Confused between a contactor and a relay? Learn the real differences in current rating, duty and use, plus a sizing checklist for low-voltage panels.
Contactors and relays are both electromagnetically operated switches, and to a buyer glancing at a catalogue they look nearly identical: a coil, a set of contacts, a DIN-rail clip. The confusion is so common that many enquiry emails simply ask for "a relay" when they actually need a contactor, or vice-versa. Getting this wrong does not just waste a purchase order - it can leave a motor unprotected or a control circuit unable to drive the load.
This guide explains, in plain engineering terms, what each device is built for, where the two overlap, and how to specify the right one for a low-voltage panel. The principles below apply to the AC contactors and general-purpose relays most panel builders source for 230 V / 400 V systems.
A relay is a low-signal switching device. Its coil is energised by a small control current (often 12 V, 24 V DC or 24 V, 110 V, 230 V AC), and its contacts switch another circuit on or off. The defining feature is isolation: the control side and the load side are electrically separated, so a fragile PLC output or pushbutton can safely command a higher-power circuit.
General-purpose relays such as the LY, MY and MK series typically handle 5 A to 15 A per contact and are used for signalling, interlocking, alarm loops and logic. They are not intended for repeatedly switching large inductive loads like motors - the contacts would weld or erode quickly.
A contactor is a heavy-duty switching device designed to make and break high currents many times per hour. A 3TF, 3TB, S-N, B-series or LC1-D AC contactor will routinely switch 9 A to several hundred amperes of motor or heater load, and it is built with arc chutes and reinforced contacts to survive the punishment.
Because the loads are inductive and noisy, contactors include features relays lack: blow-out magnets or arc chambers, often a built-in auxiliary contact for feedback, and a mechanical life rated in the millions of operations. They are the device you use to start a pump, a compressor or a conveyor.
| Attribute | Relay | Contactor |
|---|---|---|
| Typical load current | 5-15 A | 9 A - 630 A+ |
| Primary job | Switch signals / isolate circuits | Switch power to motors & heaters |
| Arc suppression | Minimal | Arc chute / blow-out magnet |
| Duty cycle | Intermittent, low count | High, thousands per hour |
| Coil options | Wide (DC & AC low volt) | AC/DC, often 24/110/230 V |
| Cost per pole | Low | Higher (built to last) |
Often yes - but indirectly. A contactor's main contacts are over-specified for signal work, but its auxiliary contact (the small 1NO/1NC block clipped to the side) behaves exactly like a relay contact. Panel designers therefore use a small interface relay to drive the contactor coil from a PLC, combining the relay's gentle input with the contactor's muscle. For pure logic, though, a relay is cheaper and smaller.
| Application | Device | Why |
|---|---|---|
| Start/stop a 7.5 kW pump | Contactor + overload | High current, frequent duty |
| PLC controls a valve | Relay | Low current, isolation |
| Emergency stop feedback | Auxiliary contact on contactor | State signal to controller |
| Indicator / alarm loop | Relay | Small, cheap, many poles |
Not safely for any meaningful motor. Relays are rated for signal loads; a motor's starting current will arc and weld the contacts. Use a contactor (or a contactor with overload) for any motor above a fraction of a horsepower.
A power relay bridges the gap - rated for higher currents (30-60 A) than a signal relay but without the arc chamber of a true contactor. For continuous motor duty a contactor is still the safer choice; a power relay suits resistive heating or infrequent switching.
Match the coil to your control transformer secondary - most panels run 24 V DC or 230 V AC control. Check the contactor's coil label; using the wrong voltage will either fail to pull in or overheat the coil.
Yes for motors. The contactor only switches; motor protection comes from a thermal overload relay or electronic motor protector mounted alongside it. Relays driving small loads usually rely on the upstream breaker.
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.