Sizing a Contactor by Category AC-1 or AC-3?
Whether a contactor switches a motor or a purely resistive load decides its frame size. This guide explains the utilization categories AC-1 and AC-3 per IEC 60947-4-1, how to match rated current to motor power and what to watch for when sizing.
View contactorsWhat do utilization categories AC-1 and AC-3 mean?
The utilization category per IEC 60947‑4‑1 describes the conditions under which the main contacts of a contactor make and break. It defines the current the contacts must handle safely during switching and is therefore the basis of every sizing decision.
AC-1 covers purely resistive or slightly inductive loads with a power factor cos φ ≥ 0.95, such as heaters, incandescent lamps or resistance furnaces. Almost no inrush current flows, so the full rated current Ie can be used.
AC-3 covers squirrel-cage motors switched off while running. On start-up the motor draws 5 to 7 times its rated current as inrush, and on breaking nearly full voltage sits across the opening contacts. This stresses the contacts far more heavily.
How to match motor-protective circuit breakers and overload relays to the contactor.
Read the guideHow do you match motor power to rated current?
A contactor carries two figures: the thermal rated current for AC-1 and a lower AC-3 current with an associated motor power in kW. Under inductive load the permissible current drops because the arc is harder to quench when the contacts open.
The same contactor can carry a much higher current in AC-1. A type rated 9 A in AC-3 often handles 20 A or more in AC-1, because resistive loads barely stress the contacts. Use the AC-1 value for heaters and the AC-3 value for motors, without exception.
What matters in selection and accessories?
Beyond category and current, the coil voltage, auxiliary contacts and environment shape the choice. Check the coil voltage (for example 24 V DC or 230 V AC) and the electrical life expressed in operating cycles.
- Choose the coil voltage to match the control system, usually 24 V DC or 230 V AC.
- Add auxiliary contacts (NO/NC) for interlocking, signalling and latching.
- Set the overload relay or motor-protective breaker to the motor rated current.
- For frequent switching, pick one frame size larger to extend service life.
- Ambient temperature above 40 °C reduces the permissible current, apply derating.
Frequently asked questions
How do AC-1 and AC-3 differ?
AC-1 applies to resistive loads with cos φ ≥ 0.95, where almost no inrush current flows. AC-3 applies to motors with high starting current and stresses the contacts more, so the permissible current is lower.
Why does the contactor state a kW value?
The kW value refers to AC-3 and a standard squirrel-cage motor at 400 V. It simplifies selection but does not replace checking the actual motor rated current.
Can I use an AC-3 contactor for a heater?
Yes, resistive loads use the higher AC-1 current of the same contactor. Check the AC-1 value in the datasheet; it is usually well above the AC-3 value.
How do I account for frequent switching?
Frequent switching shortens contact life. Choose one frame size larger or a contactor with a higher rated switching cycle count to build in reserve.
Looking for the right contactor for your load?
We supply power contactors and accessories for AC-1 and AC-3, tested to IEC 60947-4-1, including auxiliary contacts and overload relays.
Standard-tested
All contactors meet IEC 60947-4-1.
Motor-ready
kW and AC-3 values clearly stated in the datasheet.
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Auxiliary contacts and relays from one source.
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