To size a motor starter, match the motor's Full Load Amps (FLA) and nominal voltage to the NEMA ICS 2 or IEC 60947-4-1 utilization category chart. For a standard 10 HP, 460V 3-phase motor drawing 14A FLA, you need a NEMA Size 1 (rated 27A at 460V) or an IEC S00 frame contactor (rated 18A for AC-3 duty). Always size the contactor to the FLA, not the breaker size, and apply environmental derating if your ambient temperature exceeds 40°C (104°F).
How to Read This Motor Starter Sizing Chart
Before jumping to the numbers, you must know which column applies to your installation. The global market splits motor starters into two distinct philosophies: NEMA (National Electrical Manufacturers Association) and IEC (International Electrotechnical Commission).
- The NEMA Columns (HP & Amps): NEMA sizes (00 through 4) are physically oversized, ruggedized, and rated by Horsepower (HP) at specific voltages (230V and 460V). If you are in North America working on heavy industrial machinery, high-vibration environments, or replacing legacy panels, use the NEMA HP and 460V Amp columns.
- The IEC Columns (Frame & AC-3 Amps): IEC frames (S00 through S4) are compact, 'exact-fit' components rated in kilowatts (kW) or Amps under specific utilization categories. For standard squirrel-cage induction motors starting and stopping under load, use the AC-3 Amp column. (Do not use the AC-1 resistive heating column for motors).
NEMA and IEC Motor Starter Sizing Chart
The following table cross-references NEMA ICS 2 sizes with IEC 60947-4-1 frames for 3-phase AC induction motors. Source standards: NEMA ICS 2 and IEC 60947-4-1.
| NEMA Size | IEC Frame | Max HP @ 230V | Max HP @ 460V | AC-3 Current (A) @ 460V | Typical Coil Voltage |
|---|---|---|---|---|---|
| 00 | S00 (9A) | 1.5 HP | 2 HP | 5A | 24VDC / 120VAC |
| 0 | S00 (18A) | 3 HP | 5 HP | 10A | 24VDC / 120VAC |
| 1 (Most Common) | S0 (25A) | 7.5 HP | 10 HP | 14A (Rated 27A) | 24VDC / 120VAC |
| 2 (Most Common) | S2 (32A) | 10 HP | 25 HP | 34A (Rated 45A) | 24VDC / 240VAC |
| 3 | S3 (65A) | 20 HP | 50 HP | 65A | 120VAC / 480VAC |
| 4 | S4 (95A) | 40 HP | 100 HP | 125A | 120VAC / 480VAC |
Note: The AC-3 Current column lists the standard motor FLA for that HP rating. The 'Rated' value in parentheses is the maximum continuous thermal current the contactor can handle without derating at 40°C ambient.
Derating and Environmental Modifiers
The base values in the chart above assume an ambient temperature of 40°C (104°F) and standard AC-3 duty (starting a motor and switching it off while running). If your installation deviates from this, derating rows modify the base value significantly.
- High Ambient Temperature: If the panel is located near a furnace, in direct sunlight, or in a poorly ventilated room exceeding 40°C, the contactor's thermal capacity drops. At 50°C, multiply the base current rating by 0.90. At 60°C, multiply by 0.80. Example: A 25A IEC S0 contactor at 55°C ambient derates to roughly 21A. If your motor draws 22A, you must step up to an S2 frame.
- High Altitude: Above 2,000 meters (6,600 ft), air density drops, reducing both cooling efficiency and dielectric strength. You must derate the voltage rating by 10% and the current rating by 10% for every 1,000 meters above the baseline.
- Jogging and Plugging (AC-4 Duty): If the motor is rapidly reversed (plugging) or inch-started (jogging) to position a load like a crane or conveyor, the contacts must interrupt locked-rotor current while the motor is still spinning. According to IEC utilization categories, AC-4 duty requires you to multiply the motor FLA by at least 2.0 when selecting the contactor frame.
Decision Path: Picking Your Exact Contactor Part Number
Use this decision-tree-table to terminate your selection process and land on a concrete, orderable part number.
| Application Condition | Sizing Rule | Concrete Part Pick (460V, 10HP/14A Motor) |
|---|---|---|
| Standard HVAC, Pump, or Fan (AC-3, <40°C ambient, low vibration) | Match FLA to IEC AC-3 column exactly. Select compact frame. | Siemens Sirius 3RT2016-1BB41 (IEC S00, 18A, 24VDC coil) |
| Heavy Industrial, High Vibration, or Legacy Panel Replacement | Select NEMA size based on HP column. Ignore IEC frames. | Eaton Freedom C25DND230A (NEMA Size 1, 27A, 120VAC coil) |
| Conveyor, Crane, or Press with Jogging/Plugging (AC-4 duty) | Multiply FLA by 2.0. Step up two IEC frame sizes for contact survivability. | Schneider TeSys LC1D25 (IEC S2, 25A AC-4 rating, 24VDC coil) |
| High Ambient Temp Enclosure (55°C / 131°F) | Calculate derated FLA (Base Rating × 0.85). Step up one frame size. | ABB AF26-30-11-13 (IEC S0, 26A base, wide AC/DC coil) |
Default Recommendation: If you are building a new standard industrial control panel in North America for a typical 3-phase machine, default to the Siemens Sirius 3RT2 or Schneider TeSys D IEC lines. They offer the best balance of panel real estate, integrated auxiliary contacts, and global availability. Only specify NEMA (like Eaton Freedom or Allen-Bradley Bulletin 100) if the facility maintenance team explicitly requires the oversized, field-repairable contacts of legacy NEMA gear.
What This Chart Cannot Tell You
A motor starter sizing chart gets you the correct contactor frame, but it is only one-third of a complete motor starter assembly. The chart cannot tell you the following critical parameters:
- Overload Relay Heater Sizing: The contactor handles the switching; the overload relay protects the motor from burning out. You cannot size the overload relay using the chart above. You must read the exact Service Factor (SF) and FLA off the physical motor nameplate and set the bimetallic or electronic overload dial to that exact number.
- Short Circuit Current Rating (SCCR): The contactor alone will weld its contacts shut and explode under a massive fault current. To achieve a safe SCCR (e.g., 65kA), you must use a Type 2 Coordinated combination. This means pairing the specific contactor model with a specific class of fuse (e.g., Class RK5 or Class J) or a specific molded case circuit breaker listed together in the manufacturer's coordination tables.
- Control Circuit Voltage vs. Line Voltage: The chart lists line voltage (460V) for the power poles. It does not dictate your coil voltage. Always use a 24VDC or 120VAC control circuit via a step-down transformer for safety and PLC compatibility, rather than pulling 480V directly through the pushbuttons and PLC relays to the contactor coil.
