What is this motor current tool used for?
This tool estimates full-load current (FLA) from a motor's nameplate power, then applies an entered design margin to give the design current used for contactor, overload relay and motor cable selection.
It differs from the kW to Ampere tool by focusing on the motor selection workflow and adding a design margin directly to the result; use the kW to Ampere tool for general load-power checks.
Formulas used
- Single-phase: I = P / (V x cos phi x eta)
- Three-phase: I = P / (sqrt(3) x V x cos phi x eta)
- Design current: I_design = FLA x (1 + margin)
Variables and meaning
- P
- Rated power from the motor nameplate.
- V
- Supply voltage or line-to-line voltage.
- cos phi
- Motor power factor, taken from the nameplate or catalog value.
- eta
- Motor efficiency, taken from the nameplate or catalog value.
- FLA
- Calculated approximate full-load current.
- Design margin
- Extra design headroom applied for contactor, overload relay and cable selection.
Unit reference tables
Power units
| Unit name | Symbol | SI equivalent | Typical use |
|---|---|---|---|
| Watt (W) | W | 1 W | Base SI heat-transfer-rate unit |
| Kilowatt (kW) | kW | 1,000 W | Larger heat loads |
| Megawatt (MW) | MW | 1,000,000 W | Very large heat-transfer rates |
| Kilocalorie per hour (kcal/h) | kcal/h | 1.16222222222 W | Legacy heat-load and process calculations |
| Btu per hour (Btu/h) | Btu/h | 0.293071070172 W | HVAC and Imperial/US heat loads |
Voltage units
| Unit name | Symbol | SI equivalent | Typical use |
|---|---|---|---|
| Millivolt (mV) | mV | 0.001 V | Sensors and low-level signals |
| Volt (V) | V | 1 V | Base SI voltage unit |
| Kilovolt (kV) | kV | 1,000 V | High-voltage transmission |
Current units
| Unit name | Symbol | SI equivalent | Typical use |
|---|---|---|---|
| Milliampere (mA) | mA | 0.001 A | Electronic circuit currents |
| Ampere (A) | A | 1 A | Base SI current unit |
| Kiloampere (kA) | kA | 1,000 A | Industrial power systems |
Worked examples
5.5 kW, 400 V, three-phase, 0.85 power factor, 90 efficiency, 15% margin
FLA = 5500 / (1.732 x 400 x 0.85 x 0.9) which gives about 10.38 A. The design current is about 11.93 A.
1.1 kW, 230 V, single-phase, 0.8 power factor, 75 efficiency, 15% margin
FLA = 1100 / (230 x 0.8 x 0.75) which gives about 7.97 A. The design current is about 9.17 A.
Typical applications
- Preselection of contactors and overload relays
- Motor cable and fuse sizing
- Quick full-load current checks during project estimation
- Verifying current from an existing motor's nameplate values
Assumptions and limitations
- This tool gives an approximate full-load current (FLA); locked-rotor starting current can typically reach 6-8 times FLA and is not included here.
- The result depends on the power factor and efficiency values entered rather than the motor's actual nameplate figures; use nameplate data when available.
- Final contactor, overload relay and cable selection should follow the relevant product catalog and applicable electrical code.