Motor Full-Load Current Calculator
Estimate the full-load current of a three-phase or single-phase motor from kW, voltage, power factor and efficiency, with the formula and worked example.
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Estimate only. Not engineering or safety advice. Check the result against the applicable standard and the equipment data, and have a competent engineer review it before it is relied on. Terms of use.
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About this calculator
Use this when you know the motor rating in kW but not the current, for example while choosing a starter, a cable or a breaker before the nameplate is available. Efficiency and power factor are inputs because the line current depends on both, not on the shaft power alone.
Limits of this calculator
This is a planning estimate. When the motor nameplate gives a full-load current, use the nameplate value. Starting current is not included; the motor starting calculator covers it.
Cross-check: 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review.
Verification status: Engineering reference
Engineering reference. Formula and method documented and tested against separately coded reference calculations. Not yet Verified: the evidence listed on this page is still needed.
What this calculator checks
- The arithmetic of the published formula for the values you entered, in the units shown.
What remains unverified
- That the inputs suit your application. Take ratings, resistances and constants from the manufacturer data sheet or project specification.
Verification register entry
| Calculation author | Rahul Sharma (author) |
|---|---|
| Technical review | No independent technical review has been carried out |
| Individual professional approval | Not recorded |
| Formula reference | not recorded |
| Standard and edition | IEC 60034-1, 2026 Ed 15 |
| Standard review state | Public-scope screened; licensed clause review still pending |
| Scope-screening note | Rotating-machine rating and performance context. The equation estimates input current from output power, efficiency, power factor and voltage; nameplate data takes precedence. |
| Cross-check test cases | 6 reference vectors (tests/reference_vectors.json) |
| Review date | not recorded |
| Next review date | not recorded |
| Status | Engineering reference |
Needed before this calculator can be marked Verified: reviewer name, qualification and review date; licensed clause-by-clause standard review; exact formula reference; formula source (textbook or standard); next review date.
Formula
Worked example
| Input | Value |
|---|---|
| Shaft power | 250 kW |
| Line voltage (L-L) | 415 V |
| Power factor | 0.88 |
| Efficiency | 0.95 |
| Phase | Three-phase |
- Input power = 250 kW / 0.95 = 263.16 kW
- I = 263,158 / (1.7321 × 415 × 0.88) = 416.03 A
Assumptions
- Rated load
- Nameplate current takes precedence over this value
- Starting current is 5–8× and not included
Nameplate values should be preferred. Use this for planning when only kW is known.
Inputs and results
| Input | Unit | Allowed or shown | Note |
|---|---|---|---|
| Shaft power | kW | greater than 0 kW | |
| Line voltage (L-L) | V | greater than 0 V | |
| Power factor | (number) | greater than 0, up to 1 | Rough starting point only. Resistive heating and incandescent loads are 1.0; a fully loaded induction motor is typically 0.8 to 0.88 and falls sharply when lightly loaded; discharge lighting depends on its ballast or driver. For real work use the nameplate or a measured value. |
| Efficiency | (number) | greater than 0, up to 1 | |
| Phase | (choice) | Three-phase; Single-phase |
| Result | Unit | Allowed or shown | Note |
|---|---|---|---|
| Full-load current | A | Shown to 1 decimal place | |
| Electrical input power | kW | Shown to 1 decimal place |
How it is calculated
Each input is converted to the base unit of its quantity, the formula above is applied in the order shown in the worked example, and each result is shown in the unit you choose. Results are held at full precision and rounded only for display, except where a step selects a standard size, as the worked example shows. A value outside its allowed range is refused with a message instead of giving a result.
Typical values
| Shaft power (kW) | Full-load current (A) | Electrical input power (kW) |
|---|---|---|
| 5.5 | 9.2 | 5.8 |
| 11 | 18.3 | 11.6 |
| 22 | 36.6 | 23.2 |
| 55 | 91.5 | 57.9 |
| 110 | 183.1 | 115.8 |
| 250 | 416 | 263.2 |
| 400 | 665.6 | 421.1 |
Other inputs are the same as in the worked example above.
Standards and further reading
- IEC 60034-1 (rotating electrical machines, rating and performance)
- IEC 60034-30-1 (efficiency classes)
Where the motor nameplate or data sheet gives a value, use it in place of a calculated one. These are named as background reading. Elecmetric does not embed clause text or rating tables, and the reviewer record for this calculator is separate from this list.
Frequently asked questions
What do I need to enter?
Enter shaft power, line voltage (L-L), power factor and efficiency. Change the unit of a value with its selector and the result stays the same.
Why is the current higher than the rated kW divided by the voltage?
The kW rating is shaft power. The motor draws more electrical power than that because of its losses (efficiency), and for the same power a lower power factor means more current. The formula divides by efficiency, power factor, the voltage and √3 for three-phase.
Should I use the nameplate current instead?
Yes, when you have it. The nameplate figure is measured for that motor. Use the calculated value only for planning when the nameplate or data sheet is not available yet.
Does this give the starting current?
No. Starting current is several times the full-load current and depends on the starting method. Use the motor starting current calculator with the full-load current from this page.
How accurate are the results?
Every figure comes from the same tested engine as the Elecmetric calculators and is meant for estimating. 6 reference cases, calculated from textbook formulas coded separately from the engine by the same author, agree with this calculator. This is not an independent review. Check design values against the standard and the equipment data you work to.
Formula guides and reference
Longer explanations with worked examples and tables, written for the same topic.
- Induction Motor Formulas: Induction motor formulas for synchronous speed, slip, shaft torque and full-load current
- Motor Efficiency Formulas: Input and Savings: Motor efficiency formulas: input power and losses from shaft power
- Motor Locked-Rotor Code Letter Formulas: NEMA motor code letter formulas: the locked-rotor kVA per horsepower
- Inverter-Fed Motor Cable Formulas: VFD motor cable formulas: the DC bus voltage, the reflected-wave overshoot at the motor
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