Motor Overload Protection: Complete Sizing Guide with Charts & Selection Criteria

Motor overload protection is the most critical component in preventing costly motor failures and ensuring safe, reliable operation of electrical equipment. Motor overload relays protect against sustained overcurrent conditions that cause dangerous overheating, insulation breakdown, and premature motor failure. Whether you're installing a 3-phase motor starter with overload protection for a 3 HP, 5 HP, or 10 HP motor, proper sizing and selection directly impacts motor life expectancy and system uptime.

Incorrect overload relay sizing leads to two equally problematic scenarios: undersized protection fails to prevent motor damage, while oversized protection causes nuisance tripping that disrupts operations. This comprehensive guide provides detailed motor overload sizing charts, step-by-step selection procedures, and practical examples for properly protecting motors across all common industrial and commercial applications.

Understanding Motor Overload Protection

What is Motor Overload Protection?

Motor overload protection is a protective device that monitors motor current and disconnects power when sustained overcurrent conditions exceed safe operating limits. Unlike instantaneous short circuit protection, overload relays provide time-delayed tripping that allows for normal motor starting currents while protecting against harmful sustained overloads.

How Motor Overload Relays Work

Operating Principle:

1. Current sensing: Monitors motor current through heater elements or electronic sensors
2. Thermal modeling: Simulates motor heating based on current draw
3. Trip calculation: Compares accumulated heat to trip threshold
4. Contact operation: Opens circuit when trip point reached
5. Manual reset: Requires operator intervention after trip

Types of Motor Overload Relays

Type	Operating Principle	Typical Applications	Advantages	Limitations
Thermal Overload	Bimetallic strip heated by current	Small to medium motors	Simple, reliable, self-compensating	Fixed trip characteristics
Magnetic Overload	Electromagnetic coil operation	Large motors, precise protection	Adjustable, repeatable	Doesn't track motor temperature
Electronic Overload	Microprocessor-based protection	All motor sizes, advanced features	Highly accurate, multiple functions	Higher cost, requires power
Solid-State Overload	Current sensing with electronic trip	Modern motor control centers	Compact, programmable, diagnostic	Complex troubleshooting

Motor Protection vs. Circuit Protection

Understanding the distinction between different protection types is essential:

Overload Protection:

• Protects against 110-150% sustained overcurrent
• Time-delayed response (10-30 seconds typical)
• Prevents motor overheating
• Sized at 115-125% of motor full load amperage (FLA)

Short Circuit Protection:

• Protects against 500-10,000% instantaneous fault current
• Immediate response (0.01-0.1 seconds)
• Prevents conductor and equipment damage
• Sized at 150-400% of motor FLA (depending on type)

Ground Fault Protection:

• Detects leakage current to ground
• Prevents shock hazards and equipment damage
• Typical settings: 5-30mA for personnel, higher for equipment
• Required by NEC in many applications

Motor Overload Sizing Charts

Standard Motor Full Load Current Values

The foundation of overload relay sizing is accurate motor full load current data:

Three-Phase Motor FLA Chart (230V/460V)

Motor HP	FLA at 230V	FLA at 460V	Overload Range (115%)	Overload Range (125%)
1/2 HP	2.0 A	1.0 A	2.3 A	2.5 A
3/4 HP	2.8 A	1.4 A	3.2 A	3.5 A
1 HP	3.6 A	1.8 A	4.1 A	4.5 A
1.5 HP	5.2 A	2.6 A	6.0 A	6.5 A
2 HP	6.8 A	3.4 A	7.8 A	8.5 A
3 HP	9.6 A	4.8 A	11.0 A	12.0 A
5 HP	15.2 A	7.6 A	17.5 A	19.0 A
7.5 HP	22 A	11 A	25.3 A	27.5 A
10 HP	28 A	14 A	32.2 A	35.0 A
15 HP	42 A	21 A	48.3 A	52.5 A
20 HP	54 A	27 A	62.1 A	67.5 A
25 HP	68 A	34 A	78.2 A	85.0 A
30 HP	80 A	40 A	92.0 A	100.0 A

Single-Phase Motor FLA Chart (115V/230V)

