NEMA motor starter sizes range from Size 00 (up to 2 HP at 230V) to Size 9 (up to 2000 HP at 460V). Selecting the correct size requires matching the motor's Full Load Amps (FLA) and horsepower to the NEMA frame, not just the nameplate kW. While IEC contactors are rated strictly by continuous current and utilization categories, NEMA starters are built with heavy-duty, standardized physical frames designed to withstand the massive inrush currents and mechanical stress of across-the-line AC induction motor starting.
The NEMA Sizing Framework: Matching HP to Full Load Amps
The National Electrical Manufacturers Association (NEMA) standardizes starter enclosures and contactor assemblies by physical size. As the physical frame increases, so does the contact surface area, arc chutes, and thermal mass of the overload relay block. You must never size a starter solely on horsepower; the Full Load Amps (FLA) dictate the thermal limits of the overload heaters or adjustable dial.
NEMA Starter Size Reference Table
| NEMA Size | Max HP @ 230V (3-Phase) | Max HP @ 460V (3-Phase) | Max FLA Rating (Amps) | Typical 2026 Pricing |
|---|---|---|---|---|
| Size 00 | 2 HP | 5 HP | 9 A | $120 - $180 |
| Size 0 | 5 HP | 10 HP | 18 A | $150 - $250 |
| Size 1 | 10 HP | 25 HP | 45 A | $250 - $400 |
| Size 2 | 25 HP | 50 HP | 90 A | $450 - $700 |
| Size 3 | 50 HP | 100 HP | 180 A | $800 - $1,200 |
| Size 4 | 100 HP | 200 HP | 360 A | $1,500 - $2,200 |
Source: Standard NEMA ICS 2 frame ratings; pricing reflects 2026 distributor averages for open-type combination starters.
Sizing Rule of Thumb and Worked Load Example
The Rule: Size the NEMA starter so the motor's nameplate FLA falls within the adjustable range of the overload relay block, while ensuring the motor HP does not exceed the NEMA size maximum for your specific line voltage. Always leave a 15-20% thermal margin in the enclosure if ambient temperatures exceed 104°F (40°C).
Worked Example: You are wiring a 15 HP, 460V 3-phase air compressor motor. The nameplate states an FLA of 19.2A and a code letter G (indicating high locked-rotor inrush).
1. Look at the table: A Size 0 starter maxes out at 18A and 10 HP at 460V. It is too small.
2. A Size 1 starter handles up to 45A and 25 HP at 460V. This is our target frame.
3. We select a NEMA Size 1 combination starter (e.g., Allen-Bradley Bulletin 509 or Eaton Freedom C306DN3).
4. We install an overload relay block with an adjustable dial range of 14A to 22A, and set the dial exactly to 19.2A.
Motor Types, Load Profiles, and Controller Demands
Standard NEMA electromechanical starters are designed specifically for across-the-line (DOL) starting of AC induction motors. They close all three phases simultaneously, subjecting the motor and the mechanical load to full line voltage instantly. The motor type dictates whether a standard NEMA starter is viable, or if the inrush current will cause unacceptable voltage sag on your facility's bus.
Motor Type and Load Profile Comparison
| Motor Type (NEMA Design) | Torque Curve Profile | Control / Starter Needs | Relative Cost | Ideal Load Profile |
|---|---|---|---|---|
| Design B (Standard Induction) | Normal starting torque (~150%), low slip, high inrush (600%) | Standard NEMA electromechanical starter | $ | Centrifugal pumps, fans, blowers |
| Design C (High Starting Torque) | High starting torque (~225%), lower starting current | NEMA starter (up-rated contacts) or Soft Starter | $$ | Loaded conveyors, compressors, crushers |
| Design D (High Slip) | Very high starting torque (~275%), high slip | Reversing NEMA starter with heavy-duty contacts | $$$ | Punch presses, hoists, elevators |
| PMSM / BLDC (Synchronous) | Zero-speed max torque, high efficiency | VFD with sensorless vector or encoder FOC (No NEMA starter) | $$$$ | Precision extruders, EV traction, CNC spindles |
Matching the Driver to the Load
If your load is a NEMA Design B fan or pump, the mechanical load is low at startup and increases with speed. A standard NEMA magnetic starter is perfect here; the high inrush current only lasts a second or two before the motor reaches synchronous speed.
