Introduction #

Motor starter sizing is a critical aspect of industrial electrical system design that directly impacts motor reliability, protection, and system performance. An undersized starter can fail prematurely or fail to provide adequate protection, while an oversized starter wastes money and may not provide optimal protection characteristics. Understanding how to properly size motor starters based on motor characteristics, application requirements, and NEC Article 430 is essential for designing reliable motor control systems.

This comprehensive guide covers motor starter sizing fundamentals, NEC requirements, starter type selection, sizing calculations, and practical design considerations. Whether you're selecting starters for new motor installations or retrofitting existing systems, this guide provides the knowledge you need to make informed decisions.

What is a Motor Starter? #

A motor starter is an electrical device that controls the starting, stopping, and protection of electric motors. It consists of:

  1. Contactor: Electrically operated switch that controls motor power
  2. Overload Relay: Thermal or electronic device that protects motor from overload
  3. Auxiliary Contacts: For control and indication circuits
  4. Enclosure: Provides protection and mounting

Motor Starter Functions #

Motor starters perform three primary functions:

  1. Starting Control: Provides controlled motor starting (direct-on-line, reduced voltage, etc.)
  2. Stopping Control: Allows safe motor shutdown
  3. Overload Protection: Protects motor from excessive current that could cause damage

Motor Starter Types #

1. Direct-On-Line (DOL) Starters #

Characteristics:

  • Simplest and most economical
  • Full voltage applied immediately
  • High starting current (6-8× FLC)
  • Suitable for small motors (< 10 HP typically)

Applications:

  • Small pumps, fans, compressors
  • Motors with low starting torque requirements
  • Applications where high starting current is acceptable

2. Reduced Voltage Starters #

Types:

  • Star-Delta (Wye-Delta): Reduces starting current to 33% of DOL
  • Soft Starters: Gradually increases voltage using solid-state control
  • Autotransformer: Reduces voltage using transformer taps

Applications:

  • Large motors (> 10 HP)
  • Applications with limited available fault current
  • Systems where voltage drop during starting must be minimized

3. Variable Frequency Drives (VFDs) #

Characteristics:

  • Provides variable speed control
  • Smooth starting with controlled acceleration
  • Energy savings at reduced speeds
  • Higher cost but superior control

Applications:

  • Pumps and fans requiring flow control
  • Conveyor systems
  • Applications requiring speed variation

NEC Article 430 Requirements #

Starter Size Based on Motor FLC #

NEC Article 430.82 requires motor starters to be sized based on motor full-load current (FLC), not starting current:

Basic Rule:

  • Starter must be rated for at least 125% of motor FLC
  • Exception: Starters rated for 100% of FLC are acceptable if marked for continuous duty

Example:

  • 50 HP motor at 480V, 3-phase
  • FLC from NEC Table 430.250: 65 A
  • Minimum starter rating: 65 × 1.25 = 81.25 A
  • Select: 100 A starter (next standard size)

Starter Interrupting Rating #

NEC Article 430.109 requires starters to have adequate interrupting rating:

Requirement:

  • Starter must interrupt available fault current
  • Typically requires 10 kA or higher interrupting rating for industrial applications
  • Must coordinate with upstream protective devices

Motor Starter Sizing Process #

Step 1: Determine Motor Full-Load Current (FLC) #

Method 1: Use NEC Tables

  • NEC Table 430.250 (3-phase motors)
  • NEC Table 430.248 (single-phase motors)
  • Based on motor HP and voltage

Method 2: Use Nameplate Data

  • Use actual nameplate FLC if available
  • More accurate than table values

Method 3: Calculate from Motor Data

FLC (A) = (HP × 746) / (√3 × V × PF × Efficiency)

Example:

  • Motor: 50 HP, 480V, 0.85 PF, 0.92 efficiency
  • FLC = (50 × 746) / (1.732 × 480 × 0.85 × 0.92)
  • FLC = 37,300 / 650.3 = 57.4 A

Step 2: Apply NEC Sizing Factor #

Standard Starters:

  • Minimum rating: 125% of FLC
  • Select next standard size

100% Rated Starters:

  • Can be sized at 100% of FLC
  • Must be marked for continuous duty
  • Typically more expensive

Example:

  • FLC: 65 A
  • Minimum rating: 65 × 1.25 = 81.25 A
  • Standard sizes: 30, 45, 90, 135, 270 A
  • Select: 90 A starter

Step 3: Select Starter Type #

Considerations:

  • Motor size and starting current
  • Available fault current
  • Voltage drop limitations
  • Application requirements
  • Cost constraints

Selection Guide:

