The nameplate full-load current takes priority over any calculation. Cables, overload settings and protective devices must be selected by a competent person to BS 7671 or local rules.
Formula and breakdown
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This motor full load current calculator works out the current an electric motor draws at its rated output from its power in kW or hp, the supply voltage, power factor and efficiency, and estimates the starting current, for electricians, maintenance engineers, students and machinery buyers in the UK, Europe and elsewhere. It covers three-phase motors on 400 V and single-phase motors on 230 V, and shows the input power, kVA and kVAr behind the result.
Quick answer: A motor full load current calculator divides the electrical input power (output kW divided by efficiency) by 1.732 x voltage x power factor for three-phase, or by voltage x power factor for single-phase. Example: a 7.5 kW motor at 400 V, power factor 0.85 and 90% efficiency draws 14.15 A per line.
What Is a Motor Full Load Current Calculator?
A motor full load current calculator is a tool that estimates the current a motor takes from the supply when it delivers its rated shaft power. The kW or hp on a motor’s nameplate is mechanical output, so the electrical input is larger by the losses. In plain words: input kW equals output kW divided by efficiency; three-phase current equals input watts divided by (1.732 x line voltage x power factor); single-phase current equals input watts divided by (voltage x power factor).
Both power factor and efficiency matter. The Wikipedia article on induction motors gives full-load efficiency of 85% to 97% and a power factor of about 0.85 to 0.90 at full load, falling to around 0.20 at no load. Small motors sit at the lower end of both ranges, which is why they draw more current per kW than large ones.
How Do You Use the Motor Full Load Current Calculator?
- Choose three-phase or single-phase.
- Enter the rated output from the nameplate in kW or hp.
- Enter the supply voltage: line-to-line for three-phase (400 V in the UK and EU), or 230 V for single-phase.
- Enter the full-load power factor (cos ฯ) and efficiency from the nameplate or datasheet. The defaults are example values.
- Choose the starting method and, if you have it, the maker’s starting current ratio.
- Read the full-load current, input power, kVA, kVAr, losses and estimated starting current.
How Do You Calculate the Full Load Current of a Three-Phase Motor?
Divide output power by efficiency, then divide by 1.732 x line voltage x power factor: for an 11 kW motor at 400 V, 0.85 and 90%, that is 11,000 / 0.9 / (1.732 x 400 x 0.85) = 20.8 A. The 1.732 is the square root of 3, which applies to balanced three-phase loads measured with line voltage and line current. For loads that are not motors, the kW to amps calculator does the same sum without the efficiency step.
The table gives full-load current at 400 V three-phase and 230 V single-phase for common motor sizes, using example values of 0.85 power factor and 90% efficiency throughout. Real motors differ, especially small ones, so use the nameplate figure where you have it.
| Output (kW) | Three-phase 400 V | Single-phase 230 V |
|---|---|---|
| 0.37 | 0.7 A | 2.1 A |
| 0.75 | 1.4 A | 4.3 A |
| 1.5 | 2.8 A | 8.5 A |
| 2.2 | 4.2 A | 12.5 A |
| 3 | 5.7 A | 17.1 A |
| 4 | 7.5 A | Usually three-phase |
| 5.5 | 10.4 A | Usually three-phase |
| 7.5 | 14.2 A | Usually three-phase |
| 11 | 20.8 A | Usually three-phase |
| 15 | 28.3 A | Usually three-phase |
| 22 | 41.5 A | Usually three-phase |
| 30 | 56.6 A | Usually three-phase |
| 45 | 84.9 A | Usually three-phase |
| 75 | 141.5 A | Usually three-phase |
Why Is the Nameplate Current Different From the Calculation?
The nameplate current is measured by the manufacturer for that exact motor, so it already includes its real efficiency and power factor; a calculation with typical values can differ noticeably, particularly for small motors whose efficiency and power factor are lower. Always use the nameplate figure for overload settings and cable sizing. A calculation is useful when the plate is missing or unreadable, when comparing motors before buying, or when checking whether a supply can take an extra machine.
The current also depends on load. A motor running well below its rated output draws less current, but its power factor falls sharply, so the reduction is smaller than you might expect. The power factor calculator shows how kVAr and capacitor correction relate to this.
