About Three-Phase Power Calculator — kW, kVA, Amps & HP
The three-phase power calculator converts between the quantities that size real circuits: real power in kW, apparent power in kVA, line current in amps, and motor horsepower. Pick the direction — kW to amps for a heater or feeder, kVA to amps for a transformer or generator rating, amps to kW for a measured load, or HP to amps for a motor — choose single- or three-phase, set the voltage from the presets (120, 208, 230, 240, 400, 480, 600 V) or type your own, and enter the power factor. The three-phase current formula is I = P × 1,000 / (√3 × V × PF) with V line-to-line; single-phase drops the √3.
Two details the tool gets right that quick mental math often misses: amps from a kVA rating do not involve the power factor at all (kVA is already volts × amps × √3, so a 100 kVA transformer at 480 V delivers 120 A regardless of the load PF), and motor horsepower is shaft output, so the electrical input is HP × 0.7457 kW divided by the motor efficiency before any current is computed — a 10 hp motor at η 0.9 draws the current of an 8.29 kW load, not 7.46 kW.
How It Works
- Choose the conversion: kW → amps, kVA → amps, amps → kW, or HP → amps. The input field switches to the chosen quantity.
- Set the phase toggle. Three-phase uses I = P·1000/(√3·V·PF) with line-to-line voltage; single-phase uses I = P·1000/(V·PF). At the same voltage and power, three-phase current is √3 (about 1.73×) lower.
- Pick a voltage preset or enter any value up to 1,000 V, and set the power factor (0.05–1; 0.8 is a common assumption for mixed loads, 1 for resistive heaters). In HP mode, also set the motor efficiency (default 0.9).
- Read all three sides of the power triangle: amps, kW, and kVA. In HP mode the shaft horsepower is echoed alongside the electrical input kW so the efficiency step is visible.
Worked Example
What does a 10 kW, 400 V three-phase load at power factor 0.8 draw? Current I = 10 × 1,000 / (√3 × 400 × 0.8) = 10,000 / 554.26 = 18.04 A, and the apparent power the supply must carry is S = P/PF = 10/0.8 = 12.5 kVA. Note the power factor pushed the current up: at unity PF the same 10 kW would draw only 14.43 A — the reason utilities and generator sizing care about PF, and the entire business case for power factor correction.
kVA to amps chart: three-phase 400 V and 480 V
Full-load line current for common transformer and generator kVA ratings on the two dominant three-phase LV systems, computed through the calculator via I = kVA × 1,000 / (√3 × V). No power factor appears in this conversion — a kVA rating already is √3 × volts × amps, which is why nameplate currents match these figures at any load PF.
| Rating (kVA) | Amps @ 400 V 3φ | Amps @ 480 V 3φ |
|---|---|---|
| 10 kVA | 14.43 | 12.03 |
| 25 kVA | 36.08 | 30.07 |
| 50 kVA | 72.17 | 60.14 |
| 100 kVA | 144.34 | 120.28 |
| 200 kVA | 288.68 | 240.56 |
Formulas
- Three-phase current from kW
I = P × 1000 / (√3 × V × PF)- Single-phase current from kW
I = P × 1000 / (V × PF)- Apparent power and current from kVA
S = P / PF; I = S × 1000 / (√3 × V) (1φ: drop the √3)- Motor HP to electrical input
P(kW) = HP × 0.7456999 / η
Standards & References
- AC power triangle relationships (P = √3·V·I·PF for 3φ, P = V·I·PF for 1φ) — definitional; V is line-to-line for three-phase
- Mechanical horsepower = 745.6999 W per NIST SP 811 (2008), Appendix B — the tool treats motor HP as shaft output and divides by efficiency for electrical input
- For code-compliant motor branch-circuit conductor sizing, the NEC requires table full-load currents (NEC 430.247–250), which run above the ideal-formula values — use this tool for load estimation and the conductor ampacity tool for wire sizing
Frequently Asked Questions
How do I convert kW to amps on a three-phase system?
I = kW × 1,000 / (√3 × V × PF) with V line-to-line. A 10 kW load at 400 V and PF 0.8 draws 10,000 / (1.732 × 400 × 0.8) = 18.04 A. At unity power factor the same load would draw 14.43 A — lower PF always means more amps for the same real power.
How many amps is a 100 kVA transformer at 480 V?
100 × 1,000 / (√3 × 480) = 120.3 A full-load secondary current. No power factor enters this conversion: kVA is by definition √3 × V × I, so the nameplate kVA fixes the amps at a given voltage regardless of what PF the load runs at. That is also why transformers and generators are rated in kVA, not kW.
Why does the HP-to-amps conversion need a motor efficiency?
Nameplate horsepower is what the shaft delivers, not what the line supplies. The electrical input is HP × 0.7457 kW ÷ efficiency: a 10 hp motor at η 0.9 takes 8.29 kW from the line, and at 460 V, PF 0.85, three-phase, that is 12.23 A. Skipping the efficiency step understates the current by 10–15% on typical motors.
What power factor should I assume?
Resistive loads (heaters, incandescent lighting) are PF 1. Fully loaded modern motors run about 0.85–0.9; lightly loaded motors fall to 0.6 or below. For a mixed panel with no measurement, 0.8 is the customary planning assumption — the tool defaults to it. When the actual PF matters (utility penalties, generator sizing), measure it, and see the power factor correction calculator to fix a low one.
Is the voltage line-to-line or line-to-neutral?
Line-to-line for every three-phase formula here — 208, 400, or 480 V, not 120, 230, or 277 V. The √3 in the formula already accounts for the phase geometry. For single-phase loads, use the voltage the load actually sees across its two terminals (120 or 230/240 V typically).
Can I size motor branch-circuit conductors with these amps?
Use them for load estimation, not code sizing. The NEC requires conductors for a single motor to be sized at 125% of the table full-load current from NEC 430.247–250 — table values that run conservatively above the ideal-formula result. Compute the physics here, then size the wire with the conductor ampacity calculator and check the run with the voltage drop calculator.