How to Use the Three-Phase Motor Current Calculator
A three-phase motor's full-load current is the baseline number used to select circuit breakers, contactors, and conductor sizes. This calculator takes motor output (kW), supply voltage, power factor, and efficiency and computes full-load current (A) with the standard three-phase power formula.
The formula is I = (P x 1000) / (√3 x V x PF x efficiency). The kW rating on a motor nameplate is the mechanical output at the shaft, while the electrical input power drawn is higher — dividing by efficiency corrects for that difference. A lower power factor also means more current is needed to deliver the same power, which is why it's included too.
NEC Table 430.250 publishes tabulated full-load current values for standard motor horsepower ratings at 208/230/460V, and those tabulated values — not the nameplate or a calculated figure — are what NEC code compliance is based on for conductor and overload sizing. This calculator is useful for early-stage sizing or when you only have a kW rating rather than a standard motor HP, but always cross-check against Table 430.250 or the actual nameplate current for a code-compliant installation.
In practice, if the motor nameplate already lists a rated current, that value takes priority. This calculator is a supporting tool for spec review when nameplate data isn't yet available. Inrush current runs several times higher than full-load current, so breaker and contactor selection needs to account for that separately.
Frequently Asked Questions
The kW rating on a motor nameplate is the mechanical output at the shaft, but the electrical input power is higher than that. Dividing by efficiency converts shaft output into the electrical input needed to get an accurate current value.
This full-load current is a reference value only — inrush current, overload protection coordination, and conductor ampacity all need separate review. Have a licensed electrical engineer confirm the final breaker and cable size per NEC.