When sizing a magnetic motor starter for a 3-phase AC induction motor, the direct answer relies on matching the motor’s Full Load Amps (FLA) and Horsepower (HP) to the standardized NEMA frame sizes. For a standard 460V motor, a 5 HP load requires a NEMA Size 1 starter, a 10 HP load requires a Size 2, and a 50 HP load requires a Size 4. This reference guide provides the complete motor starter size chart based on NEMA ICS 2 standards, detailing exactly how to read the columns, apply environmental derating, and avoid the most common sizing failures on the jobsite.
How to Read the NEMA Motor Starter Size Chart
The National Electrical Manufacturers Association (NEMA) standardizes motor starters into discrete physical sizes, designated from 00 to 5. These sizes are defined by their maximum continuous current rating and their horsepower capacity at specific voltages.
Which column applies to your installation? Always size the starter based on the actual supply voltage and the motor nameplate FLA, not just the HP rating. If your motor nameplate reads 460V, use the 460V column. If you are running a 208/230V dual-rated motor on a 208V supply, you must use the 200V column, as the motor will draw higher current at the lower voltage. If the motor's FLA exceeds the maximum continuous current of a given NEMA size, you must step up to the next physical size, even if the HP rating suggests otherwise.
- Most Common (Size 1): Covers up to 25 HP at 460V / 27 Amps max.
- Heavy Duty (Size 2): Covers up to 50 HP at 460V / 45 Amps max.
| NEMA Size | Max Continuous Current (FLA) | Max HP @ 200V | Max HP @ 230V | Max HP @ 460V | Max HP @ 575V |
|---|---|---|---|---|---|
| 00 | 9 A | 1.5 | 2 | 3 | 3 |
| 0 | 18 A | 3 | 5 | 10 | 10 |
| 1 | 27 A | 7.5 | 10 | 25 | 25 |
| 2 | 45 A | 10 | 15 | 50 | 50 |
| 3 | 90 A | 15 | 25 | 100 | 100 |
| 4 | 135 A | 30 | 50 | 200 | 200 |
| 5 | 270 A | 50 | 75 | 400 | 400 |
Applying Derating Factors and Environmental Adjustments
The base values in the motor starter size chart assume a standard industrial environment: an ambient temperature of 40°C (104°F), standard duty cycling, and an altitude below 3,300 feet. When your installation deviates from these baselines, you must apply derating factors that modify the base continuous current value.
How derating modifies the base value: If your control panel is located in a hot environment (e.g., a boiler room or an outdoor enclosure in direct sun) where the internal ambient temperature reaches 50°C (122°F), the thermal capacity of the overload relays and contactor coils drops. Most manufacturers, such as Siemens and Eaton, require a 10% to 15% derating of the continuous current at 50°C. For a Size 2 starter rated at 45A, a 15% derate drops your usable capacity to 38.2A. If your motor draws 40A, you must step up to a Size 3 starter.
Similarly, at altitudes above 3,300 feet, the thinner air reduces convective cooling. You must derate the continuous current by approximately 5% for every additional 1,000 feet of elevation.
This chart dictates thermal and continuous current limits, but it does not account for Short Circuit Current Rating (SCCR), voltage drop, or Locked Rotor Amps (LRA) coordination. A NEMA Size 2 starter can handle 45A continuously, but if the available fault current at the panel is 65 kA, the starter will violently fail unless paired with the exact class and size of upstream current-limiting fuses specified by the manufacturer. Always verify the SCCR of the complete combination starter assembly against your facility's fault current study.
Frequently Asked Questions
What size NEMA starter do I need for a 10 HP 3-phase motor?
The correct size depends entirely on the operating voltage. If the motor is wired for 460V, it will draw approximately 14A. A NEMA Size 1 starter (rated for 27A max and up to 25 HP at 460V) is the correct, cost-effective choice. However, if that same 10 HP motor is wired for 230V, it will draw roughly 28A. Because 28A exceeds the 27A limit of a Size 1, you must install a NEMA Size 2 starter (45A max), even though the horsepower rating for Size 2 at 230V is 15 HP.
How do I size a motor starter for a high-inertia load?
Standard NEMA sizing assumes a normal starting time of under 10 seconds. High-inertia loads—such as large centrifugal fans, rock crushers, or heavy flywheels—take much longer to accelerate to full speed. During this extended acceleration phase, the motor draws locked-rotor current, generating massive heat in the starter's overload heaters. To prevent nuisance tripping and thermal damage, you must increase the NEMA size by one step (e.g., use a Size 3 instead of a Size 2) and swap the standard Class 10 overload relays for Class 20 or Class 30 overload relays, which feature a longer thermal trip delay curve. Always consult the NEMA ICS 2 guidelines for specific trip class selections.
What is the difference between NEMA and IEC motor starter sizing?
NEMA (predominant in North America) uses standardized physical frame sizes (00 through 5) designed for heavy-duty, long-life, and harsh environments. NEMA contactors are physically larger, have higher fault withstand ratings, and are often oversized for the specific motor to ensure longevity. IEC (International, governed by standards like IEC 60947-4-1) uses utilization categories (such as AC-3 for squirrel cage motors) and sizes contactors exactly to the motor's kW and FLA. IEC starters are much more compact, modular, and cost-effective, but they are less tolerant of severe voltage sags, heavy jogging, and extreme environmental abuse. Choose NEMA for critical, high-abuse industrial applications; choose IEC for space-constrained, standard-duty OEM machinery.
