Example 1: Three-phase pump motor
Power = 15 kW, voltage = 400 V, power factor = 0.85, efficiency = 90%
Current = 15,000 / (1.732 x 400 x 0.85 x 0.90) = 28.30 A
This gives a useful first estimate for breaker, cable, and overload planning.
Motor Full Load Current Calculator
Estimate single-phase or three-phase motor full-load current from motor power, voltage, efficiency, and power factor.
This motor full-load current calculator gives a planning estimate from electrical power relationships. Final motor current selection should also consider nameplate data, service factor, design code tables, and starting conditions.
What Current This Calculator Estimates
Motor full-load current is one of the most useful starting values in electrical design. It affects breaker sizing, cable sizing, overload setting, feeder loading, and voltage-drop checks. Even when a designer later uses a motor code table or a manufacturer nameplate, a quick current estimate is still valuable for understanding whether the design direction makes sense.
This motor full load current calculator converts motor output power into input current by allowing for motor efficiency and power factor. It works for both single-phase and three-phase systems and accepts power in either kilowatts or horsepower. That makes it useful for pump motors, fans, compressors, conveyors, workshop machines, and many other industrial or building services applications.
The result is a planning-level current value, not a substitute for code tables or nameplate information. Still, it is often the fastest way to check whether a proposed motor size is in the right current range before selecting protective devices and conductors.
Calculation Formula
Input power: Pin = Pout / eta
Single-phase current: I = Pout / (V x eta x PF)
Three-phase current: I = Pout / (sqrt(3) x V x eta x PF)
Single-phase current: I = Pout / (V x eta x PF)
Three-phase current: I = Pout / (sqrt(3) x V x eta x PF)
Meaning of the Inputs
| Input | Meaning | Unit |
|---|---|---|
| Motor Power | Rated shaft output power | kW or HP |
| Voltage | Supply voltage | V |
| Power Factor | Electrical power factor at the operating point | 0 to 1 |
| Efficiency | Motor conversion efficiency | % |
| System Type | Single-phase or three-phase current relationship | - |
Unit Guide
If the motor rating is given in horsepower, the calculator converts it to kilowatts using the common mechanical relationship of 1 HP = 0.746 kW. The current result is shown in amperes. A second output line also reports the approximate electrical input power, which helps explain why motor input is always higher than the motor shaft output when efficiency is less than 100 percent.
Worked Examples
Example 2: Single-phase compressor
Power = 5 HP, voltage = 230 V, power factor = 0.9, efficiency = 88%
Output power = 5 x 0.746 = 3.73 kW
Current = 3,730 / (230 x 0.9 x 0.88) = 20.45 A
A designer would then compare that value with starting current, cable length, and local selection rules.
Practical Design Notes
The result on this page is not the same thing as a code-table motor current. Some wiring rules use standard table values rather than exact nameplate values for branch-circuit and feeder design. In the same way, a motor running at partial load may draw less than the current calculated here, while a heavily loaded or low-efficiency motor may draw more. The biggest short-term current difference usually appears during starting, not during steady operation.
Even with those limits, the calculator remains very useful because it turns basic motor data into a realistic current estimate that can be cross-checked against breakers, cable sizes, VFD input planning, and three-phase power calculations. It is a practical engineering tool for early design, maintenance estimates, and quick checks when only motor rating and supply details are known.