Quick technical guide: This article explains thermal overload relay trip class: why class 10, 20 and 30 are not interchangeable for panel builders, contractors, maintenance teams and industrial electrical buyers.
A motor starts, runs normally, and never exceeds its expected running current.
Yet its overload relay trips during startup.
The immediate reaction is often to turn the current dial higher.
That can be the wrong fix.
The problem may not be the current setting at all. It may be the trip class.
Class 10, Class 20 and Class 30 overload relays are designed to tolerate different motor starting characteristics before operating. The correct choice depends on the motor, its starting time and the mechanical load it has to accelerate.
This distinction matters because an overload relay has two jobs that can conflict if it is selected incorrectly: allow normal motor starting, but disconnect a sustained overload before excessive heating damages the motor.
What does a thermal overload relay protect against?
A thermal overload relay is primarily a motor-overload protection device.
In a conventional contactor starter, motor current passes through the overload relay. When an overload persists, the relay operates its control contacts, which can interrupt the contactor coil circuit and stop the motor.
ABB describes its thermal overload relays as electromechanical devices providing motor protection against overload and phase failure. Its thermal designs use bimetal elements that respond to heating caused by motor current.
ABB Group
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That is a different job from short-circuit protection.
A thermal overload relay should not be treated as a replacement for the appropriate fuse, circuit breaker or other short-circuit protective device.
Why doesn't the relay trip immediately when motor current rises?
Because a motor normally draws increased current during acceleration.
If overload protection reacted instantly whenever current exceeded normal running current, many motors could never start.
Thermal overload protection therefore has a time-current characteristic.
As overload current increases, the permitted operating time generally becomes shorter.
That gives a healthy motor time to accelerate while still providing protection against a sustained abnormal load.
The required delay is not identical for every machine.
A lightly loaded pump may accelerate quickly. A high-inertia conveyor, centrifuge or similar load can take substantially longer.
This is where trip class becomes important.
What does "trip class" actually mean?
Trip class describes the overload relay's tripping-time characteristic under defined test conditions.
Under IEC-based terminology, ABB identifies standard thermal-overload classes including 10A, 10, 20 and 30 and states that the class defines maximum tripping time under specified multiples of the relay setting current.
ABB Library
Schneider Electric likewise explains that IEC 60947-4-1 defines different overload-relay classes because motor starting times can range from a few seconds to several tens of seconds depending on the load.
Schneider Electric
The class number is therefore not an ampere rating.
A Class 20 relay is not "twice as powerful" as a Class 10 relay.
It has a different time-current characteristic.
Class 10: typical shorter-start applications
Class 10 overload protection is commonly associated with motors that reach operating speed relatively quickly.
ABB's current thermal overload range provides a useful real-world example: applicable relays use Class 10 protection with adjustable current settings and phase-loss sensitivity.
ABB Group
Schneider describes Class 10A and Class 10 as suitable for normal-duty motor applications in its application guidance.
Schneider Electric
That does not mean every ordinary motor automatically requires Class 10.
The actual starting behaviour still needs to be checked.
Class 20: when the motor needs longer to accelerate
Some machines place greater mechanical demand on the motor during startup.
The motor may legitimately remain at elevated current for longer while accelerating.
If the overload relay's trip characteristic is too fast for that normal start, nuisance tripping can occur even though the motor and machine are healthy.
Class 20 protection provides a longer starting allowance under its defined characteristic.
Schneider's application guidance associates Class 20 with heavier-duty starting conditions.
Schneider Electric
Current products demonstrate that this is not just theoretical. Siemens currently publishes SIRIUS solid-state motor overload relays with Class 20E characteristics for applicable motor-protection applications.
Siemens Mall
The presence of Class 20 does not mean it should replace Class 10 whenever nuisance trips occur.
First establish why the motor is taking longer to start.
Class 30: very long starting conditions
Some applications have particularly long acceleration periods.
Schneider identifies Class 30 as applicable where very long motor starting times require a corresponding overload characteristic.
Schneider Electric
That extra tolerance has an obvious consequence.
The protection is deliberately allowing elevated motor current for longer under the conditions defined by its trip curve.
Class 30 should therefore be selected because the motor application requires it—not because a technician wants the overload relay to "stop tripping."
The protection curve must remain appropriate for the motor's thermal capability and starter design.
