About this calculator
How many amps does a 2 kW heater draw? What power does a 16 A circuit carry, or a 3 × 25 A main fuse? This calculator converts between active power P (W, kW), current I (A) and apparent power S (VA, kVA) for a DC, single-phase or three-phase circuit, taking the power factor cos φ into account, and shows the formula for every result.
Formula
DC: P = U × I
single-phase: S = U × I
three-phase: S = √3 × U × I
AC, both: P = S × cos φ
Q = S × sin φ
U— DC voltage; the phase voltage (230 V) for single-phase; the line-to-line voltage (400 V) for three-phase.cos φ— power factor: 1 for a resistive load (heater, kettle, oven element), lower for motors, compressors and many electronic power supplies — see the appliance's rating plate.S— apparent power, what cables and fuses must carry;P— active power, what does the work and what the meter counts;Q— reactive power.sin φ = √(1 − cos² φ).
The three-phase formulas assume a balanced load — the same current in each phase.
Worked example
A 10 kW three-phase heater (a resistive load, cos φ = 1) on 400 V:
S = P = 10 000 VA
I = 10 000 / (√3 × 400) = 10 000 / 692.8 = 14.43 A
Q = 0
A heat pump whose rating plate gives 3 kW electrical input at cos φ = 0.9, single-phase 230 V:
S = 3000 / 0.9 = 3333 VA
I = 3333 / 230 = 14.49 A
Q = 3333 × √(1 − 0.9²) = 3333 × 0.436 = 1453 var
A motor's rating plate states its shaft power. The electrical power it
draws is larger: P = P_shaft / η, with η the motor efficiency from the same
plate. Enter that electrical power here, not the shaft power.
And the other way: a 16 A single-phase circuit at 230 V and cos φ = 1
carries 230 × 16 = 3680 W; a 3 × 25 A three-phase supply carries
√3 × 400 × 25 = 17.3 kW.
FAQ
What is the difference between W and VA? Watts (P) are the power actually converted into heat or work. Volt-amperes (S) are voltage times current — what the wiring carries. They are equal only when cos φ = 1.
Which voltage do I enter for three-phase? The line-to-line voltage: 400 V in a Lithuanian low-voltage network. The phase voltage (230 V) is shown in the result.
Why is cos φ hidden for DC? DC has no phase angle; power is simply voltage times current, and there is no separate apparent power — choose power or current for DC.
Does this size my cable or breaker? No — it only converts. Use the cable cross-section calculator to choose the cable and breaker, and the voltage drop calculator for long runs.
Assumptions and limits
- Sinusoidal voltage and current; harmonics are ignored.
- Three-phase loads are balanced; the neutral current is zero.
cos φis entered by you; nothing is assumed about the load type.