An electric motor starter size chart maps a motor's horsepower (HP) and operating voltage to a standardized physical frame size. In North America, this means the NEMA ICS 2 standard sizes (00 through 5). The direct answer for sizing is straightforward: match your motor's nameplate voltage and horsepower to the chart below to find your base NEMA size, then adjust for environmental derating and starting duty cycles. The starter frame handles the physical arc and thermal mass, while the internal overload relay must be dialed to the motor's exact Full Load Amps (FLA).
The Master NEMA Electric Motor Starter Size Chart
This reference table is built on the NEMA ICS 2 standard for industrial motor controllers. How to read this table: Locate your motor's nominal 3-phase voltage across the top columns (230V, 460V, or 575V). Follow the column down to your motor's horsepower rating, then read left to find the required NEMA Size. The final column lists the maximum continuous current the starter's power contacts are rated to carry without degrading. Note that single-phase motors require different sizing logic and are not covered in this 3-phase chart.
| NEMA Size | Max HP @ 230V (3-Phase) | Max HP @ 460V (3-Phase) | Max HP @ 575V (3-Phase) | Continuous Current Rating (Amps) |
|---|---|---|---|---|
| 00 | 1.5 HP | 2 HP | 2 HP | 9A |
| 0 | 3 HP | 5 HP | 5 HP | 18A |
| 1 | 7.5 HP | 10 HP | 10 HP | 27A |
| 2 | 15 HP | 25 HP | 25 HP | 45A |
| 3 | 30 HP | 50 HP | 50 HP | 90A |
| 4 | 50 HP | 100 HP | 100 HP | 135A |
| 5 | 75 HP | 150 HP | 150 HP | 225A |
Applying the Chart: Which Column and Derating Factors Apply
The most common mistake on the jobsite is reading the wrong voltage column or ignoring the installation environment. Which column applies to you? Always use the voltage column that matches the actual measured supply voltage, not just the motor nameplate. A motor nameplated at 460V is designed for a 480V utility supply. If your facility measures 480V at the panel, use the 460V column. If you are running a 208V system, you must use the 230V column and accept the lower HP rating for that frame.
The chart assumes a standard installation: an ambient temperature of 40°C (104°F) and an altitude below 3,300 feet (1,000 meters). When conditions exceed these baselines, the thermal mass of the starter contacts cannot dissipate heat fast enough, requiring you to modify the base value.
- High Ambient Temperature: If the enclosure interior exceeds 40°C (common in steel panels in direct sunlight or near boilers), upsize the starter by one full NEMA frame to prevent contact welding and premature overload tripping.
- High Altitude: Above 3,300 feet, the thinner air reduces dielectric strength and cooling. Derate the continuous current column by 1% for every 330 feet above 3,300 feet, or simply upsize one frame.
- High Switching Frequency: If the motor starts and stops more than 5 times per hour, the arc-quenching limits of the contacts are exceeded. Upsize one frame or switch to a solid-state soft starter.
Decision Path: Selecting Your Exact Starter Frame
Use this decision-tree-table to terminate your sizing process with one concrete pick. Do not stop at the base chart if your application involves high-inertia loads.
| Application Condition | Base Chart Result | Required Action | Final Concrete Pick |
|---|---|---|---|
| Standard centrifugal pump, 460V, 25 HP, normal start/stop | Size 2 (45A) | No modification needed. | NEMA Size 2 |
| High-inertia load (centrifuge, punch press), 460V, 25 HP | Size 2 (45A) | Upsize one frame to handle extended starting current (AC-4 duty). | NEMA Size 3 |
| Standard conveyor, 230V, 10 HP, 40°C ambient | Size 2 (45A) | No modification needed (Size 1 maxes at 7.5HP @ 230V). | NEMA Size 2 |
| Standard fan, 460V, 50 HP, installed at 5,000 ft elevation | Size 3 (90A) | Derate for altitude (approx. 5% loss). Upsize for safety margin. | NEMA Size 4 |
Quick-Jump Reference: Most Queried Motor Sizes
For the most common 3-phase industrial motors (at standard 460V), here are your immediate frame assignments. Bookmark these quick-jump rows to skip the full table lookup:
- 1 HP @ 460V: NEMA Size 00. (Use a Size 0 if you want room to upgrade to 5 HP later without changing the enclosure).
- 5 HP @ 460V: NEMA Size 0. (The workhorse size for small machine tools and coolant pumps).
- 10 HP @ 460V: NEMA Size 1. (Standard for mid-range compressors and conveyors).
- 50 HP @ 460V: NEMA Size 3. (Heavy industrial territory; ensure your upstream breaker and wire gauge can handle the 65A+ inrush).
Remember: The NEMA size dictates the contact frame. You must still select the correct overload relay heater or electronic trip module based on the motor nameplate FLA. A Size 2 starter can house overload modules ranging from 12A all the way up to 45A.
What the Chart Cannot Tell You (NEMA vs. IEC Limits)
While the NEMA electric motor starter size chart gives you the physical frame and thermal limits, it has blind spots that require supplemental engineering data. According to Schneider Electric's motor control guidelines, NEMA sizing is inherently conservative, built around heavy-duty thermal mass and standardized physical dimensions. However, the chart does not provide the Short Circuit Current Rating (SCCR).
A NEMA Size 2 combination starter might only have a 5 kA SCCR if paired with standard Class H fuses, but that exact same enclosure jumps to a 65 kA or 100 kA SCCR when fitted with specific Class J or Class RK1 fuses and a verified contactor. Always check the manufacturer's combination starter SCCR tables to ensure the starter can survive a dead short on the available fault current of your facility's busbar.
Furthermore, the chart does not translate directly to IEC standards (IEC 60947-4-1). IEC starters are sized by utilization categories rather than discrete physical frames. An IEC starter rated for AC-3 (squirrel cage motors, starting and switching off at rated speed) will fail rapidly if used for AC-4 duty (plugging, reversing, or inching high-inertia loads) without massive derating. NEMA sizes inherently absorb AC-4 abuse better due to their oversized contacts.
The Final Default Recommendation: When specifying a starter for a general-purpose 3-phase induction motor in an industrial environment, use the NEMA ICS 2 chart to find your base size. If the load involves high inertia, frequent jogging, or high ambient heat, default to upsizing by exactly one NEMA frame. This single step provides the necessary thermal mass to prevent contact welding and extends the mechanical life of the starter by a factor of three, saving you from premature replacement costs and unplanned downtime.