If your load is a NEMA Design C rock crusher or a conveyor belt that might be fully loaded when the power trips, a standard NEMA starter will cause massive voltage dips. The high starting torque requirement demands either a solid-state soft starter (to ramp voltage and limit inrush to 300%) or a Variable Frequency Drive (VFD).
Note on Stepper and Servo Motors: Stepper and servo motors demand entirely different pulse/direction or field-oriented control (FOC) drivers. They are never paired with NEMA magnetic starters. Attempting to switch a servo motor's power with a NEMA contactor will destroy the drive's regenerative braking circuits and fault the controller.
Wiring, Terminal Identification, and Failure Signatures
A NEMA combination starter integrates a disconnect switch (or circuit breaker), a magnetic contactor, and an overload relay into one enclosure. Proper terminal identification and torque are critical to preventing arc flashes and nuisance trips.
Terminal Identification Guide
- Line Side (Incoming Power): L1, L2, L3. Connects to the top of the disconnect or breaker block.
- Load Side (To Motor): T1, T2, T3. Connects to the bottom of the overload relay block. Always torque these to the manufacturer's spec (e.g., 14 in-lbs for 12 AWG THHN on a Size 1) to prevent high-resistance heating.
- Contactor Coil: A1 and A2. Energizing these pulls the main contacts closed. Common voltages are 120V AC or 24V DC.
- Auxiliary Contacts: Used for control logic. Normally Open (NO) is typically designated 13/14. Normally Closed (NC) is designated 21/22.
Diagnosing Failure Signatures
When a NEMA starter or motor fails, the physical symptoms point directly to the electrical or mechanical root cause. Do not simply reset the overload and walk away.
1. The Contactors Hums or Chatters
Cause: Low coil voltage or a broken shading coil. AC contactors rely on a copper shading ring embedded in the armature to prevent the magnetic field from dropping to zero 120 times a second. If the shading coil cracks, the armature chatters violently.
Fix: Measure voltage at A1/A2 while the coil is pulled in. If it reads below 85% of nominal (e.g., < 102V on a 120V coil), check for voltage drop across the control circuit wiring or transformer. If voltage is good, replace the contactor coil or the entire contactor assembly.
2. Overheating at the Starter or Motor Terminals
Cause: Loose T1-T3 lug connections or an incorrectly set overload dial. A loose 10 AWG wire on a T1 terminal will arc and generate enough localized heat to melt the plastic overload block, eventually causing a phase-to-ground fault.
Fix: De-energize, lock out, and verify dead. Inspect the T1-T3 lugs for blued copper or melted insulation. Re-strip, re-terminate, and torque to spec. Verify the overload dial is set exactly to the motor nameplate FLA, not the HP rating.
3. Motor Stalls and Overload Trips Instantly
Cause: Single-phasing or mechanical seizure. If one incoming phase drops (e.g., a blown fuse on L2), the motor will attempt to maintain rotation by drawing 200% to 300% current on the remaining two phases. The bimetallic overload relays will sense this asymmetry and trip the starter within seconds to save the windings.
Fix: Check all three phases at L1, L2, and L3 with a multimeter. If you have 460V phase-to-phase on L1-L2 and L1-L3, but 0V on L2-L3, you have a blown disconnect fuse or a broken utility line. For mechanical stalls, uncouple the motor from the load and spin the shaft by hand to check for seized bearings.
For deeper technical reference on NEMA frame standardization and utilization categories, consult the Rockwell Automation NEMA and IEC Motor Controller Selection Guide. For foundational theory on why AC induction motors draw such high locked-rotor current during across-the-line starting, review the Three-Phase Induction Motors chapter on All About Circuits. Always defer to the manufacturer's specific sizing FAQs and your local AHJ for code-compliant installations.