Motor HP	FLA at 115V	FLA at 230V	Overload Range (115%)	Overload Range (125%)
1/6 HP	4.4 A	2.2 A	5.1 A	5.5 A
1/4 HP	5.8 A	2.9 A	6.7 A	7.3 A
1/3 HP	7.2 A	3.6 A	8.3 A	9.0 A
1/2 HP	9.8 A	4.9 A	11.3 A	12.3 A
3/4 HP	13.8 A	6.9 A	15.9 A	17.3 A
1 HP	16 A	8 A	18.4 A	20.0 A
1.5 HP	20 A	10 A	23.0 A	25.0 A
2 HP	24 A	12 A	27.6 A	30.0 A
3 HP	34 A	17 A	39.1 A	42.5 A
5 HP	56 A	28 A	64.4 A	70.0 A

Overload Relay Selection Chart

Standard thermal overload relay ranges for common motor sizes:

Relay Size	Adjustment Range	Suitable for Motor HP	Heater Element
OL-01	0.16 - 0.26 A	1/6 - 1/4 HP (230V)	H01 Series
OL-02	0.24 - 0.40 A	1/4 - 1/3 HP (230V)	H02 Series
OL-03	0.38 - 0.63 A	1/3 - 1/2 HP (230V)	H03 Series
OL-05	0.6 - 1.0 A	1/2 - 3/4 HP (460V)	H05 Series
OL-08	1.0 - 1.6 A	3/4 - 1 HP (460V)	H08 Series
OL-12	1.5 - 2.6 A	1.5 - 2 HP (460V)	H12 Series
OL-20	2.5 - 4.0 A	2 - 3 HP (460V)	H20 Series
OL-32	4.0 - 6.3 A	3 - 5 HP (460V)	H32 Series
OL-50	6.3 - 10 A	5 - 7.5 HP (460V)	H50 Series
OL-80	10 - 16 A	7.5 - 10 HP (460V)	H80 Series

Overload Trip Class Selection

Trip Class	Trip Time at 600% FLA	Typical Applications
Class 10	Maximum 10 seconds	Standard motors, pumps, fans, conveyors
Class 10A	10-20 seconds	Hermetic compressors, some HVAC
Class 20	Maximum 20 seconds	High inertia loads, centrifuges, flywheels
Class 30	Maximum 30 seconds	Extremely high starting loads, crushers

Step-by-Step Overload Relay Sizing

Sizing Process Overview

Follow this systematic approach for accurate overload relay selection:

• Step 1: Obtain motor nameplate data (HP, voltage, FLA)
• Step 2: Calculate overload setting range (115-125% of FLA)
• Step 3: Select overload relay with appropriate adjustment range
• Step 4: Choose trip class based on application
• Step 5: Verify compatibility with motor starter contactor
• Step 6: Set and test overload relay

Example 1: Sizing Overload for 3 HP Motor

Motor Specifications:

• Power: 3 HP
• Voltage: 230V, 3-phase
• Full Load Current: 9.6 A (from motor nameplate)
• Service Factor: 1.15
• Application: Centrifugal pump (standard duty)

Calculation Steps:

Step 1 - Verify Motor FLA: Motor nameplate shows 9.6A at 230V three-phase ✓

Step 2 - Calculate Overload Setting Range:

Minimum setting:

$$\text{OL}_{\text{min}} = \text{FLA} \times 1.15 = 9.6 \times 1.15 = 11.04\text{ A}$$

Maximum setting:

$$\text{OL}_{\text{max}} = \text{FLA} \times 1.25 = 9.6 \times 1.25 = 12.0\text{ A}$$

Overload setting range: 11.0 - 12.0 A

Step 3 - Select Overload Relay: Choose thermal overload relay with adjustment range covering calculated values:

• Relay Model: OL-80 or equivalent
• Adjustment Range: 10-16 A ✓
• Set Point: 11.0 A (recommended for pumps)

Step 4 - Select Trip Class:

• Application: Centrifugal pump (normal starting)
• Trip Class: Class 10 (standard protection)

Step 5 - Verify Starter Compatibility:

• Contactor Size: NEMA Size 1 (suitable for 3 HP at 230V)
• Overload Relay: Compatible with Size 1 contactor ✓

Example 2: Sizing Overload for 5 HP Motor

Motor Specifications:

