A NEMA motor starter size is selected by matching the motor's horsepower (HP) and operating voltage to the continuous current ratings defined in the NEMA ICS 2 standard. For example, a standard 10 HP, 460V AC three-phase motor requires a NEMA Size 1 starter, which is rated for 27 continuous amps and up to 10 HP at 460V. You do not size a starter based solely on the motor's Full Load Amps (FLA); you must use the HP rating at your specific voltage to ensure the contacts can survive the inrush current during startup.

Safety Warning: Motor starters control high-voltage, high-current industrial loads. Always de-energize the main disconnect, apply lockout/tagout (LOTO) procedures, and verify zero energy with a calibrated multimeter before inspecting or wiring starter contacts. Local electrical codes may require a licensed electrician for installations over 600V or in hazardous locations.

The Complete Motor Starter NEMA Size Chart (NEMA ICS 2)

The table below provides the baseline sizing matrix for standard AC induction motors. This data is derived directly from the National Electrical Manufacturers Association (NEMA) ICS 2 standard, which governs industrial controllers, contactors, and overload relays.

How to Read This Table

Locate your motor's nameplate voltage and phase (single-phase or three-phase). Read across to the corresponding HP column. The Continuous Amps column represents the maximum steady-state current the starter's power contacts can carry indefinitely without thermal degradation at a standard 40°C ambient temperature. Always verify that your motor's FLA is below this continuous amp rating, but always defer to the HP column for the final size selection to account for locked-rotor inrush currents.

NEMA Size Continuous Amps Max HP @ 115V (1Ø) Max HP @ 230V (3Ø) Max HP @ 460V (3Ø) Max HP @ 575V (3Ø)
00 9 A 1/3 HP 1.5 HP 2 HP 2 HP
0 18 A 1 HP 3 HP 5 HP 5 HP
1 27 A 2 HP 10 HP 10 HP 10 HP
2 45 A 3 HP 20 HP 25 HP 25 HP
3 90 A - 40 HP 50 HP 50 HP
4 135 A - 75 HP 100 HP 100 HP
5 270 A - 150 HP 200 HP 200 HP
Bookmark Quick-Jump: The most commonly queried sizes in commercial HVAC and light manufacturing are Size 1 (10 HP @ 460V / 27A) and Size 2 (25 HP @ 460V / 45A). If your motor falls exactly on the boundary (e.g., exactly 10 HP), standard practice allows the use of that size, but upsizing to the next tier is recommended if the motor has a high service factor (1.15 or greater).

Applying the Chart: Derating and Real-World Loads

The chart above assumes standard duty—defined by NEMA as starting the motor and running it continuously without excessive jogging, plugging, or high-inertia acceleration. In real-world installations, you must apply application-specific derating factors which effectively modify the base HP value of the starter.

Which Column Applies to Your Installation?

Always match the column to the motor nameplate's operating voltage, not the facility's nominal supply voltage. If your facility supplies 480V but the motor is tapped and nameplated at 460V, use the 460V column. For single-phase motors (common in fractional HP shop tools and small pumps), use the 115V or 230V single-phase column. Note that NEMA sizes 3 and above generally do not support single-phase applications because the massive inrush currents of large single-phase motors require specialized contactor designs or soft starters.

How Duty Cycles Modify the Base Value

NEMA ICS 2 includes application tables that act as derating multipliers against the base HP rating. If your application involves severe duty, you must upsize the starter. Here is how the derating logic works in practice:

  • High-Inertia Loads (Fans, Blowers, Rock Crushers): Acceleration times exceed 10 seconds. The contacts endure prolonged arcing. Action: Upsize by one full NEMA size.
  • Jogging/Inching (Cranes, Hoists, Indexing Conveyors): Rapidly breaking the circuit while the motor is still near locked-rotor current. Action: Upsize by one full NEMA size, or use a starter specifically rated for 'Jogging Duty'.
  • Plugging (Reversing the motor to stop it quickly): Generates massive electromechanical stress and heat. Action: Upsize by two full NEMA sizes.

Worked Numeric Example

Imagine you are sizing a starter for a 20 HP, 230V, 3-phase motor driving a heavy flywheel punch press. The press requires frequent jogging to align the material.

  1. Base Lookup: The chart shows a NEMA Size 2 starter handles up to 20 HP at 230V (45A continuous).
  2. Derating Application: Because the application requires frequent jogging, NEMA guidelines dictate upsizing by one tier to prevent premature contact welding.
  3. Final Selection: You must purchase a NEMA Size 3 starter (rated for 40 HP @ 230V / 90A).

While a Size 2 might cost around $250 and a Size 3 jumps to $450, the cost of replacing welded contacts and dealing with unplanned downtime on a Size 2 will far exceed the initial upsizing premium.

What the NEMA Chart Cannot Tell You

While the motor starter NEMA size chart is the undisputed starting point for component selection, it is not a complete system design tool. Relying solely on this chart leaves three critical engineering gaps:

1. Short Circuit Current Rating (SCCR)

The NEMA size dictates thermal endurance and mechanical horsepower switching capability, but it does not define the assembly's ability to survive a dead short circuit. A NEMA Size 2 starter might have an SCCR of 5,000 amps with a standard thermal fuse, but 65,000 amps when paired with a specific class of current-limiting fuses or a properly sized Molded Case Circuit Breaker (MCCB). You must consult the manufacturer's combination tables (e.g., Eaton or Allen-Bradley SCCR guides) to ensure your panel meets the NEC 110.10 fault-current requirements.

2. Ambient Temperature Derating

The continuous amp ratings in the table assume a maximum ambient temperature of 40°C (104°F). If your control panel is located near a furnace, in direct desert sunlight, or lacks adequate ventilation, the thermal mass of the contacts and the bimetallic strips in the overload relay will trip prematurely or degrade. For every 10°C above 40°C, you typically must derate the continuous current capacity by 10% to 15%, or install forced ventilation on the enclosure.

3. NEMA vs. IEC Philosophy

It is vital to understand that NEMA sizing is inherently conservative. A NEMA Size 2 contactor is physically massive compared to an IEC-rated contactor handling the exact same 20 HP load. NEMA designs prioritize long mechanical life (millions of operations), ease of maintenance (replaceable silver-cadmium oxide contact tips), and robust fault tolerance. IEC standards (IEC 60947) size contactors based on exact application categories (e.g., AC-3 for squirrel cage motors) resulting in much smaller, cheaper, but less field-serviceable components. Choose NEMA when you need heavy-duty industrial reliability and field-repairability; choose IEC when panel space and initial capital cost are the primary constraints.