Why you should not memorize one "seconds" rule for every standard
A common explanation says:
Class 10 = trips within 10 seconds
Class 20 = within 20 seconds
Class 30 = within 30 seconds
There is some basis for this, but the test basis matters.
Schneider's NEMA overload guidance defines Class 10, 20 and 30 using operation at 600% of the relevant thermal-current rating or dial setting.
Schneider Electric
ABB's IEC-oriented motor-protection documentation instead explains the IEC classes using specified conditions including 7.2 times setting current.
ABB Library
Those descriptions should not be mixed casually.
When specifying equipment, use the trip curves and standard basis documented for the actual relay.
The practical lesson remains the same: trip class describes time-current behaviour, not the motor's normal current rating.
Trip class and current setting are two different decisions
Suppose a motor nameplate shows a rated current of 18 A.
The overload relay must have an adjustment range suitable for the required motor-protection setting.
That is one decision.
Whether the application needs Class 10, Class 20 or another characteristic is another.
Turning an adjustable relay from 18 A to 22 A does not convert Class 10 into Class 20.
It changes the current setting.
That distinction is important during commissioning.
Schneider's application guidance states that the overload relay current setting should be based on the motor's nameplate full-load current, while the trip class is chosen according to motor starting characteristics and calculated starting time.
Schneider Electric
Do not solve a timing problem by casually increasing the current setting.
Why motor nameplate current matters
The motor nameplate provides the rated electrical information for the motor's specified operating conditions.
When selecting an overload relay, the relay's adjustment range needs to cover the required motor current setting.
For example, ABB publishes thermal overload relays with defined adjustable current ranges rather than one relay covering every motor size.
Empower
That is why ordering an overload relay from motor kW alone is weak purchasing practice.
Motor current varies with voltage, motor design and operating conditions.
For replacement work, photograph the motor nameplate and existing relay setting.
A nuisance trip may indicate a mechanical problem
Suppose a motor previously started comfortably with its existing overload protection.
Months later, the relay begins tripping during acceleration.
Changing to a slower trip class without investigation could hide the real fault.
Possible causes include:
Increased mechanical load
Tight or damaged bearings
Jammed conveyor or driven equipment
Supply-voltage problems
Phase imbalance or phase loss
Excessive starting frequency
Incorrect overload setting
Deteriorated motor condition
Measure motor current during startup and normal running using appropriate procedures.
Compare all phases.
Also inspect the machine being driven.
Electrical protection can be the first component to reveal a mechanical problem.
Phase loss is another reason overload relays matter
Three-phase motors can be exposed to abnormal current conditions when one phase is lost.
Many thermal overload relays therefore include phase-loss sensitivity.
ABB's current thermal overload documentation specifically identifies overload and phase-failure protection and lists phase-loss sensitivity among applicable relay features.
ABB Group
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Do not assume every overload relay responds identically to phase loss.
Verify the feature for the exact product.
A separate phase-monitoring relay may still be required where the control system needs phase-sequence, undervoltage, overvoltage or other supply-monitoring functions.
Thermal vs electronic overload relay
Not every overload relay uses bimetal elements.
Electronic or solid-state overload relays measure motor current electronically.
This can provide additional adjustment and monitoring functions depending on the product.
For example, Schneider's Motor Logic solid-state overload range includes phase-loss and unbalance protection and selectable Class 10/20 characteristics on applicable models.
Product Info
Siemens also currently publishes electronic SIRIUS overload relays in defined trip classes, including Class 20E products.
Siemens Mall
Those features belong to specific product families.
"Electronic overload relay" does not automatically mean adjustable trip class, communications, ground-fault detection or any other advanced function.
Check the datasheet.
Manual reset or automatic reset?
Many overload relays offer reset options.
ABB's current thermal overload products include manual or automatic reset selection on applicable models.
ABB Group
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The choice affects machine behaviour after an overload trip.
With manual reset, intervention is required before the control circuit can return to its normal state.
Automatic reset can allow the relay to reset after the required recovery condition is reached.
Whether automatic reset is appropriate depends on the machine and control architecture.
A motor should not unexpectedly restart merely because an overload relay has cooled and reset. The complete control system must determine safe restart behaviour.
The overload relay normally trips the contactor—not the main power directly
In a conventional starter, the overload relay's auxiliary contact is wired into the contactor control circuit.
