Motor current calculation for practical electrical design
This motor calculator helps maintenance electricians, panel builders and automation engineers estimate electrical demand before selecting a feeder, branch-circuit conductor or disconnect. Enter shaft horsepower, supply voltage, expected motor efficiency and power factor. The calculator returns an estimated full-load amperage, a preliminary breaker rating and a copper cable suggestion.
Motor nameplate data should always take priority in a final design. A higher supply voltage generally reduces current for the same output power, while lower efficiency and lower power factor increase input current.
Formula and assumptions
Mechanical horsepower is converted to watts using 746 watts per HP. For a three-phase motor, current is calculated as I = HP × 746 ÷ (√3 × V × efficiency × power factor). For single phase, the √3 factor is omitted. The suggested breaker starts at 125% of calculated full-load current and rounds upward to the next five-ampere rating.
Frequently asked questions
Why is my nameplate amperage different? Manufacturer data, service factor and design class vary; use the nameplate or applicable code tables for final selection. Is the breaker result a motor-protection setting? No. Motor protective devices follow specific code rules and may be sized for starting current. Can this size a VFD? Use the VFD manufacturer’s input-current rating and installation instructions.
Safety note
This is an estimating tool, not a protection study. Confirm short-circuit rating, overload relay setting, starting method, conductor derating, grounding and all local code requirements with a qualified electrical professional before energizing equipment.