Contactors & Overload Relays

Selection Tables, Typical Applications, Coil Voltages

Contactors and thermal relays are essential elements in any industrial installation. They provide control, protection and automation of motors, heaters and industrial loads. This page provides practical tables for selecting contactors according to motor power, as well as typical settings of thermal relays.


1. Contactor Selection for Motors (AC‑3 Duty)

Contactors in AC‑3 mode are designed to control asynchronous motors — switching on under load and switching off at rated current.

Contactor Selection for Motors (AC‑3 Duty)
Motor Power (kW) Rated Current (A) Typical Contactor Size
0.75 2 6–9 A
1.1 3 6–9 A
1.5 4 6–9 A
2.2 5 9 A
3 6 9–12 A
4 8 12 A
5.5 11 12–18 A
7.5 15 18 A
11 21 25 A
15 29 32 A
18.5 37 40 A
22 42 50 A
30 58 65 A

2. Thermal Overload Relay Settings

Thermal relays protect the motor from overload. The setting must correspond to the rated current of the motor.

Thermal Overload Relay Settings
Motor Power (kW) Motor Current (A) Overload Relay Setting
0.75 2 1.6–2.5 A
1.1 3 2.5–4 A
1.5 4 3–5 A
2.2 5 4–6 A
3 6 5–7 A
4 8 6–9 A
5.5 11 9–13 A
7.5 15 12–18 A
11 21 18–25 A
15 29 24–32 A
18.5 37 30–40 A
22 42 36–45 A
30 58 48–63 A

3. Common Coil Voltages

Contactor Coil Voltages
Voltage Usage
24 V DC Control panels, PLC outputs
24 V AC Older control systems
110 V AC Industrial machines (US/Asia)
230 V AC EU industrial and building automation
400 V AC Direct line control (rare today)

4. Practical Notes

• The contactor must be selected according to its AC‑3 current rating, not the mechanical frame size.

• The thermal overload relay must be adjusted to the motor’s nominal current, not the cable rating.

• For heavy‑duty starts (pumps, compressors, conveyors), choose a larger contactor size to handle higher inrush currents.

• When using frequency inverters, contactors should be rated for AC‑1 duty, unless switching on the output side is required.

• The coil voltage must always match the control circuit voltage to ensure reliable operation.