The correct NEMA motor starter size is determined by matching the motor's Full Load Amps (FLA) and operating voltage to the NEMA ICS 2 standard, not just the horsepower rating. For example, a 10 HP motor at 230V requires a NEMA Size 2 starter (rated for 15 HP / 45A at 230V), but that exact same 10 HP motor at 460V only needs a NEMA Size 1 starter (rated for 15 HP / 27A at 460V). Always size by the continuous current rating and voltage column, using horsepower only as a secondary sanity check.
How to Read the NEMA Starter Sizing Chart
Before picking a part number, you need to understand which column actually governs your installation. The NEMA ICS 2 standard defines starter sizes by their maximum continuous current capability and their ability to interrupt locked-rotor currents.
Which column applies to you? Look at the motor nameplate. Find the FLA (Full Load Amps) for your specific supply voltage. Match that FLA to the 'Continuous Current' column in the chart below. The Horsepower columns are simply the maximum HP a starter can safely switch at that specific voltage, assuming standard motor efficiency and power factor. If your motor has an unusually low power factor or high FLA for its HP rating, the FLA column overrides the HP column.
Bookmark these quick-jump rows for the most common fractional and integral horsepower sizes:
- Size 00 & 0: Fractional HP & Small Pumps (1.5 - 5 HP @ 230V)
- Size 1: The 230V Workhorse (7.5 HP @ 230V / 15 HP @ 460V)
- Size 2: Mid-Range Compressors & HVAC (15 HP @ 230V / 30 HP @ 460V)
- Size 3: Heavy Industrial & Large Fans (30 HP @ 230V / 50 HP @ 460V)
The Master NEMA Motor Starter Sizing Chart (ICS 2)
This table covers NEMA sizes 00 through 5, which account for over 95% of commercial and industrial applications. Sizes 6 through 9 exist for massive multi-hundred-horsepower medium-voltage applications and require custom engineering. Data is sourced from NEMA ICS 2-2009 (R2018) for 60Hz, 3-phase induction motors.
| NEMA Size | Max HP @ 115V (1Ø) | Max HP @ 230V (3Ø) | Max HP @ 460V (3Ø) | Max HP @ 575V (3Ø) | Continuous Current (Amps) |
|---|---|---|---|---|---|
| 00 | 1.5 | 3 | 7.5 | 10 | 18 A |
| 0 | 2 | 5 | 10 | 15 | 18 A |
| 1 | 3 | 7.5 | 15 | 20 | 27 A |
| 2 | 7.5 | 15 | 30 | 40 | 45 A |
| 3 | 15 | 30 | 50 | 75 | 90 A |
| 4 | 30 | 50 | 100 | 150 | 135 A |
| 5 | 50 | 100 | 200 | 300 | 270 A |
Note: NEMA Size 0 and Size 00 share the same 18A continuous current rating, but Size 0 features heavier contact assemblies tested for higher horsepower switching transients. Always choose Size 0 over 00 if the motor is exactly 5 HP at 230V.
Decision Path: Sizing for Edge Cases & Derating
The chart above assumes a standard NEMA Design B motor, normal starting torque, a maximum of 10 starts per hour, and standard ambient conditions. When your application deviates from this baseline, you must modify the base value. Use this decision tree to finalize your part number.
| Application Condition (IF...) | Required Action (THEN...) | Concrete Example |
|---|---|---|
| Motor is standard duty (pumps, conveyors, standard fans) | Select exact NEMA size from chart based on FLA. | 10 HP / 460V motor (14A FLA) → Size 1. |
| Application requires 'Jogging' (rapid inching for alignment) | Size up ONE NEMA size. Jogging causes repeated inrush current without cooling time. | 5 HP / 230V motor (15A FLA) normally Size 0 → Select Size 1. |
| Application requires 'Plugging' (reversing motor to stop it quickly) | Size up TWO NEMA sizes. Plugging generates extreme arc energy and contact welding risk. | 3 HP / 230V motor normally Size 00 → Select Size 1. |
| High Inertia Load (long acceleration time > 10 seconds) | Size up ONE NEMA size AND change overload relay to Class 20 or Class 30. | 40 HP / 460V blower (52A FLA) normally Size 2 → Select Size 3 with Class 20 overloads. |
| Installation Altitude is above 3,300 ft (1,000 m) | Derate continuous current by 10%, or size up one NEMA frame. Thinner air reduces contact arc quenching and thermal dissipation. | 25A FLA motor at 5,000 ft elevation → Treat as 27.5A → Select Size 2 (45A) instead of Size 1. |
What the Chart Cannot Tell You (And How to Fix It)
A common mistake on the bench is treating the NEMA sizing chart as a complete bill of materials. The chart gives you the physical frame size and contactor rating, but it leaves three critical variables undefined that will cause your starter to trip nuisance faults or burn out if ignored.
The chart does not specify the thermal overload trip class. Class 10 trips in 10 seconds at 600% of FLA (used for submersible pumps). Class 20 is the default for general-purpose motors. Class 30 allows 30 seconds at 600% FLA and is mandatory for high-inertia loads like large centrifugal fans or rock crushers. If you put a Class 10 overload on a high-inertia fan, it will trip before the motor reaches full speed. Check the motor datasheet for the 'Locked Rotor Time' and match the trip class accordingly.
The NEMA chart assumes an ambient temperature of 40°C (104°F) maximum. If you are mounting the starter inside an unventilated control cabinet sitting in a 120°F Texas pump house, the bimetallic overload relays will trip prematurely. For environments exceeding 40°C, you must either apply a thermal derating curve (found in the manufacturer's datasheet, like Schneider Electric's TeSys guides) or use remote-mounted overloads placed in a cooler environment while keeping the contactor in the hot cabinet.
Do not confuse NEMA sizes with IEC utilization categories (like AC-3). NEMA starters are deliberately overbuilt with larger, heavier contacts and air gaps designed for brutal, high-fault-current North American grids and harsh environments. An IEC-rated starter might be physically half the size of a NEMA Size 2 for the same horsepower. If you are replacing a failed IEC starter in a high-vibration or high-dust environment, upgrading to the equivalent NEMA frame size will drastically increase the mean time between failures (MTBF), even if it requires modifying the DIN rail or backplate mounting holes.
