To size an electric motor starter, match the motor’s nameplate Full Load Amps (FLA) and operating voltage to the starter’s continuous current rating, then select the next available NEMA or IEC frame size. For example, a standard 5 HP, 460V 3-phase motor drawing ~7.6A requires a NEMA Size 1 (27A max) or an IEC AF16 (16A max) starter. Always size by the exact FLA on the nameplate, not just the horsepower rating.
How to Read the Electric Motor Starter Sizing Chart
Motor starters are categorized by two primary global standards: NEMA (National Electrical Manufacturers Association, standard ICS 2) common in North America, and IEC (International Electrotechnical Commission, standard 60947-4-1) common in Europe and increasingly in global OEM equipment. NEMA starters are physically larger, built with heavy-duty contacts designed for broad application ranges and harsh environments. IEC starters are compact, application-specific, and precisely sized to the motor’s exact duty cycle.
How to read this table: First, identify your supply voltage and locate the corresponding HP column (230V or 460V 3-phase). Find the row where the Max HP meets or exceeds your motor’s horsepower. Next, verify that the Max Continuous Amps column is strictly greater than your motor’s nameplate FLA. If your motor has a high service factor (e.g., 1.15), multiply the FLA by the service factor before comparing it to the continuous amps column. The source standards for the baseline ratings below are NEMA ICS 2 and IEC 60947-4-1 as detailed in Rockwell Automation's application guides.
| NEMA Size | IEC Frame (Approx.) | Max Continuous Amps | Max HP @ 230V (3Ø) | Max HP @ 460V (3Ø) |
|---|---|---|---|---|
| 00 | AF09 | 18 A | 1.5 HP | 3 HP |
| 0 | AF12 / AF16 | 18 A / 22 A | 3 HP | 5 HP |
| 1 (Most Common) | AF26 / AF40 | 27 A | 7.5 HP | 10 HP |
| 2 (Heavy Duty) | AF52 / AF65 | 45 A | 15 HP | 25 HP |
| 3 | AF80 / AF96 | 90 A | 30 HP | 50 HP |
| 4 | AF116 / AF145 | 135 A | 60 HP | 100 HP |
| 5 | AF185 / AF205 | 270 A | 125 HP | 200 HP |
Applying Derating Factors and Environmental Adjustments
The continuous amp ratings in the chart above assume a standard ambient temperature of 40°C (104°F) and free air circulation. In real-world installations, you must apply derating factors when environmental conditions exceed these baselines.
- High Ambient Temperature: If the starter is mounted inside an unventilated control panel located on a hot factory floor (e.g., 50°C / 122°F ambient), the thermal capacity of the bimetallic overload relays and the contact insulation degrade. A NEMA Size 1 starter rated for 27A at 40°C will typically derate to roughly 22A or 23A at 50°C. Always consult the manufacturer’s specific temperature-current curve.
- Panel Grouping and Heat Sources: Mounting a starter directly above a variable frequency drive (VFD) or a bank of power resistors exposes it to convective heat. If you cannot relocate the starter, you must upsize to the next NEMA frame or install a forced-air cooling fan in the enclosure.
- Altitude: Air density drops at high elevations, reducing its cooling efficiency. For installations above 3,300 feet (1,000 meters), IEC standards typically require a 1% current derating for every 100 meters above the baseline, or an upsizing of the contactor frame.
What the Sizing Chart Cannot Tell You
A sizing chart is a starting point, not a complete engineering specification. Relying solely on HP and FLA will lead to premature contactor failure in specific applications. Here is what the table hides:
1. Starting Torque and High-Inertia Loads
Standard charts assume a normal starting time of under 10 seconds for a standard squirrel-cage induction motor. If you are starting a high-inertia load like a large centrifuge, a rock crusher, or a heavily loaded conveyor belt, the motor may take 20+ seconds to reach full speed. During this time, the starter contacts must carry 600% of the FLA (Locked Rotor Amps). For high-inertia loads, you must upsize the NEMA contactor by one or two frames to prevent the contacts from welding shut due to prolonged arc energy.
2. Duty Cycle and Jogging
If your application involves "plugging" (reversing the motor to stop it quickly), "jogging" (rapidly starting and stopping to inch a load), or reversing more than 5 times per minute, a standard IEC starter will fail rapidly. NEMA standards define specific duty classes. For severe jogging and plugging, you must select a NEMA starter rated for that specific duty, which often requires moving up two NEMA sizes from the standard chart.
3. Short Circuit Coordination (SCCR)
The starter protects the motor from overloads, but it does not protect itself from short circuits. The chart does not tell you the maximum fault current the starter can safely interrupt without exploding. You must coordinate the starter with the correct class of upstream fuses (e.g., Class J or CC) or a Motor Circuit Protector (MCP) to achieve the required Short Circuit Current Rating (SCCR) for the panel. Proper coordination testing ensures the upstream breaker clears the fault before the contactor contacts vaporize.
Frequently Asked Questions
How do I size a motor starter for a single-phase 120V/240V motor?
Single-phase motors draw significantly more current per horsepower than 3-phase motors. A 3 HP, 230V single-phase motor will draw roughly 17A to 20A, whereas a 3 HP, 230V 3-phase motor draws only about 9.6A. When sizing a starter for single-phase, ignore the 3-phase HP columns in the chart. Instead, calculate the exact FLA, multiply by 1.15 (for the service factor), and select a 2-pole or 3-pole contactor where the Max Continuous Amps exceeds your calculated value. For 3-pole contactors used on single-phase, wire L1 and L2 through two poles, and loop the L2 wire back through the third pole to ensure the thermal overload relay trips correctly on all phases.
What is the difference between a motor starter and a contactor?
A contactor is simply an electrically controlled switch used to make or break the power circuit to the motor. A motor starter is a combination device that includes the contactor plus an overload relay. The contactor handles the heavy lifting of switching the current, while the overload relay monitors the heat/current and drops out the contactor coil if the motor overloads. You cannot legally or safely start a motor across-the-line with just a contactor; you must have overload protection.
Can I use an IEC starter for a high-inertia load like a rock crusher?
Generally, no. IEC starters are designed for "Utilization Category AC-3," which covers standard squirrel-cage motors starting and switching off after reaching speed. High-inertia loads, plugging, and jogging fall under "AC-4" duty. While some IEC manufacturers publish AC-4 ratings, the physical frame size required to handle the arc energy of an AC-4 duty cycle often negates the space-saving benefits of IEC designs. For heavy, high-inertia industrial loads, a robust NEMA-rated starter is the industry-preferred choice due to its larger contact surface area and heavier arc chutes.
Do I need to size the overload relay separately from the starter?
In modern modular designs (like the Schneider TeSys or Allen-Bradley 100-C lines), the contactor and the overload relay are sold as separate part numbers but snap together physically. You must size the contactor based on the continuous amp rating (as shown in the chart), but you must size the thermal or electronic overload relay specifically to the motor’s exact nameplate FLA. Overload relays have adjustable ranges (e.g., 9A to 13A). Ensure the motor's FLA falls in the middle 60% of the relay's adjustment range for the most accurate trip response.
