3-Phase Power Calculator
Compute balanced three-phase kW and kVA from line V, line A, and PF using the √3 relationship—use Quick Examples for glove-friendly presets (for example a 50 HP fan upgraded from 40 HP: match nameplate FLA, not the old breaker label). Advanced motor inrush, harmonics, and unbalance still need field measurements and coordination studies.
Input Parameters
Quick Examples:
About this calculator
Converts balanced three-phase voltage, line current, and PF into real and apparent power for motor lines and feeders before you lock harmonics or starting multiples. For the rest of the kW/kVA/current chain, use the power calculators hub.
If you start from kW or kVA without line current, use kW to kVA first, then kVA to amps for balanced line A; use factory load when sizing from device counts. After you have design amps, screen cable size (ampacity + installation derating on your tables) and voltage drop on long feeders; breaker size ties to continuous vs non-continuous duty per NEC or IEC 60364 practices.
Calculation Results
Balanced three-phase, steady-state line current; does not replace motor inrush, harmonic, or unbalance studies.
Engineering disclaimer
This calculator provides preliminary power calculations only. The motor FLA / breaker table below is illustrative—always use manufacturer nameplate FLA/LRA and IEC 60034 / NEC 430 family rules for thermal and short-circuit protection, selective coordination, and fault levels. For final electrical system design, equipment sizing, and compliance with local electrical codes, consult a licensed electrical engineer or certified professional. Actual requirements may vary based on detailed load analysis, harmonics, motor starting currents, and specific application requirements.
Balanced 3φ power — quick √3 reference
Illustrative kW at PF 0.85 from kW = √3 × VL-L × I × PF ÷ 1000 (line quantities). Use the calculator above for your exact V, I, and PF.
| VL-L | Line I (A) | Approx. kW @ PF 0.85 |
|---|---|---|
| 400 | 10 | 5.9 |
| 400 | 50 | 29.4 |
| 480 | 100 | 70.7 |
3-Phase Power Formula & Explanation
- VL-L
- Line-to-line voltage in volts (measured at the equipment or bus).
- I (or IL)
- Balanced line current in amperes (RMS), one conductor of the three-phase set.
- PF (cos φ)
- Displacement power factor: ratio of real power to apparent power for fundamental frequency; harmonics need separate treatment.
- kW / kVA
- Real and apparent power; kVAR (reactive) fills the triangle when PF < 1.
Balanced line quantities: kW = √3 × VL-L × I × PF ÷ 1000 and kVA = √3 × VL-L × I ÷ 1000 so kW = kVA × PF. Example: 400 V, 10 A, PF 0.85 → about 5.9 kW and 6.9 kVA. The green next-step card points to kVA to amps at your voltage and phase.
kW vs kVA vs kVAR: kW is real power; kVA is apparent; kVAR is reactive. Conductors and sources must carry kVA; utility and PFC discussions often focus on kVAR when PF lags.
Motor note: running FLA follows the formulas above; locked-rotor, thermal overloads, and branch protection follow NEC 430 and IEC 60034 / IEC 60364 practices (or your local code)—size devices with those rules, not this steady-state line alone. Fault current and selective coordination are outside this page.
- Ampacity: tabulated conductor current must be derated for installation method, ambient, grouping, and harmonics on project tables.
- Balanced load assumed; unbalance needs per-phase work.
- √3 ≈ 1.732 ties line values to total three-phase power.
More: 3-phase power formula & examples · 3-phase power guide · Factory load · kW to kVA · kVA to amps · Cable size · Voltage drop · Power Calculators Hub
kVA² = kW² + kVAR²; with PF known, PF = kW ÷ kVA.
Motor FLA / breaker quick reference (400 V class)
Illustrative running FLA and order-of-magnitude LRA from nameplate practice; verify NEC 430.52 / 430.22, IEC 60034 nameplate currents, and nameplate LRA before selecting devices.
| Motor size | Typical FLA (400V) | Typical LRA | Screening breaker (inverse-time) |
|---|---|---|---|
| 5 HP | 7–8 A | 42–56 A | 20 A class |
| 10 HP | 14–16 A | 84–112 A | 30 A class |
| 25 HP | 35–40 A | 210–280 A | 60 A class |
| 50 HP | 70–75 A | 420–560 A | 200 A class |
| 100 HP | 140–150 A | 840–1200 A | 400 A class |
From HP use 1 HP ≈ 0.746 kW, then FLA ≈ kW × 1000 ÷ (√3 × V × PF × η) with nameplate η when known. This table is not a substitute for OEM data, cable ampacity, or fault studies.
Frequently Asked Questions
What is the difference between line current and phase current in three-phase systems?
Line current flows in each supply conductor; phase current flows in each winding. In delta, line current equals phase current times √3; in wye (star), line current equals phase current. Field work usually starts from line quantities.
How do I calculate kVA from kW in a three-phase system?
kVA = kW ÷ power factor. Example: 10 kW at PF 0.85 gives about 11.76 kVA. The calculator shows both once V, I, and PF are set.
What power factor should I use for different types of loads?
Resistive loads are near PF 1.0; motors often 0.8–0.9; electronics often 0.9–0.95. For mixed plants use a weighted average or the lowest PF for a conservative envelope; 0.85 is a common placeholder when data is thin.
Why is the recommended breaker rating higher than the calculated current?
Protective devices are sized above steady-state FLA for continuous-duty margins (often 125% where codes require), inrush, coordination, and future load growth—always follow NEC, IEC 60364 family rules, or your local code and manufacturer tables.
Can I use this calculator for single-phase systems?
No. This page assumes balanced three-phase with √3 in the power equation. Single-phase real power uses P = V × I × PF without √3; use a single-phase calculator for those branches.
How do I calculate 3-phase current from kilowatts?
I (A) = (kW × 1000) ÷ (√3 × V_L-L × PF). Example: 10 kW at 400 V and PF 0.85 is about 17 A. Always use measured V and PF when commissioning.
What is the difference between kW and kVA in 3-phase systems?
kW is real power; kVA is apparent power. kW = kVA × PF. Size transformers and generators on kVA; utility demand charges often track both kW and PF.
How do I convert horsepower to kilowatts for 3-phase motors?
1 HP ≈ 0.746 kW. Convert HP to kW, then use the three-phase current formula with nameplate PF and efficiency when known; remember locked-rotor and starting multiples are not modeled here.
What voltage should I use for 3-phase calculations?
Use line-to-line (phase-to-phase) voltage. Nominal 380/400/415 V is common in IEC markets; 480 V is common in North America. Prefer measured bus voltage at the equipment terminals when harmonics or long feeders skew the nameplate assumption.
How do I calculate 3-phase power factor from current and voltage?
If kW and kVA are known, PF = kW ÷ kVA. If kW, line-to-line voltage, and line current are known, PF = (kW × 1000) ÷ (√3 × V × I). Example: 10 kW at 400 V with 20 A line current is about 0.72 PF. Very low PF may trigger correction or utility penalties.
Why is my 3-phase current calculation different from measured values?
Mismatch comes from nominal versus measured voltage, true PF versus assumed PF, phase unbalance, harmonics, VFD waveforms, and instrument accuracy. Use clamp-on RMS readings at the load and reconcile with the site energy meter when disputes arise.