How to size a contactor for a motor using AC-1 and AC-3 utilisation categories, why the AC-3 rating is the one to quote, and how coil voltage and auxiliary contacts enter the order.
Two contactors with the same "25 A" on the box can differ by a factor of two in the load they will switch reliably, because the headline current is normally the AC-1 rating — a purely resistive load. Motors are not resistive loads.
| Category | Typical load | What it means for sizing |
|---|---|---|
| AC-1 | Non-inductive or lightly inductive: heaters, resistive loads, distribution | The number printed largest on the label; roughly the thermal current |
| AC-2 | Slip-ring motors, starting | Rare in modern panels |
| AC-3 | Squirrel-cage motors: starting and switching off while running | The rating to use for a motor — accounts for starting inrush |
| AC-4 | Inching, plugging, reversing, high operating rate | Severe duty; expect a much larger frame or a dedicated motor-rated device |
A motor starting direct-on-line draws roughly 6–8 times its full-load current for a second or so. The contacts must close into that current repeatedly without welding, and must interrupt it when the motor is running, which is where the arc is at its worst. AC-3 exists precisely to express that.
The AC-3 rating falls with voltage class and with the number of operations per hour. A contactor good for 40 A at AC-3 in a 400 V system is not the same as one in a 690 V system, and the datasheet's operations-per-hour column is part of the rating, not a footnote.
The coil is a separate specification. Common choices are 24 V AC or DC (from a control transformer, standard for safety reasons), 110 V, 230 V and 400 V AC. DC coils are quieter and give faster drop-out but draw more current. If your control circuit is 24 V DC from a power supply, order a 24 V DC coil, not a 230 V AC one with a transformer in front of it. Also state the frequency for AC coils: a 50 Hz coil on a 60 Hz supply will hold in but draws less and can chatter, and will not be reliable as a drop-out device.
Auxiliary contact blocks are rated for control current, not motor current, and must never be used to switch the motor. They are for indication, hold-in and interlocking. For reversing duty, specify the mechanical interlock plus an electrical interlock in the control circuit — a mechanical interlock alone can still be defeated by a welded contact.
A contactor does not provide overload protection. You need a thermal overload relay set to the motor's FLC (or a manual motor starter / MPCB), and a short-circuit protective device upstream with a type-coordination table from the contactor manufacturer confirming that the combination has been tested. Choosing the three parts from three brands without checking coordination is how a "correctly sized" starter still fails on a short circuit.
ELEHPD supplies AC contactors, thermal overload relays, auxiliary contact blocks, timers and motor control accessories across the common frame sizes, with coil voltages specified per order.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.