Three-phase current equation
For balanced three-phase systems, current can be calculated from either real power in kW or apparent power in kVA.
where:
- I
- Balanced three-phase line current[A]
- P
- Real power[kW]
- S
- Apparent power[kVA]
- VL-L
- Line-to-line voltage[V]
- PF
- Power factor
- η
- Efficiency as a decimal
The equation assumes a balanced load. Motor starting current, harmonics, and unbalanced phase loading require separate checks.
How to calculate three-phase current
Balanced three-phase current can be calculated from real power in kW or apparent power in kVA. A kW input requires line voltage, power factor, and efficiency; a kVA input requires only line voltage because apparent power already represents voltage-current loading.
- Choose whether the known power is kW or kVA.
- Enter line-to-line voltage.
- For kW, enter power factor and efficiency.
- Calculate line current using the √3 three-phase relationship.
- Check nameplate current, starting duty, and phase balance before final sizing.
Three-phase current calculation example
A 30 kW balanced load at 400 V, 0.85 power factor, and 92% efficiency draws approximately 55.4 A. The corresponding input apparent power is about 38.4 kVA.
I = 30,000 / (√3 × 400 × 0.85 × 0.92) = 55.4 A
S = 30 / (0.85 × 0.92) = 38.4 kVA
| Input or result | Value |
|---|---|
| Output power | 30 kW |
| Line voltage | 400 V |
| Power factor | 0.85 |
| Efficiency | 92% |
| Line current | 55.4 A |
Three-phase amps from kW vs. kVA
When power is entered in kW, current increases as power factor or efficiency decreases. When power is entered in kVA, do not divide by power factor again because kVA already represents apparent input power.
| Known value | Current equation |
|---|---|
| Output kW | I = kW × 1000 / (√3 × V × PF × η) |
| Input kVA | I = kVA × 1000 / (√3 × V) |
Line voltage and phase voltage
This calculator expects line-to-line voltage, such as 400 V or 480 V. Do not substitute phase-to-neutral voltage unless the formula and load connection are changed accordingly.
Apply three-phase current to equipment sizing
Use the result for preliminary cable, breaker, contactor, and busbar checks. Starting current, unbalance, harmonics, cyclic loading, ambient temperature, and equipment nameplate data may require a different final rating.
kW, kVA and Amp CalculatorMotor Current CalculatorCircuit Breaker Size Calculator
Assumptions
- Balanced three-phase load
- Line-to-line voltage input
Important Warnings
- Starting current and harmonics are not included.
- Use actual equipment nameplate data for final selection.