What this electric motor current calculator does
This calculator estimates full-load motor current (amps) from motor output power, voltage, power factor, and efficiency. It supports single-phase AC, three-phase AC, and DC motors. It is useful for quick sizing checks during design, maintenance planning, and troubleshooting.
If you are selecting breakers, contactors, overloads, or cable sizes, this tool provides a practical starting point before applying code-specific rules (such as NEC/IEC tables and local standards).
Formulas used
1) AC single-phase motor current
I = P / (V × PF × η)
- I = current in amps
- P = motor output power in watts
- V = voltage in volts
- PF = power factor
- η = efficiency (decimal form)
2) AC three-phase motor current
I = P / (√3 × V × PF × η)
The factor √3 is used for balanced three-phase systems.
3) DC motor current
I = P / (V × η)
DC calculations do not use power factor.
How to use the calculator
- Select the motor type (single-phase AC, three-phase AC, or DC).
- Enter motor output power and choose the unit (kW, HP, or W).
- Enter system voltage.
- Enter power factor (AC only) and efficiency.
- Click Calculate Current to view estimated full-load amps.
Practical notes for better accuracy
- Use the nameplate values for power factor and efficiency when available.
- For VFD-fed motors, input current can differ from direct-on-line values.
- Startup/inrush current is usually much higher than full-load current.
- Ambient temperature, altitude, and harmonics can affect real operating current.
Example calculations
Example A: Three-phase motor
A 15 kW, 400 V, PF 0.86, 92% efficient motor:
I = 15000 / (1.732 × 400 × 0.86 × 0.92) ≈ 27.3 A
Example B: Single-phase motor
A 2 HP, 230 V motor, PF 0.8, 85% efficient:
2 HP = 1492 W, so I = 1492 / (230 × 0.8 × 0.85) ≈ 9.5 A
Example C: DC motor
A 3 kW DC motor at 120 V, 88% efficiency:
I = 3000 / (120 × 0.88) ≈ 28.4 A
FAQ
Is this current value exact?
It is an engineering estimate based on entered parameters. Real-world load, supply quality, and motor condition may change actual current.
Can I use this for breaker sizing?
Use it as a preliminary calculation only. Final breaker and conductor sizing must follow the applicable electrical code and manufacturer data.
What if I only know horsepower?
No problem. Choose HP in the unit dropdown. The calculator converts HP to kW using 1 HP = 0.746 kW.