When the relay trips, that contact changes state and the contactor coil is de-energized.
ABB's thermal-overload documentation identifies both an NC trip contact and an NO signal contact on applicable products.
Empower
This arrangement also allows a separate signal to be used for indication or control logic.
Do not confuse the overload relay's auxiliary-contact rating with the motor current flowing through its main sensing path.
They are separate electrical functions.
What happens after an overload trip?
Do not repeatedly press RESET and START.
The trip is information.
Check the motor current, driven equipment, supply conditions and relay setting.
If the motor trips again after resetting, continuing to restart it can increase thermal stress without addressing the cause.
For an established machine that previously ran normally, a new overload trip deserves investigation before protection settings are changed.
A practical example: conveyor starts tripping
Consider a conveyor motor protected by a Class 10 overload relay.
The machine has operated correctly for two years.
It suddenly begins tripping during startup.
Changing immediately to Class 20 would be difficult to justify.
The original Class 10 protection successfully tolerated the motor's starting characteristic for years.
Something has changed.
The conveyor may be overloaded. A bearing may be deteriorating. Material may be creating excessive resistance. Supply conditions may have changed.
Now consider a different situation.
A newly engineered high-inertia machine has a documented normal acceleration time incompatible with the selected Class 10 characteristic.
Here, reviewing the specified trip class can be entirely appropriate.
The same symptom—tripping during startup—can therefore point to two very different engineering decisions.
Contactor coordination still matters
The overload relay does not exist independently of the starter.
Siemens explicitly notes in current electronic overload-relay instructions that the motor, conductors and contactor need to be selected for the corresponding tripping class.
Siemens Industry Support
Schneider likewise publishes motor-control coordination tables built around defined overload-relay classes and warns that applications requiring another class need the appropriate relay selection.
Schneider Electric
Replacing a Class 10 relay with Class 20 or Class 30 is therefore not merely changing a dial.
Review the complete starter and coordination data.
What to check before ordering an overload relay
A useful specification should include:
Motor nameplate current
Motor supply voltage
Required overload setting range
Required trip class
Starting time
Driven load or machine type
Contactor model
Mounting method
Manual or automatic reset requirement
Required auxiliary contacts
Existing relay model for replacement work
If the required trip class is not known, provide the motor and application data rather than guessing.
For an existing machine, include measured starting time and current if available.
What to send when requesting an overload relay from INTEKS
For replacement work, send clear photographs of the motor nameplate, contactor and existing overload relay—including the current setting and complete model reference.
If the machine is experiencing nuisance trips, describe when the trip occurs: during startup, after several minutes, under heavy production load, or apparently at random.
For new panels, provide the motor current, starter arrangement, starting characteristics and required trip class where established by the electrical design.
INTEKS supplies industrial electrical protection and control components for panel builders, contractors, maintenance teams and industrial users. Motor overload protection should be selected from the motor and machine requirements rather than current range alone.
Frequently asked questions
What is the difference between Class 10 and Class 20 overload relays?
They have different time-current trip characteristics. Class 20 permits a longer high-current starting condition than Class 10 under the relevant defined test conditions, making it suitable for applications whose normal motor acceleration requires that characteristic.
Schneider Electric
Should I increase the overload setting if the relay trips during motor startup?
Not without diagnosing the cause. The current setting and trip class solve different problems. Verify motor nameplate current, actual starting current, acceleration time, supply conditions and mechanical load before changing protection.
Is Class 30 better protection than Class 10?
No. A higher class is not a quality ranking. It provides a different trip characteristic intended for motors with longer starting requirements. The correct class depends on the motor and application.
Does a thermal overload relay protect against a short circuit?
Its primary purpose is motor overload protection, with phase-failure sensitivity available on applicable products. Appropriate short-circuit protection still has to be provided by the starter's protective design.
ABB Group
Can I replace a Class 10 overload relay with Class 20?
Only after confirming that Class 20 is appropriate for the motor, starting characteristics and complete starter coordination. Siemens and Schneider documentation both show that trip class is part of the starter design, not an isolated replacement choice.
Siemens Industry Support
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For project requirements, component identification or a panel enquiry, contact the INTEKS team with the relevant rating, application and existing product details.
For the applicable standard and current edition, consult the official IEC publication search.
