Contactor vs relay: how to choose, and how to size a contactor

A contactor and a relay both close a contact when a coil is energised, and that is where the similarity ends. They are built to different standards, tested against different duties and sized by different methods — and the most consequential difference is that a contactor’s current rating is not one number. It depends on what the contactor is switching, and the same device carries figures that differ substantially.

Three device families, three standards

The vocabulary is confusing because three overlapping products all get called “relay” in conversation. A general-purpose electromechanical relay — the plug-in type on a socket base — is covered by IEC 61810: a signal and small-load device, typically a single break with a modest gap and no arc chute. A contactor relay (control relay, auxiliary contactor) looks like a small contactor and is covered by IEC 60947-5-1, the standard for control-circuit devices; it is the correct device for interlocking and control logic. A contactor proper is covered by IEC 60947-4-1, and that standard — which defines the utilisation categories, the AC-3 endurance requirements and the coil operating limits — is what makes a contactor a contactor rather than a large relay.

Utilisation categories: why there are two current figures

IEC 60947 defines utilisation categories because the difficulty of switching a load depends almost entirely on what it does at the moment of making and of breaking.

AC-1 covers non-inductive or slightly inductive loads — resistance heating being the archetype — with a power factor of at least 0.95. The contactor makes and breaks at essentially rated current, with no large inrush at closing and with current and voltage close to in phase at opening, so the arc extinguishes readily at the natural current zero.

AC-3 covers squirrel-cage motors — starting them, and switching them off while running — and is much harder in both directions. At closing the contacts make onto locked-rotor current, several times rated current, and must not weld under the electrodynamic forces and heating that produces. At opening the current is near rated, but the still-turning motor acts briefly as a generator and the recovery voltage across the parting contacts is severe.

AC-4 is harsher still: plugging and inching, breaking the motor at or near locked-rotor current, repeatedly. Others you will meet are AC-5a and AC-5b for lamp loads, AC-6a for transformers, AC-6b for capacitor banks, and AC-15 and DC-13 for control circuits.

The practical consequence: for the same physical contactor the AC-1 rating is typically substantially higher than the AC-3 rating, because AC-1 never asks it to make onto locked-rotor current. Both figures appear on the datasheet, and reading the AC-1 figure for a motor is how contactors get welded shut. DC categories are a separate matter again — a DC arc has no natural current zero, so a device used on DC must declare a DC category and a DC rating at that voltage.

Sizing a contactor, in order

1. Identify the load, and therefore the category. Motor: AC-3, or AC-4 if it will be plugged or inched. Heater bank: AC-1. Capacitor bank: AC-6b, on a device specifically rated for it. Lamp circuit: AC-5a or AC-5b.
2. Take full-load current from the motor’s nameplate, at the actual supply voltage — not from a kW-to-amps table, which assumes an efficiency and power factor your motor may not have.
3. Select on the rated operational current in that category at that voltage. The datasheet lists I_e per category per voltage.
4. Check the duty cycle — operations per hour, against the electrical endurance curve for the category.
5. Check ambient and mounting. IEC 60947-1 takes -5 °C to +40 °C as standard conditions with a 24-hour mean not exceeding 35 °C; inside a closed panel the air around the device is well above room temperature, and grouping devices side by side derates them further.
6. Choose the coil, and count the auxiliary contacts you need for interlocking, status and seal-in — checking whether they are built in or need an add-on block.
7. Co-ordinate with the short-circuit protection. A contactor is not a short-circuit protective device. IEC 60947-4-1 defines type 1 and type 2 co-ordination, and the valid combinations are published as tested tables, not derived.

The coil is part of the selection

An AC coil draws a large inrush while the armature is open and the magnetic circuit has a large air gap, then settles to a much smaller sealed current once closed. Both figures are on the datasheet in VA, and both matter: the inrush VA sizes whatever switches the coil — a PLC output, a relay contact, a control transformer — and the sealed VA is the continuous burden. A DC coil draws the same current open or closed, set by resistance; larger ones often include an electronic economiser that reduces holding power after pick-up.

IEC 60947-4-1 sets the operating limits: a contactor must close reliably at any control voltage between 85 % and 110 % of its rated control supply voltage, and must drop out somewhere between 20 % and 75 %. That drop-out band is why a long control cable with significant volt-drop, or a supply that sags when several contactors pull in together, produces contactors that chatter or fail to release. The control circuit’s voltage regulation is part of the design.

Mirror contacts and safety functions

Where a contactor forms part of a safety function, the controller must know whether the power contacts really opened. A mirror contact, defined in Annex F of IEC 60947-4-1, is a normally-closed auxiliary contact that cannot be closed at the same time as any normally-open main contact, so a welded main pole holds it open and the monitoring circuit sees the fault. It is a tested property, not an inference from the mechanism: a plain auxiliary contact on the same carrier is not equivalent.

So when is a relay the right answer?

Use a control relay or contactor relay when the job is logic and signalling: interposing between a PLC output and a contactor coil, providing extra contacts, isolating a field circuit, or switching a small resistive load well within its contact rating. Use a contactor when the job is switching power.

The intermediate case worth naming is a motor of a few hundred watts. It may sit within a large relay’s contact rating on paper, but the relay has no AC-3 rating and no arc control, so that paper rating does not describe the duty being asked of it. A small contactor is the correct device.

How to check this on the product page

Contactors and relays on this site carry a specification table under the product image, with the category-dependent ratings listed separately rather than collapsed into one current figure. Look for:

• Rated operational current (Ie) in AC-1 and in AC-3, each against the voltage it applies at.
• Rated operational voltage (Ue) and rated insulation voltage (Ui).
• Number of poles and auxiliary contact configuration.
• Control supply voltage, and coil type, AC or DC.
• Utilisation categories declared, and whether mirror contacts are.
• Standard, which tells you whether this is a contactor, a contactor relay or a general-purpose relay.

Every value is a link that filters the catalogue to everything sharing it, so you can see what is available at a given AC-3 rating or coil voltage. Where a field is absent we do not hold a verified figure — take it from the manufacturer’s datasheet and ask us if you would like it confirmed. Endurance curves and type 1 / type 2 co-ordination tables are published per combination; we can supply the document for a specific pairing. Add your shortlist to a quote request and the sales desk will confirm stock, lead time and price.

A closing note on responsibility

Contactor selection depends on the actual load, the duty cycle, the ambient and enclosure conditions, the control supply, co-ordination with upstream protection and — where a safety function is involved — the relevant functional-safety standard. Those are properties of a specific design and cannot be settled from a catalogue page. Final selection is the responsibility of the designing engineer, working from actual application data and the manufacturer’s published data and co-ordination tables for the specific devices. RMS can assist with selection, supply datasheets and co-ordination tables, and check a proposed combination against them — but that assistance supports competent design, it does not replace it.