Thermal Overload Relay Setting Guide: How to Set It Correctly

Correct motor protection starts with proper thermal overload relay setting. If the relay is adjusted too low, it may trip during normal operation. If set too high, the motor may overheat before protection activates. This makes thermal overload relay setting one of the most important steps in motor control panel setup.

A thermal overload relay protects motors from overheating caused by overload current, phase imbalance, and prolonged starting conditions. It works with motor starters and contactors to disconnect power before damage happens.

Whether you are working on a small pump motor, industrial conveyor, or HVAC system, understanding the right thermal overload relay setting helps improve motor life, system reliability, and safety compliance.

In this guide, you will learn how thermal overload relays work, how to adjust them correctly, and how to troubleshoot common trip issues.

What Is a Thermal Overload Relay?

A thermal overload relay is a motor protection device designed to protect electric motors against excessive current over time. It operates based on heat generated by current flow.

Unlike electronic overload relays, thermal overload relays use mechanical parts and bimetal strips to sense overload conditions. This makes them simple, cost-effective, and widely used in industrial motor starters.

The relay is installed in series with the motor circuit. When the motor draws more current than its rated full load current for a certain period, the relay trips and disconnects the contactor.

Know more about motor overload protection chart

Main Functions of a Thermal Overload Relay

Thermal overload relays are common in:

• Motor control centers (MCC)
• Pump systems
• Compressors
• Conveyor belts
• Air handling units

Their simple design makes thermal overload relay setting easy for technicians and electricians.

How Thermal Overload Relays Work

The working principle is based on heat expansion.

Inside the relay, each phase current passes through a heater element connected to a bimetallic strip. When motor current increases, the heat generated rises.

The bimetal strip bends because it is made of two metals with different expansion rates. If the heat reaches the trip point, the strip activates the trip mechanism.

Working Sequence

1. Motor starts and draws normal current.
2. Current passes through heater elements.
3. Heat builds in the bimetal strip.
4. Excess current increases heat.
5. Strip bends and trips the contactor.

Thermal Relay Response Table

This delayed response allows motors to handle inrush current during startup.

That is why thermal overload relay setting must match motor characteristics.

Use our online tool motor cable size calculator

How to Set a Thermal Overload Relay

This is the most important part of thermal overload relay setting. A correct setting depends mainly on the motor full load amperes (FLA), service factor, and operating environment.

Step 1: Find the Motor Nameplate Current

Look at the motor nameplate and identify:

• Rated voltage
• Full load current (FLC)
• Service factor
• Power rating

Example:

The FLA is the base for thermal overload relay setting.

Find the exact setting using our Motor Overload Setting Table which includes 3 Phase Chart & Complete Guide

Step 2: Locate the Relay Adjustment Dial

Most thermal overload relays have an adjustable dial marked in amps.

Example:

Relay range = 24A – 32A

If your motor FLA is 28A, this relay is suitable.

Always choose a relay range that includes the motor’s rated current.

Step 3: Apply the Correct Setting Formula

Use this standard rule:

For motors with service factor 1.15 or more:

Overload Setting = FLA × 125%

For motors with service factor less than 1.15:

Overload Setting = FLA × 115%

Example calculation:

Motor FLA = 28A

Service Factor = 1.15

Setting = 28 × 1.25 = 35A

If the relay maximum is below 35A, select the nearest suitable relay.

Use our Motor Current Calculator

Quick Setting Table

Step 4: Test the Motor

After thermal overload relay setting:

• Start the motor
• Measure running current
• Check startup duration
• Monitor temperature

If the motor trips under normal load, fine-tune slightly.

Best Practices

• Never set below motor FLA
• Avoid oversizing relay range
• Verify actual load current
• Check manufacturer data

Proper thermal overload relay setting improves protection and avoids nuisance trips.

Use our online tool Motor Overload Setting Calculator

Thermal Overload Relay Setting for 3-Phase Motors

For three-phase motors, thermal overload relay setting follows the same principle, but phase balance becomes critical.

Each phase must carry nearly equal current. A difference greater than 5% may indicate imbalance.

3-Phase Setting Checklist

For quick motor sizing and overload values, refer to the main hub guide’s overload relay setting table.

In 3-phase motors, single phasing can cause one phase current to rise sharply. A properly adjusted thermal relay can detect this condition.

This makes thermal overload relay setting more critical in industrial three-phase systems.

Adjusting for Ambient Temperature

Ambient temperature directly affects thermal relay performance.

Thermal relays respond to heat. If the surrounding panel temperature is already high, the relay may trip earlier.

This is common in:

• Outdoor panels
• Boiler rooms
• Hot climates
• Crowded control cabinets

Ambient Temperature Effect Table

Know more about sizing motor overload protection

Two Relay Types

Ambient-Compensated Relays

These adjust internally for surrounding temperature changes.

Best for variable environments.

Non-Compensated Relays

These are more affected by room temperature.

Manual thermal overload relay setting adjustments may be needed.

Adjustment Tip

If the panel temperature is above 40°C, monitor motor current carefully before increasing the setting.

Never increase blindly.

Find all about How to Size Motor Overload Protection Step-by-Step Guide

Common Thermal Overload Relay Trip Issues

Improper thermal overload relay setting can cause unwanted trips or no trip at all.

1. Nuisance Tripping

This happens when the relay trips too often.

Common causes:

• Setting too low
• High ambient temperature
• Long motor starting time
• Voltage imbalance

Solution:

Check actual running current and adjust properly.

2. Relay Fails to Trip

This is dangerous.

Causes:

• Setting too high
• Wrong relay size
• Mechanical fault

Solution:

Inspect and recalibrate.

3. Single Phasing

Loss of one phase increases current in remaining phases.

Symptoms:

Solution:

Check supply fuses and contactor terminals.

4. Frequent Reset Requirement

If the relay keeps resetting:

• Check motor load
• Inspect bearings
• Verify cable size
• Measure voltage drop

These checks often solve thermal overload relay setting issues.

Know more about how to calculate overload relay for motor

Thermal Overload Relay vs. Electronic Overload Relay

Both protect motors, but their working method differs.

Comparison Table

Thermal relays are better for:

• Standard industrial motors
• Budget systems
• Simple motor starters

Electronic relays are better for:

• Critical loads
• Variable speed drives
• Precise protection systems

Thermal overload relay setting is simpler, but electronic relays offer more control.

Find all about overload relay working principle

Frequently Asked Questions

Final Thoughts

A correct thermal overload relay setting is essential for safe motor operation. It protects against overload, overheating, and phase failure while reducing maintenance costs.

Always start with the motor nameplate current. Select the right relay range, adjust based on service factor, and test under real operating conditions.

For three-phase motors, always verify current balance and ambient conditions. Small mistakes in thermal overload relay setting can lead to motor damage or unnecessary shutdowns.

When set correctly, thermal overload relays provide reliable and affordable motor protection for almost any industrial application.

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