Motor Starting Current Calculator

This calculator converts a motor's horsepower nameplate rating into the electrical characteristics an electrician needs for circuit design: full load running current and the much larger inrush current at the moment of starting. Horsepower is first converted to watts (1 HP = 746 W), then divided by voltage, the Motor Efficiency you enter, and a fixed 0.85 assumed power factor to get Full Load Amps (FLA) — three-phase motors also divide by the square root of 3. Both the 0.85 power factor and the efficiency you provide directly shape the result: a lower efficiency input pushes FLA up (the motor draws more current to deliver the same shaft power), and since 0.85 is baked into the formula rather than exposed as an input, a motor whose actual power factor differs meaningfully from that assumption will show an FLA here that drifts from the NEC Table 430.250 nameplate value, which is what code actually requires for circuit sizing.

Starting Current, also called Locked Rotor Amps (LRA), is simply FLA multiplied by a Starting Current Multiplier you select based on how the motor is started: direct-on-line starting typically produces 6–8× FLA, a soft starter cuts that to 2–4×, and a VFD limits inrush to as little as 1–1.5× FLA. The Estimated Voltage Drop is a simplified approximation scaling with starting current relative to a 480V reference — it is not a substitute for an actual voltage-drop study using your specific source impedance, but it is a useful early flag: drops above roughly 5% can upset sensitive control circuits, and above 10% may prevent the motor from starting at all. Starting KVA Demand tells you the transient apparent-power draw the upstream transformer and generator must be able to supply for the brief starting interval, which is often the true sizing constraint for standby power systems.