• Power: 5 HP
• Voltage: 460V, 3-phase
• Full Load Current: 7.6 A
• Service Factor: 1.15
• Application: Air compressor (high starting load)

Calculation Steps:

Step 1 - Motor FLA Verification: Nameplate FLA: 7.6A at 460V ✓

Step 2 - Calculate Overload Range:

$$\text{OL}_{\text{min}} = 7.6 \times 1.15 = 8.74\text{ A}$$ $$\text{OL}_{\text{max}} = 7.6 \times 1.25 = 9.5\text{ A}$$

Overload setting range: 8.7 - 9.5 A

Step 3 - Relay Selection:

• Relay Model: OL-50 series
• Adjustment Range: 6.3-10 A ✓
• Recommended Setting: 9.0 A

Step 4 - Trip Class Selection:

• Application: Air compressor (extended starting)
• Trip Class: Class 20 (allows longer start time)
• Reason: Compressors have high starting loads and may require 15-20 seconds to reach full speed

Step 5 - Starter Verification:

• Contactor: NEMA Size 1 (7.5 HP at 460V rating)
• Compatibility: Confirmed ✓

Example 3: Sizing Overload for 10 HP Motor

Motor Specifications:

• Power: 10 HP
• Voltage: 230V, 3-phase
• Full Load Current: 28 A
• Service Factor: 1.0 (no service factor)
• Application: Conveyor motor (frequent starts)

Calculation Steps:

Step 1 - FLA Verification: Motor nameplate: 28A at 230V ✓

Step 2 - Calculate Overload Settings:

For motors without service factor (SF = 1.0), use tighter range:

$$\text{OL}_{\text{min}} = 28 \times 1.15 = 32.2\text{ A}$$ $$\text{OL}_{\text{max}} = 28 \times 1.20 = 33.6\text{ A}$$

Overload setting range: 32 - 34 A

Step 3 - Select Appropriate Relay:

• Relay Type: Electronic overload relay (recommended for frequent starts)
• Model: EOL-125 or equivalent
• Range: 25-40 A ✓
• Setting: 32 A

Step 4 - Trip Class:

• Application: Conveyor (frequent start/stop cycles)
• Trip Class: Class 10
• Special Feature: Electronic relay with trip memory and diagnostic capability

Step 5 - Verify System:

• Contactor: NEMA Size 2 (10 HP at 230V)
• Overload: Electronic type compatible with Size 2 ✓

NEC Requirements for Motor Overload Protection

NEC Article 430: Motor Protection Standards

The National Electrical Code provides specific requirements for motor overload protection:

Section 430.32: Overload Protection

For motors with service factor 1.15 or greater:

• Overload device shall trip at 125% or less of motor nameplate full-load current

For motors with service factor less than 1.15:

• Overload device shall trip at 115% or less of motor nameplate full-load current

For motors with marked temperature rise:

• Motors marked with temperature rise 40°C or less: 125% FLA
• Motors with temperature rise over 40°C: 115% FLA

Section 430.6: Determining Motor Current

Preferred sources for motor current values (in order):

1. Motor nameplate full-load current rating
2. NEC Tables 430.247 through 430.250
3. Manufacturer's data

Special Application Requirements

Automatically Started Motors (430.32(C))

Motors that restart automatically after overload tripping:

• Overload device shall not trip at more than 115% of FLA
• Prevents dangerous automatic restart under fault conditions

Intermittent Duty Motors (430.33)

Motors operating less than 5 minutes at a time:

• May use branch circuit protection only (no separate overload)
• Must be manually started
• Load must be inherently limited

3-Phase Motor Starter with Overload Protection

Complete Starter Components

A properly specified 3-phase motor starter includes:

1. Contactor: Switching device controlled by pilot circuit
2. Overload Relay: Thermal or electronic protection
3. Enclosure: NEMA rated for environment
4. Control Circuit: Start/stop buttons, pilot lights
5. Disconnect Means: Isolates starter for maintenance

Starter Sizing Table

Motor HP	Voltage	NEMA Size	Contactor Rating	Overload Range
1-1.5 HP	230V/460V	00	9A	1.5-2.6 A (460V)
2-3 HP	230V/460V	0	18A	2.5-4.0 A (460V)
5-7.5 HP	230V/460V	1	27A	6.3-10 A (460V)
10-15 HP	230V/460V	2	45A	10-16 A (460V)
20-25 HP	230V/460V	3	90A	25-40 A (460V)
30-40 HP	230V/460V	4	135A	40-63 A (460V)