Motor Size Starter Type Reason
< 10 HP DOL Economical, acceptable starting current
10-50 HP DOL or Soft Starter Depends on system capacity
> 50 HP Soft Starter or VFD Reduce starting current, control
High Inertia Loads Soft Starter or VFD Controlled acceleration
Variable Speed Required VFD Speed control capability

Step 4: Verify Interrupting Rating #

Requirement:

  • Starter must interrupt available fault current
  • Typical industrial systems: 10-65 kA available fault current
  • Verify with short-circuit study

Standard Ratings:

  • NEMA: 10 kA, 18 kA, 30 kA, 65 kA, 100 kA
  • IEC: 10 kA, 25 kA, 50 kA, 100 kA

Step 5: Select Overload Relay #

Thermal Overload Relays:

  • Class 10: Standard trip time (10 seconds at 600% FLC)
  • Class 20: Longer trip time (20 seconds at 600% FLC)
  • Class 30: Very long trip time (30 seconds at 600% FLC)

Selection:

  • Standard motors: Class 10
  • High-inertia loads: Class 20 or 30
  • Size based on motor FLC (typically 100-125% of FLC)

Real-World Case Study #

Project: Chemical Plant Motor Control Retrofit #

Background:
A chemical processing facility was experiencing frequent motor starter failures on 75 HP process pumps. The existing starters were 90 A NEMA-rated contactors, and motors were 75 HP at 480V with 93 A nameplate FLC.

Problem:

  • Starters failing every 6-12 months
  • Overload relays tripping during normal operation
  • Production downtime due to motor failures

Analysis:

  1. Starter Sizing Check:

    • Motor FLC: 93 A
    • Minimum starter rating: 93 × 1.25 = 116.25 A
    • Existing starter: 90 A (undersized)

  2. Overload Relay Check:

    • Overload set at 100 A (107% of FLC)
    • Motor running at 88-92 A (normal load)
    • Overload tripping due to thermal cycling

  3. Starting Current Analysis:

    • Starting current: 93 × 6.5 = 605 A
    • Starting time: 3-4 seconds (high-inertia pump)
    • Contactor rated for 10,000 operations
    • Actual: 50-100 starts per day = 18,000-36,000 per year

Solution:

  1. Upgrade Starter Size:

    • Selected 135 A starter (145% of FLC)
    • Provides adequate margin for starting current

  2. Install Soft Starter:

    • Reduced starting current from 605 A to 280 A (46% reduction)
    • Extended contactor life by reducing arcing
    • Reduced voltage drop during starting

  3. Adjust Overload Settings:

    • Set overload at 110 A (118% of FLC)
    • Changed to Class 20 overload for longer starting time tolerance
    • Prevents nuisance trips during starting

Results:

  • Starter failures eliminated (3+ years operation)
  • Overload trips reduced by 95%
  • Voltage drop during starting reduced from 8% to 3%
  • Motor life extended due to reduced starting stress
  • ROI: 18 months (reduced downtime and maintenance)

Key Takeaway:
Proper starter sizing requires considering not just FLC, but also starting current, duty cycle, and application characteristics. Undersized starters fail prematurely, while proper sizing with appropriate starter types provides reliable operation.

Common Mistakes to Avoid #

1. Sizing Based on Starting Current #

Mistake:
Selecting starter based on motor starting current (6-8× FLC) instead of FLC.

Example:

  • 50 HP motor: FLC = 65 A, Starting = 390 A
  • Wrong: Select 400 A starter
  • Correct: Select 90 A starter (125% of 65 A)

Why It's Wrong:

  • NEC requires sizing based on FLC, not starting current
  • Starting current is momentary (0.5-3 seconds)
  • Starter must handle continuous FLC, not starting current

Correct Approach:

  • Size starter at 125% of FLC
  • Verify starter can handle starting current (typically can)
  • Use reduced voltage starter if starting current is problematic

2. Ignoring Duty Cycle #

Mistake:
Selecting standard duty starter for high-frequency starting applications.

Example:

  • Conveyor motor: 50 starts per hour
  • Standard starter rated for 10 operations/hour
  • Result: Premature contactor failure

Why It's Wrong:

  • Standard starters have limited operations rating
  • High-frequency starting exceeds design limits
  • Causes contactor wear and failure

Correct Approach:

  • Select heavy-duty starter for high-frequency applications
  • Consider soft starter or VFD for frequent starting
  • Verify operations rating matches application

3. Oversizing "To Be Safe" #

Mistake:
Selecting much larger starter than required "to be safe."

Example:

  • 25 HP motor: FLC = 34 A
  • Minimum starter: 45 A (125% of 34 A)
  • Selected: 135 A starter "to be safe"

Why It's Wrong:

  • Oversized starters don't provide better protection
  • Overload relay must still be sized for motor FLC
  • Wastes money without benefit

Correct Approach:

  • Size starter at 125% of FLC (minimum)
  • Select next standard size
  • Size overload relay for motor FLC

4. Not Considering Voltage Drop #

Mistake:
Selecting DOL starter for large motor without checking voltage drop.

