A three-phase motor starter can pass a cold commissioning start and still trip during repeated production starts. Increasing the overload setting may hide the symptom without resolving the cause: motor current, acceleration time, cooling intervals and relay response all affect the selection. Panel builders need to distinguish a setting problem from a starting-duty problem before changing protection. This guide explains how to use the actual motor data, evaluate trip class and restart behavior, and verify the complete starter combination.
The short answer is to size a thermal overload relay from the motor nameplate current for the intended voltage and connection, then verify that its adjustable range includes that current. Select the trip class against the measured or manufacturer-published acceleration time, check ambient-compensation limits and reset mode, and confirm the complete starter combination with the contactor and upstream short-circuit protective device (SCPD). IEC 60947-4-1 provides the relevant overload-relay trip-class framework, but it does not make one percentage setting or one trip class correct for every motor.
How does motor nameplate current determine relay size?
The first input for overload relay sizing is the rated motor current shown on the nameplate for the actual voltage and connection. A 3 hp rating does not uniquely determine current: voltage, phase, frequency, efficiency, power factor, motor design and connection all matter. A catalog full-load-current table can support preliminary panel design, but it should not silently replace the installed motor’s data.
Use a selection sequence that can be audited:
- Record the nameplate rated current for the intended supply and winding connection.
- Select a relay whose published adjustment band contains that current; do not choose a range merely because the motor horsepower appears in a distributor table.
- Set and commission the device only according to its instructions, the motor data, the starting profile and the destination rules. There is no universal percentage setting that is safe to publish for every installation.
- Compare cold and hot starting behavior with the manufacturer’s time-current curve, including the number of starts per hour and the shortest restart interval.
- Record the final setting, trip class, reset mode and tested starter combination on the panel schedule.
The hidden cost of getting this sequence wrong is rarely the relay price. An overly sensitive selection can stop a sound process during acceleration; an overly permissive selection can leave the motor exposed to sustained heating. Both outcomes create troubleshooting time, return claims and avoidable production interruption.
How does motor starting time determine trip class?
Trip class describes the relay’s time-to-trip behavior under defined test conditions; it is not the same as an ampere setting and is not a statement of breaking capacity. Schneider Electric’s Motor protection functions explains that overload relays must allow temporary starting overload and be selected using nominal motor current and starting time. The exact curve and tolerances remain device-specific. The engineering question is whether the motor can complete a normal start with margin while the relay still limits damaging stalled or prolonged-current conditions.
| Starting evidence | Trip-class direction to investigate | Decision check before approval |
|---|---|---|
| Fast, repeatable acceleration with a light inertial load | A faster class, commonly in the Class 10 family, may fit | Plot the measured start time against the exact relay curve; check hot restart and start frequency |
| Longer acceleration caused by higher inertia | A slower class, such as Class 20, may be considered | Confirm that the motor’s permitted thermal duty supports the longer acceleration; do not solve a mechanical or voltage problem by simply slowing protection |
| Very long, engineered acceleration | Class 30 or a configurable electronic function may be evaluated | Require motor thermal data, start-current data and a manufacturer-approved coordination path |
| Frequent starts, plugging, jogging, reversing or uncertain acceleration | No class should be selected from a generic table alone | Review the duty cycle, hot-state curve, contactor utilization and motor supplier limits |
This table is a decision frame, not a universal prescription. The class number by itself does not reveal the complete trip curve, cold-versus-hot response or phase-failure behavior. Compare the actual device data with the motor’s permitted locked-rotor or acceleration time. If that evidence is unavailable, hold approval rather than compensating with a higher setting.
Why are trip class and current setting different?
The current setting establishes the relay’s reference point for sustained load. Trip class describes how quickly the overload function acts at the standard test multiple defined for that class. Changing one to compensate for the other can create a protection gap: a current setting should not be raised to let a normal long start pass when the correct task is to evaluate starting duty and trip class.
Why can repeated starts trip a relay that passed a cold start?
Start duration and start frequency are separate checks. A motor may accelerate within the selected trip curve on one start, yet repeated starts with short cooling intervals can produce excessive heating. The same Motor protection functions guide distinguishes long-start protection from rapid-cycle lock-out and number-of-starts functions. Selecting a slower trip class does not establish an acceptable restart frequency.
For commissioning, record whether each start was cold or followed recent operation, its current and acceleration time, and the interval since the previous stop. Compare that sequence with the motor’s permitted duty and the relay’s cold/hot curves. For electronic relays, request the thermal-memory and reset documentation, including behavior after control-power loss; do not assume a particular model retains a thermal estimate or provides restart lock-out. A successful reset is not evidence that the motor has cooled sufficiently for another start.
