Motor Current Calculator
Calculate full-load and starting current for an electric motor.
Please note: For planning and educational use only. Electrical installation is governed by wiring regulations and is dangerous when done incorrectly. Always have work designed, installed and inspected by a qualified electrician.
What the Motor Current Calculator does
Motor nameplate ratings give shaft output power, not electrical input. Input power is higher by the efficiency loss, and the current follows from that with power factor and the three-phase √3 factor applied.
Formula
Input power = Output power ÷ EfficiencyThree phase: I = P_in ÷ (√3 × V × PF)Single phase: I = P_in ÷ (V × PF)Starting current = Full load current × Starting multiple
Inputs explained
| Input | Unit | Required | Notes |
|---|---|---|---|
| Supply type | one of 2 options | Yes | — |
| Motor rating given in | one of 2 options | Yes | — |
| Motor rating | kW or hp | Yes | Accepts more than 0. |
| Supply voltage | V | Yes | Accepts more than 0. |
| Motor efficiency | % | Yes | — |
| Power factor | number | Yes | Accepts 0.1 or more, up to 1. |
| Starting current multiple | number | Yes | Direct-on-line starting is typically 6–8× full load. Accepts 1 or more. |
How to use it
- Choose Supply type and Motor rating given in.
- Enter Motor rating, Supply voltage, Motor efficiency and Power factor and 1 more.
- Select Calculate.
Worked example
A 7.5 kW three-phase motor at 400 V, 90% efficient, 0.85 power factor, direct-on-line.
- Phase
- Three
- Rating
- 7.5 kW
- Voltage
- 400 V
- Efficiency
- 90
- PF
- 0.85
Input 8.333 kW, full load current 14.15 A, starting current 84.9 A at 6×.
Frequently asked questions
Is the nameplate rating input or output power?
Output — the mechanical power at the shaft. Electrical input is always higher, by the inverse of the efficiency.
Why is starting current so high?
At standstill the motor has no back-EMF, so it behaves almost like a short circuit until it accelerates. Reduced-voltage starting methods limit this.
Method and sources
Method. Input power from mechanical output divided by efficiency, then current from the appropriate single- or three-phase relationship with power factor applied.
Applies to. None applied — motor circuit design is governed by national wiring regulations
Assumptions
- The efficiency and power factor entered apply at the operating load; both fall at part load.
Limitations
- Efficiency and power factor are quoted at rated load. A motor running lightly loaded has a materially worse power factor, so the current is higher than a rated-load calculation suggests.
- Starting current is several times running current, and the multiple depends on the starting method — the data plate governs.
- This is a running current, not a protection or conductor selection, both of which are regulated design decisions.
Sources
- The motor data plate, and the national wiring regulations governing motor circuits — Varies by manufacturer and country. Rated efficiency, power factor and starting characteristics, and the rules for protecting a motor circuit.