Example:

  • 100 HP motor at 480V
  • DOL starting current: 600 A
  • Voltage drop during starting: 12%
  • Result: Motor may not start, or starting time excessive

Why It's Wrong:

  • High starting current causes voltage drop
  • Excessive voltage drop prevents proper starting
  • Can cause motor damage or system problems

Correct Approach:

  • Calculate voltage drop during starting
  • If > 5%, consider reduced voltage starter
  • Use soft starter or VFD for large motors

5. Incorrect Overload Relay Sizing #

Mistake:
Setting overload relay too high or too low.

Example:

  • Motor FLC: 65 A
  • Overload set at 80 A (123% of FLC) - too high
  • Motor running at 70 A (overload condition)
  • Overload doesn't trip, motor overheats

Why It's Wrong:

  • Overload relay protects motor from excessive current
  • Too high setting doesn't provide protection
  • Too low setting causes nuisance trips

Correct Approach:

  • Set overload at 100-125% of motor FLC
  • Consider service factor (SF 1.15 motors can run at 115% FLC)
  • Adjust for ambient temperature if necessary

6. Ignoring Ambient Temperature #

Mistake:
Not derating starter for high ambient temperature.

Example:

  • Starter rated for 40°C ambient
  • Actual ambient: 50°C
  • Result: Starter derating required, may be undersized

Why It's Wrong:

  • Starters have temperature ratings
  • High ambient reduces current-carrying capacity
  • Can cause premature failure

Correct Approach:

  • Check ambient temperature rating
  • Derate starter if ambient exceeds rating
  • Select higher-rated starter if necessary

7. Not Coordinating with Upstream Protection #

Mistake:
Selecting starter without considering upstream circuit breaker.

Example:

  • Motor FLC: 65 A
  • Starter: 90 A
  • Circuit breaker: 100 A
  • Available fault current: 45 kA
  • Starter interrupting rating: 10 kA (insufficient)

Why It's Wrong:

  • Starter must interrupt available fault current
  • Upstream breaker must coordinate with starter
  • Insufficient interrupting rating causes safety hazard

Correct Approach:

  • Verify available fault current
  • Select starter with adequate interrupting rating
  • Ensure proper coordination with upstream protection

Best Practices #

1. Always Use NEC Tables for FLC #

Practice:
Use NEC Table 430.250 for 3-phase motor FLC, even if nameplate differs.

Reason:

  • NEC tables are conservative and account for worst-case conditions
  • Required for code compliance
  • Provides consistent sizing basis

Implementation:

  • Look up FLC from NEC table based on HP and voltage
  • Use this value for starter sizing
  • Nameplate FLC can be used for overload relay setting

2. Select Standard Starter Sizes #

Practice:
Always select standard NEMA or IEC starter sizes.

Reason:

  • Standard sizes ensure availability and compatibility
  • Easier maintenance and replacement
  • Lower cost than custom sizes

Standard Sizes (NEMA):

  • Size 00: 9 A
  • Size 0: 18 A
  • Size 1: 27 A
  • Size 2: 45 A
  • Size 3: 90 A
  • Size 4: 135 A
  • Size 5: 270 A

3. Use Soft Starters for Large Motors #

Practice:
Consider soft starters for motors > 50 HP or high-inertia loads.

Reason:

  • Reduces starting current and voltage drop
  • Extends motor and starter life
  • Provides controlled acceleration

When to Use:

  • Motors > 50 HP
  • High-inertia loads (pumps, compressors)
  • Limited available fault current
  • Voltage drop concerns

4. Proper Overload Relay Selection #

Practice:
Select overload relay class based on application.

Reason:

  • Different applications require different trip characteristics
  • Prevents nuisance trips while providing protection

Selection Guide:

  • Standard applications: Class 10
  • High-inertia loads: Class 20
  • Very high-inertia: Class 30
  • Fast trip required: Class 5

5. Verify Interrupting Rating #

Practice:
Always verify starter interrupting rating matches available fault current.

Reason:

  • Safety requirement
  • Prevents equipment damage
  • Ensures proper protection coordination

Implementation:

  • Perform short-circuit study
  • Determine available fault current at starter location
  • Select starter with adequate interrupting rating
  • Consider future system changes

6. Consider Future Expansion #

Practice:
Size starter with some margin for future motor upgrades.

Reason:

  • Allows motor replacement with slightly larger motor
  • Reduces need for starter replacement
  • Provides flexibility

Guideline:

  • Size starter at 125-150% of current motor FLC
  • Don't oversize excessively (waste money)
  • Consider 1-2 HP size increase potential

7. Document Starter Selection #

Practice:
Document starter selection rationale and calculations.

