What is the formula for Contactor / Thermal Overload Relay Selection?

Core formula: I_KM ≥ I_e | I_FR ≈ I_e | trip class 10 / 20 / 30 by starting time. The tool returns results instantly using this formula and the selected parameters.

Contactor / Thermal Overload Relay Selection

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Select an AC-3 contactor and thermal-relay setting range from motor rated current, and recommend the trip class from starting time.

Key points

Contactor / Thermal Overload Relay Selection: Select an AC-3 contactor and thermal-relay setting range from motor rated current, and recommend the trip class from starting time. Formula: I_KM ≥ I_e | I_FR ≈ I_e | trip class 10 / 20 / 30 by starting time. Enter parameters on the DD-CAT IDE online tool page to get results instantly, with a calculation-model diagram that updates with the inputs. Select contactor current for AC-3 duty; for frequent jogging / heavy starting, upsize per the manufacturer derating curve. Do not rely only on motor nameplate current. Set the thermal relay to the motor nameplate FLA / rated current, not the contactor rating; use class 20 / 30 for long starts.

Inputs

I_KM ≥ I_e | I_FR ≈ I_e | trip class 10 / 20 / 30 by starting time

Calculation model (updates with parameters)

Motor rated current I_e	25 A	热继电器整定基准
Recommended AC-3 contactor rating	32 A	所needs ≥ 26.32 A（含Temperature降额）
Contactor current margin	1.28 ×	recommend ≥1.1×
Thermal relay setting I_set	25 A	按motor铭牌 FLA 整定，不按接触器额定Value
Recommended thermal relay range	17.0 ~ 25.0 A	选择包含 I_set 的可调范围
Recommended trip class	10 级	Starting time 8.0 s：10 级 ≤10s，20 级 ≤20s，30 级 >20s
Starting current I_st	150 A	I_e × 启动×数
Starts-per-hour check	正常 ✓	6 次/h，建议 ≤30 次/h

Engineering notes

Select contactor current for AC-3 duty; for frequent jogging / heavy starting, upsize per the manufacturer derating curve. Do not rely only on motor nameplate current.
Set the thermal relay to the motor nameplate FLA / rated current, not the contactor rating; use class 20 / 30 for long starts.
The thermal relay protects against motor overload, not short circuit; the breaker / fuse provides short-circuit protection and the two must coordinate.

FAQ

What is Contactor / Thermal Overload Relay Selection for?: Select an AC-3 contactor and thermal-relay setting range from motor rated current, and recommend the trip class from starting time.
What is the formula for Contactor / Thermal Overload Relay Selection?: Core formula: I_KM ≥ I_e | I_FR ≈ I_e | trip class 10 / 20 / 30 by starting time. The tool returns results instantly using this formula and the selected parameters.
What engineering notes should I keep in mind when using Contactor / Thermal Overload Relay Selection?: Select contactor current for AC-3 duty; for frequent jogging / heavy starting, upsize per the manufacturer derating curve. Do not rely only on motor nameplate current. Set the thermal relay to the motor nameplate FLA / rated current, not the contactor rating; use class 20 / 30 for long starts. The thermal relay protects against motor overload, not short circuit; the breaker / fuse provides short-circuit protection and the two must coordinate.