Enter motor and relay details
Use motor nameplate information whenever it is available. Fields marked optional improve the review but are not always required.
Formula used
Three-phase current estimate: I = P × 1000 ÷ (√3 × V × PF × η)
Single-phase current estimate: I = P × 1000 ÷ (V × PF × η)
Thermal load index: Thermal Index = (Measured Current ÷ Selected FLA)² × 100
Phase-current imbalance: Imbalance = Maximum Deviation ÷ Average Current × 100
The current estimate converts horsepower to kilowatts when needed. The nameplate current takes priority because it reflects the actual motor rating. The upper trip reference uses 125% for service factor 1.15 or higher, otherwise 115%. Confirm the relay’s own dial convention before applying a value.
How to use this calculator
- Read the motor nameplate and enter its supply voltage, phase, full-load current, service factor, and efficiency.
- Enter motor power only as a cross-check or when full-load current is unavailable.
- Select the relay’s setting method from the manufacturer documentation.
- Enter the relay adjustment range and check that the proposed setting fits inside it.
- Record measured running currents and all three phase currents during stable operation.
- Select a trip class after reviewing actual acceleration time, starting frequency, and motor thermal limits.
- Have a qualified electrical professional verify protection coordination before commissioning.
Reliable Motor Overload Planning
Thermal overload protection helps prevent motor winding damage during sustained overcurrent. It trips when heat exceeds its limit. This differs from short circuit protection. Fuses and circuit breakers respond to fault current. An overload relay responds to longer running overloads.
Start with the motor nameplate. The full load current is the main relay reference. Use a calculated current only when nameplate data is unavailable. Three phase calculations need voltage, power factor, and efficiency. Single phase calculations use a different current relationship. It also lets a verified nameplate current override the estimate.
Relay Setting Decisions
The selected overload setting must match the relay design. Many modern relays are programmed with nameplate full load current. Some systems use a direct trip-current approach. The relay instructions decide which value belongs on the adjustment dial. Service factor can affect the allowable upper trip level. It does not replace the relay manual. This supports reliable protection planning.
Ambient conditions matter for non-compensated thermal devices. A hot enclosure can change the relay response. A cold location can change it too. Enter an approved ambient correction factor only when the relay manufacturer supplies one. Do not invent a correction. Electronic devices may provide different thermal models. Follow the product documentation for those products.
Starting and Process Conditions
Trip class should suit the actual acceleration time. A short starting load may suit Class 10. A higher-inertia conveyor, fan, or pump can need a slower class. Choosing a slower class without checking motor heating can cause damage. Choosing a faster class can create nuisance trips. Record the starting current, run-up time, and restart frequency before selecting a class.
Running current also needs review. A motor operating near its nameplate current has little thermal margin. The thermal load index uses the current-squared heating relationship. Voltage imbalance, phase loss, poor bearings, blocked airflow, and driven-load faults can all raise temperature. Investigate the cause instead of simply increasing the setting.
Installation Checks
Confirm that the relay range contains the intended setting. Check the wiring arrangement, phase-loss capability, and reset method. Verify contactor coordination and conductor ratings separately. Confirm short-circuit protection with the applicable electrical rules. Test the control circuit after installation. Document the final dial setting and test results. A qualified electrical professional should approve every final selection.
Frequently asked questions
1. What does a thermal overload relay protect?
It protects a motor from sustained overcurrent that can overheat windings. It is not a substitute for short-circuit or ground-fault protection. A complete motor branch circuit normally requires coordinated protective devices.
2. Should I use calculated current or nameplate current?
Use the motor nameplate full-load current whenever it is available. A calculated current is an estimate. It is useful for planning and for checking data, but it may not match the actual nameplate rating.
3. Why does the calculator ask for service factor?
Service factor affects the conventional upper overload reference used here. It does not automatically authorize a higher dial setting. The relay instructions, motor data, project rules, and local electrical requirements remain controlling.
4. What is thermal trip class?
Trip class describes how quickly an overload device responds during a severe overload. Class selection should match the motor acceleration duty. Longer starts may need a different class, but motor thermal capability must be verified.
5. Can I increase the setting to stop nuisance trips?
Do not raise the setting without finding the cause. Check load, voltage, phase current, ventilation, bearings, starting time, and relay selection. A higher setting can hide a real problem and increase motor damage risk.
6. What does the ambient correction factor do?
It applies a manufacturer-approved correction for the relay environment. Use 1.00 when no approved factor exists. Non-compensated thermal relays can react differently in hot or cold enclosures.
7. Why check the relay adjustment range?
The relay must be adjustable to the intended setting. A calculated setting outside its range indicates that another relay range, heater, or protection approach may be required.
8. What is the thermal load index?
It is a planning indicator based on the current-squared heating relationship. It shows that a modest current increase can create a larger heating increase. It is not a motor-temperature measurement.
9. Does this calculator select cable size?
No. Cable ampacity, voltage drop, installation method, ambient temperature, and protective-device coordination need separate calculations. Review them under the applicable electrical rules and project specifications.
10. Can this tool be used with a variable-frequency drive?
Use caution. Drives can include electronic motor protection and model-specific parameter settings. Check the drive, motor, and relay documentation. Do not assume that a line-side relay setting applies unchanged to a drive-fed motor.
11. Who should approve the final setting?
A qualified electrical professional should verify the final setting. The review should include motor data, relay instructions, coordination, installation conditions, commissioning test results, and the applicable electrical code.