Power Conversion Calculator

When you know the kW but not the amps. Converts load capacity and current both ways, and gives apparent (kVA) and reactive (kvar) power at once.

Result

Enter a load capacity or current, then press Calculate.

What this tool does

The power conversion calculator converts between load capacity (kW, kVA, HP) and line current (A) in both directions. On site, nameplates and equipment specs usually state kW, but wire sizing, breaker selection and voltage drop all require current in amperes. This calculator bridges that gap. It handles single-phase and three-phase circuits separately, and applies power factor (cos φ) and motor efficiency to find the power actually drawn from the supply. Results include real power (kW), apparent power (kVA), reactive power (kvar) and horsepower, so the same output can feed transformer sizing or power-factor correction studies. It uses physical formulas rather than lookup tables, so there is no citation error and results follow deterministically from the inputs.

Who uses this

Starting wire and breaker sizing from a motor nameplate rated in kW
Back-calculating actual power consumption from a clamp-meter current reading
Summing apparent power to review transformer or generator capacity in kVA
Checking reactive power to size power-factor correction capacitors
Converting the HP rating of imported equipment into kW, the Korean standard

How to use (4 steps)

1Choose the direction: pick «capacity → current» if you read kW from a nameplate, or «current → capacity» if you measured amps with a clamp meter.
2Enter the value and unit: in the capacity direction choose kW, kVA or HP and enter the figure. In the current direction enter the measured amperes.
3Set circuit conditions: choose phase and voltage. The Korean low-voltage standard is 220 V single-phase and 380 V three-phase. Enter a power factor that matches the load (general 0.8, induction motor 0.85, inverter 0.95, resistive heater 1.0).
4For motors, enter efficiency: nameplate kW is shaft output, so supply input is larger by the losses. Use the nameplate efficiency if available, otherwise about 0.9. Leave it blank to treat the input as supply-side power already.

Formulas

Three-phase current: I = P × 1000 ÷ (√3 × V × cos φ) Single-phase current: I = P × 1000 ÷ (V × cos φ) Three-phase power: P = √3 × V × I × cos φ ÷ 1000 Single-phase power: P = V × I × cos φ ÷ 1000 Apparent power: S (kVA) = P (kW) ÷ cos φ Reactive power: Q (kvar) = √(S² − P²) Motor input: P_input = P_output ÷ efficiency (η) Horsepower: 1 HP = 0.7457 kW (electrical horsepower) Example: three-phase 380 V, 22 kW motor, cos φ 0.85, η 0.9 → supply input = 22 ÷ 0.9 = 24.44 kW → I = 24440 ÷ (1.732 × 380 × 0.85) ≈ 43.7 A

Worked examples

Example 1: 22 kW induction motor (three-phase 380 V)

The nameplate reads 22 kW, cos φ 0.85, efficiency 0.9. Dividing shaft output by efficiency gives a supply input of 24.44 kW, and a line current of about 43.7 A. That figure is the starting point for wire and breaker selection. If the nameplate also lists rated current, compare them — they typically agree within the 42–44 A range.

Example 2: 3 kW heater (single-phase 220 V)

A heater is a resistive load, so its power factor is 1.0. With no reactive component the calculation reduces to I = 3000 ÷ 220 = 13.6 A. Apparent power equals real power at 3 kVA and reactive power is 0 kvar. Loads at unity power factor are not candidates for power-factor correction.

Example 3: reviewing a 100 kVA transformer

If total load is 80 kW at an overall power factor of 0.8, apparent power is 80 ÷ 0.8 = 100 kVA. Transformers are rated in kVA, so at least 100 kVA is required and a margin usually pushes you one size up. Reactive power here is √(100² − 80²) = 60 kvar; raising the power factor to 0.95 would cut the required apparent power to about 84 kVA.

Frequently asked questions

What is the difference between kW and kVA?

kW is real power that does work; kVA is the apparent power the supply must deliver. Their ratio is the power factor (cos φ). Cables, breakers and transformers are sized by current, so kVA governs them, while energy billing is based on kW (strictly kWh).

What power factor should I use if I don't know it?

The nameplate value is the most accurate. Otherwise estimate from load type: 0.8 for general power loads, 0.85 for an induction motor at rated load, 0.95 for inverters and SMPS, 1.0 for heaters and incandescent lamps. A lower power factor yields a higher current, so choose a lower value if you want to err on the safe side.

Do I have to enter motor efficiency?

For motors it is worth entering. Nameplate kW is shaft output, and the supply must provide more to cover losses. A 22 kW motor at 0.9 efficiency draws 24.4 kW from the supply. For loads that consume electricity directly, such as heaters and lighting, leave the efficiency field empty.

Why does √3 appear in three-phase calculations?

In a three-phase circuit line voltage and phase voltage differ by a factor of √3, and the three phases share the load 120 degrees apart. The result is that three-phase current is lower than single-phase for the same power at the same voltage — which is why large loads use three-phase.

For HP to kW, is it 0.746 or 0.735?

This calculator uses 0.7457, the electrical horsepower. The value 0.735 (PS, metric horsepower) comes from Japanese and European mechanical practice. Korean motor nameplates use kW as standard, so HP mostly appears on imported equipment, where electrical horsepower is usual.

Can I use the calculated current directly as the breaker rating?

No. The result is normal running current, while a breaker is chosen one step up to allow for starting current and a safety factor. Direct-on-line motor starting draws five to seven times rated current, which needs its own margin. Continue with the breaker capacity calculator.

It warns that current exceeds 1,000 A.

That is beyond what a single low-voltage circuit comfortably handles. At this scale you should review the incoming supply (transformer, switchgear), circuit splitting and parallel cables — high-voltage supply may be the better option. The calculation itself still completes normally.

Cautions

•This calculator converts steady-state running conditions. It does not model motor starting current (five to seven times rated) or inrush, so breaker and protection coordination must be reviewed separately.
•Power factor and efficiency are assumptions you enter. Always use nameplate or test-report values when available — a wrong assumption can shift the current by more than 10%.
•The result is line current. For a delta-connected three-phase load, phase current is the line current divided by √3.
•Loads with high harmonic content (many inverters or rectifiers) can draw more RMS current than calculated. Neutral conductor sizing needs separate review.
•Final design must be verified by a qualified electrical engineer against KEC (Korea Electro-technical Code) and site conditions. This calculator is a design aid.

Last reviewed: 2026-09-09