How to Wire a Plug-in Relay and Socket

How do you wire a plug-in relay and socket? A plug-in relay wires in two separate circuits — a low-power coil circuit from the PLC or control device, and a load circuit through the change-over contacts to the field device — each landing on its own numbered terminal pair on the DIN-rail socket, with polarity, diode orientation and terminal torque all affecting the connection's long-term performance. Get the coil polarity or the freewheel diode backwards and a DC relay driven from a PLC transistor output can take the output card down with it. This guide covers terminal numbering, coil-circuit wiring, contact-circuit wiring, screw vs push-in technique, PLC sourcing/sinking wiring, and the wiring mistakes that generate the most field callbacks.

Socket Terminal Layout and Numbering

Every DIN-rail relay socket follows a numbering convention that separates coil terminals from contact terminals. The coil lands on A1 and A2 — A1 is typically the positive or line terminal, A2 the negative or neutral, though polarity only matters for DC coils on sockets with a built-in diode. Contact terminals use a two-digit code: the first digit identifies the pole (1, 2, 3, 4 for up to four change-over sets), the second identifies the function — 1 for common, 2 for the normally-closed contact, 4 for the normally-open contact. A relay with terminals 11, 12, 14 has one pole: 11 is common, 12 is NC, 14 is NO.

The socket, not the relay, carries the current and voltage rating stamped on the terminal block. Swap a relay rated for 10 A into a socket rated for 6 A and the socket becomes the limiting component. Check both nameplates before wiring, not just the one on the plug-in relay body.

Wiring the Coil Circuit

The coil circuit is low-power and low-current, but polarity discipline still matters on DC coils. A1 takes the positive lead from the PLC output or the 24 VDC supply, A2 returns to the common or negative rail. Get this backwards on a socket with a built-in freewheel diode and the diode blocks instead of clamping — the coil's collapse voltage spike then has nowhere to go and can push the driving PLC output past its rated voltage.

Size the coil-circuit wire and any upstream fuse or PLC output channel against the coil's actual draw, not a guess. Coil power sits in the few-hundred-mW to roughly 1 W range for miniature interface relays, and the current follows directly from the coil voltage.

A 24 VDC coil drawing 400 mW pulls under 17 mA at steady state — well inside a PLC transistor output's rating, which is why one output channel can often drive the coil directly through the socket's A1/A2 terminals without an external amplifier stage. Push the coil count per output card too high, though, and the cumulative draw across several relays wired in parallel can exceed the card's total output current even though each individual coil looks trivial on its own.

For AC coils the suppression element is an RC network or a varistor instead of a diode, and polarity at A1/A2 stops mattering — either lead can land on either terminal. See our coil suppression wiring guide for how to size and orient each suppression type.

Wiring the Load Circuit Through the Contacts

The load wiring runs through COM, NC and NO on separate terminals from the coil circuit, and that physical separation is deliberate — it keeps a 250 VAC load circuit from coupling into a 24 V control loop. Land the field device (a contactor coil, a solenoid, an indicator lamp) on COM and NO for a normally-open switching function, or on COM and NC if the control logic needs to break on energize.

Match the load to the contact's actual rating class, not just its amp figure. A contact rated 6 A / 250 VAC on a resistive load (AC1) drops well below that figure on an inductive load (AC15, a contactor coil or solenoid) because of inrush current and arc energy at break. DC loads derate further still since there is no natural current zero-crossing to help extinguish the arc — a contact good for several amps at 24 VDC resistive can fall to a fraction of an amp at 110 VDC.

Route load-circuit wire on the opposite side of the wiring duct from coil-circuit and signal wire where the panel layout allows it. Some builders run both in the same duct without issue on short runs at low switching frequency, but on panels with several dozen plug-in relays switching solenoids at once, separating the two reduces induced noise on the PLC input side.

Screw vs Push-in Terminal Wiring Technique

Screw terminals need a stripped length matched to the terminal opening — too short and strands slip out under vibration, too long and bare copper is exposed outside the cage. Torque to the value printed on the socket, typically in the 0.5-0.6 Nm range for miniature sockets; over-torquing shears the screw head or deforms the terminal cage, and under-torquing leaves a connection that loosens as the panel heats and cools through duty cycles.

Push-in (spring-cage) terminals skip the torque step entirely — insert the stripped or ferruled conductor until it seats, and the spring clamps it. Solid conductors and ferruled stranded conductors push in directly; unferruled stranded wire on some push-in designs needs a release-lever press first, so check the socket's documentation before assuming a straight push works everywhere. Push-in terminals wire faster on high-channel-count panels and hold up better under vibration, which is why plug-in relay socket systems increasingly offer push-in as the default option alongside screw.

For a side-by-side of when to specify each terminal style, see our push-in vs screw relay socket comparison.

Wiring a Plug-in Relay to a PLC Output

Most 24 VDC PLC transistor outputs are either sourcing (PNP, the output switches the positive rail to the load) or sinking (NPN, the output switches the load to the negative rail), and the interface relay's coil wiring has to match. On a sourcing output, the PLC output terminal lands on A1 and A2 goes to the 0 V common rail. On a sinking output, 24 V lands on A1 directly from the supply and the PLC output terminal takes A2, pulling the coil's return path low when the output switches on.

This depends on whether the PLC card is fixed-polarity or field-configurable — some I/O modules support either mode per channel, and getting it backwards means the coil never energizes even though the PLC logic shows the output as on. Confirm the card's output type before wiring, not after troubleshooting a relay that will not pull in.

Interface relays exist specifically to sit between the PLC's low-power I/O and field-level loads — see our PLC interface relay guide for the isolation and amplification role in more detail, and our contact configuration guide for choosing 1 CO vs 2 CO vs 4 CO for a given channel count.

Common Wiring Mistakes That Cause Field Failures

Reversed freewheel diode polarity tops the list on DC-coil sockets — the relay may still pull in on the bench under a manual push-button test, then fail in the field once the actual inductive collapse spike has somewhere to go the wrong way. Second is a retaining clip left disengaged: the relay looks seated but walks partway out of the socket under panel vibration, breaking contact continuity intermittently rather than failing cleanly.

Third is plugging the wrong coil-voltage relay into a socket wired for a different voltage. Sockets with keying prevent this by physical design in some product lines; sockets without keying rely entirely on the installer reading the coil voltage stamped on the relay body before insertion. Fourth is mixed screw torque across a bank of sockets — one under-torqued terminal in a row of forty looks identical to the rest until the panel runs a thermal cycle and that one connection loosens.

What we see in the field: most callback tickets on plug-in relay panels trace back to one of these four wiring issues rather than a defective relay or socket. Checking diode orientation, clip engagement, coil-voltage match and terminal torque during commissioning catches nearly all of them before the panel ships.

Conclusion

Wiring a plug-in relay correctly comes down to four checks: coil polarity and diode orientation on DC sockets, load-circuit contact selection matched to the actual inductive or DC rating, terminal technique — torque for screw, correct insertion for push-in — and sourcing/sinking agreement between the PLC output and the coil terminals. Get those four right and the socket-and-relay pair becomes one of the easiest components in the panel to commission and replace. For the broader isolation and amplification role these parts play between control logic and field loads, see the interface and coupling relay engineering guide, and browse current stock in our interface and control relays collection.