Wiring Configuration

Three-Phase Motor Starter Wiring:

• L1, L2, L3: Incoming power (3-phase)
• T1, T2, T3: Motor connections
• Overload contacts: Series with coil circuit
• Control circuit: Typically 120V derived from L1-L2

Coordination with Circuit Protection

Proper coordination ensures selective tripping:

Protection Hierarchy:

1. Overload relay trips: 115-125% sustained overcurrent (10-30 sec)
2. Magnetic trip/breaker: 500-1000% overcurrent (1-5 sec)
3. Instantaneous trip: >10,000% short circuit current (<0.1 sec)

Advanced Overload Protection Topics

Electronic vs. Thermal Overload Relays

Thermal Overload Advantages:

• Simple, reliable technology
• Self-powered (no auxiliary supply needed)
• Ambient temperature compensation
• Lower cost for basic applications
• Proven track record

Electronic Overload Advantages:

• Precise trip characteristics
• Adjustable trip curves
• Built-in diagnostics
• Ground fault protection options
• Communication capabilities (Modbus, Profibus)
• Trip memory and event logging
• Better accuracy (±2% vs ±10%)

Current Unbalance Protection

Phase current unbalance damages motors through:

• Negative sequence currents: Create reverse torque
• Increased heating: Temperature rise disproportionate to load
• Reduced efficiency: Power factor degradation

Electronic overload relays with unbalance protection:

• Monitor all three phases independently
• Calculate percentage unbalance
• Trip when unbalance exceeds setpoint (typically 10-20%)

Ground Fault Sensitivity

Modern electronic overload relays include ground fault detection:

Sensitivity Classes:

• Class A (High): 30mA, personnel protection
• Class B (Medium): 50-100mA, equipment protection
• Class C (Low): 1-5A, fire prevention in industrial settings

Temperature Compensation

Overload relays must account for ambient temperature:

Thermal Compensation Methods:

• Inherent: Bimetallic element naturally compensates
• Electronic: Temperature sensor adjusts trip point
• Manual: User adjusts setting for ambient conditions

Typical compensation range: 20°C to 60°C (68°F to 140°F)

Troubleshooting Motor Overload Problems

Common Overload Tripping Causes

1. Improperly Sized Overload Relay

Symptom: Trips during normal operation or won't trip during actual overload

Diagnosis:

• Measure actual motor current with clamp meter
• Compare to overload relay setting
• Check if setting matches motor nameplate FLA

Solution:

• Reset overload to proper value (115-125% FLA)
• Replace relay if adjustment range insufficient
• Verify motor nameplate matches actual motor installed

2. Mechanical Overload

Symptom: Trips consistently under load, motor current exceeds nameplate

Diagnosis:

• Check motor current under various loads
• Inspect driven equipment for binding or excessive friction
• Measure torque requirements
• Check for misalignment

Solution:

• Reduce mechanical load
• Repair or lubricate driven equipment
• Check for proper motor size for application
• Consider uprating motor if load legitimate

3. Single-Phasing Condition

Symptom: Motor hums, draws excessive current, trips on overload

Diagnosis:

• Check voltage on all three phases at motor terminals
• Verify contactor contacts closing properly
• Test for blown fuse or open conductor

Solution:

• Repair open phase (fuse, contactor contact, connection)
• Install phase loss relay for protection
• Verify proper fuse/breaker sizing

4. Voltage Problems

Symptom: Trips during starting or light load operation

Diagnosis:

• Measure supply voltage under starting conditions
• Check for voltage drop exceeding 5% during start
• Calculate voltage drop in feeder cables

Solution:

• Increase conductor size if voltage drop excessive
• Install soft starter or VFD to reduce starting current
• Correct utility voltage if supply problem
• Check connections for high resistance

5. Ambient Temperature Issues

Symptom: Trips in hot weather or near heat sources

Diagnosis:

• Measure actual ambient temperature at motor
• Check if exceeds motor rating (typically 40°C/104°F)
• Verify overload relay ambient compensation