How do ambient temperature, phase loss and reset mode affect selection?
Thermal response depends on more than motor current. A bimetal relay located beside heat-producing devices in a closed enclosure may experience a different ambient temperature from the motor. Check the selected device’s ambient-compensation range and installation instructions rather than treating compensation as permission to ignore enclosure temperature, spacing or derating.
| Selection dimension | What to verify | Why it changes the decision |
|---|---|---|
| Ambient conditions | Minimum and maximum panel temperature, enclosure ventilation, adjacent heat sources and manufacturer compensation range | Motor and relay may be at different temperatures; nuisance trips or delayed response can result if the published limits are exceeded |
| Three-phase sensing | All required poles are routed as instructed and the selected relay’s phase-failure or unbalance response is documented | A basic overload element is not automatically a full phase-monitoring relay; sensitivity and operating logic vary |
| Reset mode | Manual or automatic reset, local access, remote indication and restart interlocks | Neither reset mode replaces control logic that prevents unexpected re-energization |
| Mounting and conductor path | Direct mounting or separate mounting, conductor size, terminal torque and orientation | Heat transfer, mechanical fit and connection reliability depend on the manufacturer’s installation conditions |
| Starting method | Direct-on-line, reversing, star-delta, soft starter or drive-fed motor | The relay location and sensed current can differ; a conventional line-side selection rule may not transfer unchanged |
Phase loss deserves a clear boundary. Some three-pole thermal overloads incorporate differential or phase-failure sensitivity, but that feature and its response must be confirmed in the datasheet. Voltage unbalance, current unbalance, phase sequence and open-phase detection may require separate monitoring depending on the process and destination rules. Likewise, overload protection does not diagnose bearing damage, blocked ventilation or every high-temperature condition unless the chosen protection system has suitable sensors.
Does manual reset prevent the motor from restarting automatically?
Not by itself. Manual reset requires an operator to restore the relay after a trip, but the contactor may energize when the trip contact closes if a maintained run command is still present. Review the control circuit and PLC restart behavior so that restoring protection does not itself become a start command. The machine’s restart interlock and risk assessment must address this separately from the relay’s reset selector.
How should the relay, contactor and short-circuit device be coordinated?
A contactor makes and breaks the motor circuit during normal control operations. The overload relay detects sustained overcurrent and normally opens an auxiliary contact in the coil circuit. The upstream fuse or circuit breaker interrupts short-circuit current. These are three distinct jobs in motor starter protection; the overload relay is not the branch short-circuit protective device.
Do not copy an upstream breaker’s ampere rating or adjustment percentage onto a separate motor overload relay. Schneider Electric’s Fundamental characteristics of a circuit-breaker distinguishes rated current, overload setting and short-circuit setting within a circuit breaker. Those parameters do not replace the motor nameplate current or the separate relay’s instructions.
| Component | Primary function | Evidence needed for the combination |
|---|---|---|
| Contactor | Switch the motor according to the applicable utilization category and duty | Rated operational current, motor duty, coil voltage, operating frequency and auxiliary-contact requirements |
| Overload relay | Respond to sustained overload according to its setting and trip curve | Adjustment range, trip class, ambient limits, phase-loss behavior, reset mode and mounting compatibility |
| Fuse or circuit breaker | Interrupt short-circuit current within its ratings | Available fault current, device rating and a manufacturer-published coordination table for the exact combination |
IEC 60947-4-1 uses coordination concepts for starters and their SCPDs. Type 1 and Type 2 coordination describe the condition of the starter after a specified short-circuit test; they are outcomes for a tested combination, not labels that can be inferred from component ampere ratings. A panel builder should obtain the manufacturer’s coordination evidence for the exact contactor, overload relay and SCPD at the relevant voltage and prospective short-circuit current. Do not claim a coordination type by mixing independently suitable components.
The same discipline applies to mechanical and electrical fit. Confirm that the relay is approved for direct mounting to the selected AC contactor, that its terminals and bus geometry match, and that the normally closed trip contact is correctly incorporated into the control circuit. The related contactor-versus-control-relay guide explains why a control relay cannot replace the power-switching contactor.
Which standards and selection documents should buyers verify?
