Motor & switching
AC Contactor Sizing Calculator (Motor FLC & AC-3)
Enter the motor rating to estimate the full-load current (FLC) and the minimum AC-3 contactor rating you need. The motor nameplate current always governs the final choice — this is a fast starting point, not a substitute for the manufacturer’s table.
How the calculation works
A contactor is chosen from the motor’s full-load current under the correct utilization category, not from the motor power alone. For a three-phase motor:
For a single-phase motor: I = P / (V × η × cos φ). Horsepower is converted with 1 HP = 0.746 kW. The result estimates the range; the motor nameplate current governs the final comparison.
Why the utilization category changes the answer
| Category | Typical duty | Effect on selection |
|---|---|---|
| AC-1 | Non-inductive / resistive (heaters) | Highest current per frame; not for motors |
| AC-3 | Starting squirrel-cage motors, switching off while running | The normal basis for a direct-on-line motor contactor |
| AC-4 | Inching, plugging, reversing before speed | Much lower permissible current per frame; size up |
What this contactor does not do
A contactor switches the power circuit; it does not protect the motor. Every motor circuit still needs an overload relay for sustained overcurrent and a short-circuit protective device (breaker or fuse), coordinated for the voltage and fault level. The coil is selected separately from the control voltage and does not have to match the motor voltage.
Frequently asked questions
How do I size a contactor for a three-phase motor?
Start from the motor nameplate full-load current at the operational voltage, select AC-3 for normal starting and stopping, and choose a contactor whose AC-3 rating equals or exceeds that current. Then coordinate an overload relay and a short-circuit device.
Is the coil voltage the same as the motor voltage?
No. The main contacts are chosen for the motor circuit; the coil for the control circuit. A contactor can switch a 400 V motor with a 24 V DC or 230 V AC coil.
Why is AC-4 different from AC-3?
AC-4 covers jogging, plugging, and frequent reversing, where both making and breaking occur at high motor current — far more contact wear, so the permissible current per frame is much lower.
Ready to choose the contactor?
With the required AC-3 rating, compare configurations, coil options, and auxiliary contacts, or send the load and coil requirements for a recommendation.
Engineering estimate for guidance only. Final motor-circuit design, protection coordination, and product selection must be verified by a qualified person against the motor nameplate, the applicable standards, and the manufacturer’s datasheet.