Solution:

• Improve ventilation around motor
• Install cooling fans if necessary
• Use higher ambient-rated motor
• Relocate motor away from heat sources

Overload Relay Testing Procedures

Initial Setup Test:

1. Visual Inspection:

• Check all connections tight
• Verify proper heater elements installed
• Confirm trip class setting correct

2. Current Test:

• Start motor and measure actual current each phase
• Compare to motor nameplate FLA
• Adjust overload setting if needed

3. Trip Test:

• Temporarily increase overload setting
• Apply overload condition
• Verify relay trips in expected time
• Reset to proper setting

Periodic Maintenance:

Monthly:

• Test trip operation by pressing test button
• Verify reset mechanism functions properly

Annually:

• Verify trip calibration with current injection
• Clean contacts and check for pitting
• Test under simulated overload conditions
• Document trip times and current levels

Selection Guidelines by Application

Pumps and Fans

Characteristics:

• Generally easy starting
• Moderate to low inertia
• Steady-state operation

Overload Selection:

• Trip Class: 10
• Sizing: 115-120% FLA
• Type: Thermal or electronic

Compressors

Characteristics:

• High starting torque
• Possible liquid slugging
• Cyclic operation

Overload Selection:

• Trip Class: 20 (or 10A for hermetic)
• Sizing: 120-125% FLA
• Type: Electronic with unbalance protection preferred

Conveyors and Material Handling

Characteristics:

• Frequent starts and stops
• Variable load conditions
• Potential jam conditions

Overload Selection:

• Trip Class: 10
• Sizing: 115% FLA for better responsiveness
• Type: Electronic with trip memory
• Additional: Consider jam detection feature

HVAC Equipment

Characteristics:

• Seasonal variation in load
• Wide ambient temperature range
• Long running times

Overload Selection:

• Trip Class: 10 or 10A (hermetic compressors)
• Sizing: 125% FLA
• Type: Electronic with ambient compensation

Machine Tools

Characteristics:

• Intermittent duty
• Variable speed (often VFD controlled)
• Precision requirements

Overload Selection:

• Trip Class: 10
• Sizing: 115% FLA
• Type: Electronic with VFD compatibility
• Additional: Integrated motor protection if using VFD

Cost-Benefit Analysis

Thermal vs. Electronic Overload Comparison

Factor	Thermal Overload	Electronic Overload
Initial Cost	$50-150	$150-500
Accuracy	±10%	±2%
Features	Basic protection only	Advanced diagnostics, communication
Maintenance	Replace heater elements	Minimal, software updates
Lifespan	10-15 years	15-20 years
Energy Savings	None	Optimized settings reduce downtime
ROI Period	N/A	2-4 years in critical applications

When to Upgrade to Electronic Protection

Consider electronic overload relays for:

• Motors larger than 25 HP
• Critical process equipment
• Frequent nuisance tripping issues
• Systems requiring remote monitoring
• Applications with variable loads
• Installations requiring detailed diagnostics

Conclusion: Ensuring Proper Motor Protection

Proper motor overload relay sizing and selection is fundamental to motor longevity, operational reliability, and electrical safety. By following the systematic approach outlined in this guide—accurately determining motor FLA, applying correct sizing factors, selecting appropriate trip classes, and verifying installation—you ensure motors receive optimal protection throughout their service life.

Key Takeaways:

1. Always size overload relays to motor nameplate FLA (not NEC table values)
2. Apply 115-125% sizing factor based on motor service factor
3. Use Class 10 for 90% of applications; Class 20/30 only when specified
4. Verify overload relay adjustment range covers calculated setting
5. Consider electronic overload relays for critical or large motors
6. Test and document overload settings during commissioning
7. Maintain regular testing schedule to ensure continued protection

Whether sizing overload protection for a 3 HP pump motor, 5 HP compressor, or 10 HP conveyor, these principles ensure reliable operation and prevent costly motor failures. For complex applications or motors exceeding 50 HP, consult with motor protection specialists to optimize protection schemes.

Ready to implement comprehensive motor protection? Explore our related guides on motor control basics, three-phase power calculations, and VFD applications.

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Helpful Calculators

• Ohm's Law Calculator
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Credits

• Photo by Griffin Wooldridge on Unsplash

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