IEC 60947-4-1:2023, Low-voltage switchgear and controlgear—Contactors and motor-starters is a product-standard reference, not proof that an unspecified relay or assembled panel is certified. Request the applicable edition and reports for the supplied equipment rather than relying on a standard number in a quotation.
Applicability depends on the destination market, system voltage, installation rules, intended use and claims placed on the panel. IEC coordination evidence does not by itself establish compliance with US or other locally adopted installation and panel requirements. Buyers should verify that the model, ratings and tested combination match the supplied equipment. Unsupported certification or coordination claims can cause approval delays, rejected submittals and liability exposure.
What motor and starter data should accompany a selection request?
| Selection input | Information to provide | Document to request |
|---|---|---|
| Motor identity | Photo or transcription of rated voltage, rated current, frequency, phase, connection, power, duty and service factor when shown | Motor datasheet and permitted start/locked-rotor information |
| Starting profile | Starting method, measured or predicted acceleration time, starts per hour, hot-restart interval and load inertia | Time-current curves for the proposed relay and trip class; documented restart restrictions |
| Installation | Panel ambient range, enclosure arrangement, altitude where relevant, mounting method and conductor details | Installation, ambient-compensation and derating instructions |
| Control behavior | Manual or automatic reset, remote indication, PLC inputs and restart policy | Auxiliary-contact diagram, reset instructions and control-circuit restart review |
| Starter combination | Contactor model and rating, supply voltage, utilization duty, SCPD model/rating and available fault current | Coordination table or test evidence for the exact combination and required destination |
For a three-phase starter selection, the CQR2 Series thermal overload relay is a CHAC product-family starting point. Its suitability and contactor compatibility still require the exact model documentation, motor data, project engineering and destination-market verification.
What else should panel builders check about overload relays?
How do I calculate the size of a thermal overload relay?
Start with the motor nameplate rated current for the actual voltage and connection, then choose a relay adjustment range that includes that current. Verify trip class, starting frequency, ambient limits, phase-loss behavior and reset mode using the exact device instructions and destination rules. Finally, confirm the relay/contactor/SCPD combination with published coordination evidence; do not apply one universal percentage.
What is trip class on an overload relay?
Trip class identifies time-to-trip performance under defined test conditions, helping engineers compare the relay response with motor acceleration and stall withstand. It does not state short-circuit breaking capacity, and the full manufacturer curve, including cold and hot behavior, must be checked.
What is the overload protection size for a 3 hp motor?
Horsepower alone is insufficient to give a safe setting because a 3 hp motor’s rated current varies with voltage, phase, design, efficiency and connection. Use the nameplate current and governing installation rules; treat generic motor-current tables only as preliminary design aids until the actual motor data is available.
Can an overload relay replace a circuit breaker or fuse?
No. A conventional overload relay is intended for sustained overload response and relies on an appropriately selected upstream SCPD to interrupt short-circuit current. The complete starter combination must be coordinated for the available fault current.
Should the overload relay use manual or automatic reset?
Choose reset mode from the machine risk assessment, operating procedure and equipment instructions. Neither mode is a restart interlock: after a manual reset, a maintained run command may still energize the contactor. The control system must prevent unintended restart, and automatic reset must be permitted by the equipment instructions and destination rules.
Can I restart the motor as soon as the overload relay resets?
Not on that fact alone. Reset availability does not establish the motor’s thermal condition or permitted restart interval. Identify the reason for the trip, verify the motor’s restart limits and any documented relay lock-out or thermal-memory behavior, then follow the machine’s restart procedure.
Which sources support thermal overload relay selection?
- IEC 60947-4-1:2023, Low-voltage switchgear and controlgear—Contactors and motor-starters
- Motor protection functions, Schneider Electric Electrical Installation Guide, last edited 5 August 2026.
- Fundamental characteristics of a circuit-breaker, Schneider Electric Electrical Installation Guide, last edited 5 August 2026.
The selection principle is to protect the motor you actually have through its full operating cycle, not just one successful cold start. Establish nameplate current and winding connection first, then compare acceleration, repeated-start duty and cooling intervals with the motor and relay documentation. Verify the control circuit’s restart behavior separately from reset mode, and obtain coordination evidence for the complete starter at the intended voltage and fault level. Neither a convenient ampere range nor an IEC standard reference closes those gaps. CHAC Electric’s product families provide a starting point for component review; the final selection remains tied to the documented motor, application and destination requirements.
Send the nameplate, starting profile, panel ambient range, reset requirement, contactor details and upstream SCPD information through CHAC Electric’s contact page to request a documented selection review.