Reason:

  • Aids future maintenance and troubleshooting
  • Ensures proper replacement
  • Supports code compliance

Documentation Should Include:

  • Motor FLC (from NEC table and nameplate)
  • Starter size and type selected
  • Overload relay settings
  • Interrupting rating verification
  • Application-specific considerations

Standards & References #

IEEE Standards #

  • IEEE 141: Recommended Practice for Electric Power Distribution for Industrial Plants

    • Provides guidance on motor control system design
    • Covers starter selection and protection coordination
    • IEEE Standards

  • IEEE 519: Recommended Practices and Requirements for Harmonic Control in Electric Power Systems

    • Addresses harmonic issues with VFDs and soft starters
    • Provides limits and mitigation strategies

NEC/NFPA Standards #

  • NEC Article 430: Motors, Motor Circuits, and Controllers
    • Section 430.82: Controller rating requirements
    • Section 430.109: Controller interrupting rating
    • Section 430.32: Overload protection requirements
    • NFPA 70: National Electrical Code

NEMA Standards #

  • NEMA ICS 2: Industrial Control and Systems: Controllers, Contactors, and Overload Relays

    • Defines NEMA starter sizes and ratings
    • Provides selection guidelines
    • NEMA Standards

  • NEMA MG 1: Motors and Generators

    • Motor performance standards
    • Starting current characteristics
    • Service factor definitions

IEC Standards #

  • IEC 60947: Low-voltage switchgear and controlgear
    • Part 4-1: Contactors and motor-starters
    • Defines IEC starter ratings and characteristics
    • IEC Standards

Industry Resources #

  • Schneider Electric: Motor Control and Protection Guide

  • ABB: Motor Control and Protection

    • Starter selection and application guides
    • Technical documentation
    • ABB Resources

Engineer's Practical Insight #

From 15+ years of motor control design experience: The most common mistake I see is engineers oversizing starters "to be safe" without understanding that starter size doesn't directly correlate with protection level. A 50 HP motor (65 A FLC) doesn't need a 270 A starter. NEC requires 125% of FLC, which means a 90 A starter is sufficient. The overload relay, not the starter size, provides motor protection. Oversizing wastes money and doesn't improve protection.

Critical field observation: High-frequency starting applications (conveyors, indexing machines) require heavy-duty starters or soft starters, not standard duty starters. I've seen standard starters fail in 6 months on applications with 50+ starts per hour. The contactor contacts wear out from excessive arcing. A soft starter eliminates arcing during starting, extending contactor life by 5-10×. For a $15,000 motor installation, spending an extra $2,000 on a soft starter pays for itself in reduced maintenance and downtime.

Practical sizing strategy: Always verify starter interrupting rating matches available fault current. In one project, we had 45 kA available fault current, but the specified starters were only rated for 10 kA. This would have been a safety hazard. We upgraded to 65 kA rated starters, which cost 30% more but were essential for safety. Never assume standard interrupting ratings are adequate—always verify with a short-circuit study.

Multiple motor considerations: When sizing starters for multiple motors on the same feeder, consider the combined starting current impact. In a water treatment plant, we had six 25 HP pumps (34 A FLC each) on a 400 A feeder. If all started simultaneously, combined starting current would be 1,224 A (6 × 34 × 6), exceeding feeder capacity. We installed soft starters with staggered starting (2-second delay between motors) to limit peak starting current to 400 A. This required coordination but prevented feeder overload.

Conclusion #

Motor starter sizing is a critical aspect of industrial electrical system design that requires understanding motor characteristics, NEC requirements, and application needs. Proper sizing ensures reliable motor operation, adequate protection, and code compliance.

Key takeaways:

  1. Size starters at 125% of motor FLC (minimum), not starting current
  2. Use NEC tables for FLC to ensure code compliance
  3. Select appropriate starter type (DOL, soft starter, VFD) based on application
  4. Verify interrupting rating matches available fault current
  5. Size overload relays at 100-125% of motor FLC for proper protection
  6. Consider duty cycle and select heavy-duty starters for high-frequency applications
  7. Document selection rationale for future reference and maintenance

For quick calculations, use our 3-Phase Power Calculator to determine motor FLC, and always consult NEC Article 430 and motor nameplate data for starter sizing.

About the Author: James Chen, P.E. is a licensed electrical engineer with 15+ years of experience in industrial power systems design. He has designed motor control systems for manufacturing facilities, chemical plants, and water treatment facilities. Former Schneider Electric application engineer specializing in 3-phase motor control and power distribution. All content in this guide has been reviewed and validated by licensed engineers.