3 Phase Power Calculator: kW, kVA, and Full Load Current
A free 3 phase power calculator for kW, kVA, and kVAR; full load current of motors, transformers, and heaters; and three phase voltage imbalance per NEMA MG-1. Use it to size feeders, check measured loads against nameplate, or verify a balanced three-phase system before energizing.
Three-Phase Power Calculations
Calculate power relationships in balanced three-phase systems
Need this validated by a licensed PE?
A free calculator gets you in the ballpark. For permit-stamped, defensible work, ClarkTE delivers a PE-stamped three-phase load and feeder study from a working engineer — typically within 48 hours of receiving your one-line and load data.
What is three phase power?
Three phase power is an AC distribution system that uses three sinusoidal voltages, each offset by 120 degrees, instead of a single waveform. The three phases combine in a way that delivers power continuously rather than in pulses, which is why almost every motor over a few horsepower runs on three phase.
In North American commercial and industrial buildings the most common three phase voltages are 208Y/120 V (small commercial), 480Y/277 V (most industrial), and 600Y/347 V (Canadian industrial). The "Y" denotes a wye-connected secondary; the slash separates the line-to-line voltage from the line-to-neutral voltage, and the relationship between them is the √3 factor that shows up in every three phase formula.
Why does the √3 matter? Because when you measure the three line currents, each is in phase with its own line-to-neutral voltage but not with the line-to-line voltage you usually use as the system label. The √3 (≈1.732) bridges the two. That's the entire reason kVA = √3 × V_LL × I and kW = √3 × V_LL × I × power factor.
Three phase formulas you'll actually use
| What you want | Formula | Notes |
|---|---|---|
| Apparent power S | S (kVA) = √3 × V_LL × I / 1000 | Always positive |
| Active power P | P (kW) = √3 × V_LL × I × pf / 1000 | Multiply by power factor |
| Reactive power Q | Q (kVAR) = √(S² − P²) | Or S × sin(arccos pf) |
| Line current from kW | I = (kW × 1000) / (√3 × V_LL × pf) | Inverse of P formula |
| Motor FLA from HP | I = (HP × 746) / (√3 × V × η × pf) | Use NEC 430.250 for design |
| Voltage imbalance | % = (max deviation / avg) × 100 | Keep ≤ 1% per NEMA MG-1 |
Three-phase systems behaving badly?
If you're seeing voltage imbalance over 2%, repeat motor failures, or unexplained heating in feeders, ClarkTE's protection and control team can do a power quality study, root-cause analysis, and produce a PE-stamped recommendations report.
Three-phase power FAQ
What is three-phase power?
Three-phase power uses three sinusoidal voltages each offset by 120 degrees. The phases combine to deliver power continuously rather than in pulses, which is why almost every motor over a few horsepower runs on three-phase. Common North American three-phase voltages are 208Y/120 V, 480Y/277 V, and 600Y/347 V.
How do I convert kW to amps for a three-phase load?
I = (kW × 1000) / (√3 × V_LL × pf). The √3 factor (≈1.732) comes from the 120° phase relationship between line voltages. The Power Calculations tab returns the line current directly when you enter kW and power factor.
What's the limit on three-phase voltage imbalance?
NEMA MG-1 recommends voltage imbalance ≤ 1%. Above 2%, motors must be derated to avoid overheating. The Unbalanced Loads tab calculates imbalance as (max deviation from average / average) × 100% and flags marginal/poor results.