazine: Branch-Circuit, Feeder and Service Calculations. Accessed October 2026.
- US Department of Energy, "Determining Electric Motor Load and Efficiency" (fact sheet) - how motor load and efficiency behave across the load range, including the drop in power factor at part load. DOE: Determining Electric Motor Load and Efficiency (PDF). Accessed October 2026.