How Much Current Does a Motor Draw When Starting?
Started direct-on-line, a standard induction motor draws about 6 to 7 times its full-load current for a short time, according to the Wikipedia article on induction motors, and the Wikipedia article on soft starters notes inrush up to 7 to 10 times running current, and 10 to 15 times for some higher-efficiency motors. That surge can trip breakers, dim lights and cause voltage dips for neighbours on the same supply.
Reduced-voltage starting lowers the surge. In a star-delta starter the windings start in star, so each sees the phase voltage (1/1.732 of line voltage) and the line current is one third of what direct-on-line starting in delta would draw; the motor full load current calculator’s star-delta option divides the example DOL multiple of 6 by 3. Soft starters and variable speed drives limit current further, by an amount set in the device, so enter the maker’s figure.
How Many Amps Does a 1 hp Motor Draw?
One horsepower is 0.746 kW, so a 1 hp three-phase motor at 400 V, 0.80 power factor and 80% efficiency draws about 1.7 A per line; on 230 V single-phase at the same values it draws about 5.1 A. Small motors often have lower efficiency and power factor than the defaults, so enter the nameplate values to get a closer figure.
Who Should Size Motor Cables and Protection?
Motor circuits, overload relays, cables and isolators must be selected and installed by a competent person. In the UK that means work to BS 7671, the IET Wiring Regulations, which the IET co-publishes with BSI and which is currently the 18th Edition with Amendment 4:2026; elsewhere, follow local wiring rules. Long cable runs to motors also need a check of voltage drop, especially during starting; the voltage drop calculator gives a first estimate.
Worked Example: A 7.5 kW Pump Motor on a 400 V Supply
Example: a farm in Cumbria is adding a 7.5 kW three-phase pump motor on a 400 V supply. The datasheet, for this example, gives a full-load power factor of 0.85 and an efficiency of 90%.
- Input power: 7.5 kW / 0.90 = 8.333 kW.
- Full-load current: 8,333 / (1.732 x 400 x 0.85) = 14.15 A per line.
- Apparent power: 8.333 / 0.85 = 9.80 kVA; reactive power about 5.16 kVAr.
- Losses at full load: 8,333 minus 7,500 = 833 W of heat.
- Starting current direct-on-line at an example 6 times: 84.9 A; with star-delta, about 28.3 A.
Frequently Asked Questions
How do I calculate motor full load current?
Divide the output in kW by the efficiency to get input power. For three-phase, divide input watts by 1.732 x line voltage x power factor; for single-phase, divide by voltage x power factor. A 7.5 kW motor at 400 V, 0.85 and 90% draws 14.15 A.
What is the full load current of a 1 kW three-phase motor?
At 400 V with an example power factor of 0.85 and efficiency of 90%, a 1 kW three-phase motor draws about 1.9 A per line. Small motors often have lower efficiency and power factor, so the nameplate figure may be a little higher.
Why does motor efficiency affect the current?
The kW rating is the mechanical power at the shaft. The supply must also provide the losses as heat in the windings, core and bearings, so input power is output divided by efficiency. A lower efficiency means more input power and therefore more current for the same output.
What is the difference between full load current and starting current?
Full load current is the steady current at rated output. Starting current is the brief surge while the motor accelerates, typically several times higher when started direct-on-line. Protection must carry the starting surge without tripping while still protecting against sustained overload.
How do I convert motor hp to kW?
Multiply horsepower by 0.746 to get kilowatts, so a 10 hp motor is 7.46 kW of output. Divide kW by 0.746 to go the other way. The motor full load current calculator accepts either unit and shows the conversion alongside the current.
Can I size a motor cable from the calculated current?
Use the nameplate current where possible, and have a competent person select the cable, overload and protective device. In the UK that work must meet BS 7671, which also covers grouping, installation method, ambient temperature and voltage drop, not only the current.
Checked October 2026 by the Solaxyra Editorial Team. Sources: Wikipedia: Induction motor, Wikipedia: Motor soft starter, Wikipedia: Three-phase electric power, IET: BS 7671.