Motor Starter Sizing Guide

What Is a Motor Starter?

A motor starter is an electrical device that starts, stops, and protects electric motors. It combines a magnetic contactor (for switching power) with an overload relay (for motor protection) in a single assembly. Motor starters are required by the National Electrical Code (NEC) for most motor applications.

Motor starters serve three critical functions:

• Control: Start and stop the motor remotely via pushbuttons, timers, thermostats, or automation systems
• Protection: Disconnect the motor if it draws excessive current for too long (overload protection per NEC 430.32)
• Safety: Provide low-voltage release (LVR) to prevent automatic restart after power loss, and low-voltage protection (LVP) to disconnect during brownouts

Motor starters are rated and selected based on motor HP, voltage, service type (normal duty, heavy duty), and the number of operations per hour. Understanding proper sizing ensures reliable motor operation and code compliance.

Motor Starter Components

A complete motor starter assembly typically consists of the following components:

NEMA Motor Starter Sizes

NEMA (National Electrical Manufacturers Association) classifies motor starters by size number from 0 to 4.5. Each size has specific continuous current and HP ratings. NEMA starters are designed with conservative safety margins, making them robust and forgiving for various applications.

NEMA Starter Size Definitions

The NEMA size determines both the contactor capacity and the maximum motor HP that can be controlled:

• Size 0: Up to 2 HP at 230V (18A continuous)
• Size 1: Up to 5 HP at 230V / 10 HP at 460V (27A continuous)
• Size 2: Up to 10 HP at 230V / 25 HP at 460V (45A continuous)
• Size 3: Up to 25 HP at 230V / 50 HP at 460V (90A continuous)
• Size 4: Up to 50 HP at 230V / 100 HP at 460V (135A continuous)
• Size 4.5: Up to 100 HP at 230V / 200 HP at 460V (270A continuous)

Motor Starter Sizing Chart by HP and Voltage

Use this chart to quickly select the correct NEMA starter size for your motor. Find your motor's HP and voltage, then select the corresponding NEMA size:

Single-Phase Motors

Three-Phase Motors

Overload Protection Sizing

Overload protection is the most critical function of a motor starter. It protects the motor from sustained overcurrent conditions that would damage the windings. Overload protection is distinct from short-circuit protection (fuses/breakers).

NEC 430.32 — Overload Sizing Rules

Overload Relay Types

• Thermal (bimetallic): Most common. Uses bimetallic strips that bend when heated by motor current. Provides inherent time delay to allow motor starting. Class 10, 20, or 30 trip times.
• Electronic (solid-state): More accurate and adjustable. Digital settings for FLA, trip class, and phase-loss detection. Preferred for critical applications and VFD bypass starters.
• Melting alloy: Oldest technology. Uses a eutectic alloy that melts at a specific temperature. Less common in modern installations.

Trip Classes

The trip class defines how quickly the overload relay trips under a locked-rotor condition:

Contactor Selection for Motor Starters

The contactor is the electromechanical switch that makes and breaks the motor power circuit. Proper contactor selection ensures reliable switching and long operational life.

Contactor Rating Requirements

A motor contactor must be rated for:

• Continuous current ≥ motor FLA — The contactor's AC-3 rating should exceed the motor's full-load current
• HP rating ≥ motor HP — The NEMA HP rating at your voltage must match or exceed the motor
• Voltage rating ≥ system voltage — Coil voltage and contact voltage rating must suit the installation
• Breaking capacity ≥ available fault current — The contactor must handle the prospective fault current at the installation point

Contactor Application Categories (IEC)

• AC-1: Non-inductive or slightly inductive loads (resistive heating, lighting)
• AC-2: Slip-ring motors — starting, inching
• AC-3: Squirrel-cage motors — starting, switching off during running
• AC-4: Squirrel-cage motors — starting, plugging, inching (most severe)

For standard motor starting applications, select contactors rated for AC-3 utilization. For applications with frequent jogging or plugging, use AC-4 ratings.

NEC Article 430 Requirements for Motor Starters

NEC Article 430 contains comprehensive requirements for motor circuits. Here are the key sections relevant to motor starter sizing:

NEC 430.32 — Overload Protection

Every motor must have overload protection sized per the rules above. The overload device must be able to sense and respond to sustained overcurrent conditions.

NEC 430.35 — Shutoff During Overload

Overload protection must disconnect all ungrounded conductors of the circuit. This means three-pole overload relays for three-phase motors.

NEC 430.37 — Motor Controllers

Motor controllers (starters) must be capable of starting and stopping the motor they control and must be rated for the motor's HP at the system voltage.

NEC 430.52 — Branch-Circuit Overcurrent Protection

Maximum overcurrent device ratings for motor branch circuits:

NEC 430.102 — Disconnecting Means

A disconnect must be provided within sight of the motor location (or lockable at the motor location if not in sight). The disconnect must be rated at least 115% of the motor FLC.

NEMA vs IEC Motor Starters

Understanding the differences between NEMA and IEC starter standards helps you choose the right product:

Using a VFD as a Motor Starter

Variable Frequency Drives (VFDs) can function as motor starters, providing soft-start capability and speed control. When using a VFD as a starter:

• Soft start: VFDs ramp motor speed up gradually, eliminating high inrush current (typically 150-600% FLA at across-the-line start)
• Overload protection: Built-in electronic overload protection eliminates the need for separate overload relays
• Bypass contactor: Some installations include a bypass contactor that bypasses the VFD at full speed for efficiency
• Isolation: A disconnect switch is still required per NEC for service and maintenance

When sizing a VFD as a starter, select the VFD's continuous current rating ≥ motor FLA. For constant-torque applications (conveyors, compressors), use the VFD's constant-torque rating. For variable-torque applications (fans, pumps), the variable-torque rating is acceptable.