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A contactor is sized by what it switches, not by the current on its label, and IEC 60947-4-1 encodes that in the utilisation category. AC-1 is a resistive or lightly inductive load such as a heater bank. AC-3 is a squirrel-cage motor started and switched off while running, where the contacts see several times full load current on every start. AC-4 is inching, plugging and reversing, where they see it repeatedly and the same device is rated far lower. A contactor bought on its AC-1 figure and used on an AC-4 duty welds its contacts, and the failure looks like a bad batch rather than a specification error. Two more items complete the enquiry: the coil voltage, which is a separate variable from the switched voltage and is the most common wrong-item return on this line, and the overload relay trip class, where class 10 suits ordinary starts and class 20 or 30 exists for motors with a long run-up.
The more of this you send, the closer the first quotation lands.
| Utilisation category | AC-1, AC-3 or AC-4 — decided by the duty, not the current |
|---|---|
| Load | Motor kW, heater kW, or the switched circuit |
| Switched voltage and frequency | System voltage at 60 Hz |
| Coil voltage | AC or DC, and the control voltage — this is a separate variable |
| Auxiliary contacts | Number of normally open and normally closed contacts needed |
| Overload relay | Thermal or electronic, current range and trip class 10, 20 or 30 |
| Starting method | Direct on line, star-delta, or reversing pair with interlock |
| Switching frequency | Operations per hour — drives the electrical life |
| Relay type | For control relays: plug-in, interface, timer or safety relay |
| Mounting | DIN rail or panel, and the space available in the board |
| Ambient derating | State the panel internal temperature, not the room |
| Quantity | Units per rating, with coils and overloads matched |
Select one and it travels with your enquiry. Not sure? Leave it and describe the application instead.
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