Motor starter and contactor sizing starts with the motor’s full-load current and the actual switching duty—not simply motor horsepower or the contactor’s largest printed amp rating. The selected equipment must suit the motor load, supply voltage and phase, contactor utilization category, overload range, coil voltage, and available control power.
A contactor switches the motor circuit. A starter combines a contactor with motor overload protection. Neither selection should be based on starting current alone: starting current affects the electrical system and the starting method, while the contactor and starter still need ratings appropriate to the motor’s normal operating current and application duty.
Separate the Four Ratings
Several values are often called “motor starter sizing,” but they answer different questions.
| Item | What it describes | Why it is needed |
|---|---|---|
| Motor full-load current | The motor’s expected running current at rated load | Establishes the operating-current basis for starter and overload selection |
| Contactor operational rating | The current the contactor is rated to switch for its stated utilization category | Determines whether the contactor fits the actual load and duty |
| Coil VA | The control-power demand of the contactor coil | Determines the burden placed on the control circuit or control-power source |
| Starting current | The elevated current during acceleration or locked-rotor conditions | Helps assess the starting event and whether the starting approach is appropriate |
For a motor, begin with the nameplate full-load amperes, voltage, phase, and service-factor information where applicable. Eaton’s motor-control selection material identifies motor full-load ampere rating and service factor as nameplate information used in overload-current-range selection.
Use the Motor Starter Current Reference Calculator to organize the motor-current portion of the selection. For a closer explanation of running current, full-load current, and startup demand, see Motor Current, Starting Current, And FLC.
Select the Contactor for Its Duty
A contactor is not interchangeable across all loads that happen to have the same amperes. Its published rating must be read with the manufacturer’s stated voltage, load type, utilization category, operating duty, and expected switching frequency.
For IEC-style contactors, utilization categories identify the switching conditions the contactor is intended to handle. Schneider Electric notes that categories such as AC-1, AC-2, AC-3, and AC-4 vary with current, voltage, power factor, frequency of operation, and endurance requirements. Eaton likewise directs the selector to identify the application’s utilization category, use the matching performance curve, and compare application operational current with the contactor’s rating.
Confirm these items against the manufacturer’s data:
- Motor supply voltage and phase
- Motor full-load current
- Normal start/stop duty versus more demanding plugging, reversing, inching, or jogging duty
- Required utilization category
- Rated operational current at the actual application voltage
- Coil voltage and coil type, AC or DC
- Enclosure and environmental requirements
- Available short-circuit and ground-fault protection arrangement, disconnecting means, and the complete listed assembly requirements
A NEMA starter is commonly selected through the manufacturer’s NEMA size and motor application data. An IEC contactor is commonly selected from its operational-current rating and utilization category. Those are different catalog approaches, so do not assume that a NEMA size or an ampere value from one product family directly substitutes for another.
Account for Coil VA and Control Load
The motor power circuit and the contactor control circuit are separate sizing questions. A properly selected power contactor can still have a coil that does not match the available control voltage, or a control-power source that is inadequate for the combined load.
Verify the coil’s specified voltage before selection. The coil must match the control circuit—not the motor line voltage unless the design intentionally uses it. Then obtain the coil VA values from the exact manufacturer and coil part number, including any distinction between pickup or inrush burden and sealed or holding burden.
The Contactor Coil VA Calculator can be used to document coil power demand. For the wider control circuit, include every intended load rather than treating the contactor coil as the entire circuit: pilot devices, indicating lights, relays, timers, PLC interfaces, solenoids, and other control components can change the total requirement.
Eaton’s control-power guidance specifically treats inrush power requirement as a control-power-transformer selection factor and relates that requirement to allowable secondary-voltage performance. Where a low-voltage control circuit is used, conductor selection also depends on circuit voltage and run conditions; the 24V Wire Size Guide for Control Circuits covers that separate question.
Use the Motor Control Circuit Calculator to examine the control-side load without confusing it with motor branch-circuit current.
Treat Starting Current Separately
Starting current is a transient condition, not a replacement for full-load current. It affects voltage drop, upstream equipment performance, and the suitability of the motor-starting method, but it should not be used by itself to choose a contactor or overload range.
For a conventional motor starter, keep these decisions distinct:
- Use motor operating information to establish the running-current basis.
- Use the contactor’s manufacturer rating at the applicable utilization category and voltage to select the switching device.
- Use the specified coil VA and all connected control loads to assess the control circuit.
- Use motor starting-current information to assess the startup event and its effect on the electrical system.
The Motor Starting Current Calculator is useful when the question is specifically about startup demand. Power-factor assumptions can also affect current calculations when applicable; see Power Factor Basics for Electrical Planning: 2026 U.S. Guide.
Avoid Rating Mix-Ups
The most common selection errors come from treating unlike ratings as though they answer the same question.
- Do not size a contactor from horsepower alone without checking voltage, phase, operational current, and utilization category.
- Do not select the overload range from a generic motor estimate when the motor nameplate full-load current is available.
- Do not assume the coil voltage matches the motor supply voltage.
- Do not confuse coil VA with motor current or use coil VA to select the contactor’s power poles.
- Do not use starting current as the motor’s continuous operating current.
- Do not assume a contactor’s AC-1 rating applies to a motor duty that requires another utilization category.
- Do not confuse motor overload protection with branch-circuit protection against short circuit and ground fault.
- Do not overlook enclosure, ambient conditions, conductor count, control-voltage type, and the manufacturer’s complete equipment ratings.
A motor starter contactor sizing calculator can organize the relevant inputs and make omissions easier to spot. Final equipment selection, installation, protection coordination, permitting, and energization require verification against the exact manufacturer documentation, the adopted electrical code, and the requirements of the authority having jurisdiction.