Relays are the backbone of industrial control panels.

This guide covers how to wire up a relay from scratch: how a relay works, pin configurations for 4-pin and 5-pin relays, wiring a relay to a switch or directly to a 12V supply, reading a ladder diagram, and how safety relay circuits differ from standard relay wiring. We also cover how to wire a solid-state relay and how to wire a fan relay — two common industrial applications with their own wiring considerations.

What a relay actually does

A relay is an electrically operated switch. It uses a small control signal to switch a separate, often much larger, load circuit. The two sides are electrically isolated from each other, and that isolation is what makes relays useful.

Control circuit (coil side)

Low voltage, typically 12V or 24V DC from a PLC output, switch, or microcontroller. Energising this side pulls in the armature.

Load circuit (contact side)

The circuit being switched. Could be 120V AC, 240V AC, 48V DC, or whatever the downstream device requires. The contacts carry this voltage; the coil never sees it.

This separation is the core safety feature of relay-based control. A 5V PLC output can safely control a 240V contactor coil without the two ever touching.

Always check the relay datasheet before wiring. Coil voltage, contact current rating, and contact configuration (SPDT, DPDT, etc.) must all match your application.

Figure 1 — Relay wiring basics: control circuit, load circuit, and contact type reference

Relay pin reference

Most general-purpose relays follow a consistent pin naming convention regardless of manufacturer:

A1 / A2 — Coil terminals. A1 is positive (+), A2 is negative (−). Connect your control voltage here.

COM — Common terminal. Always in the load circuit. Start your load wiring here.

NO — Normally Open. Opens when the coil is de-energised; closes when the coil is energised. Used in most switch-on applications.

NC — Normally Closed. Closed when the coil is de-energised; opens when the coil is energised. Used in fail-safe and interlock applications.

11-pin (PLA) relay sockets use a slightly different numbering scheme; consult the socket diagram printed on the housing to understand it. For RIB relays (Functional Devices), pin layout follows the same SPDT logic but is labelled on the enclosure rather than the terminal block; how to wire a RIB relay is covered in our dedicated RIB relay guide.

4-pin vs 5-pin relays

The most common question when wiring a relay is whether you have a 4-pin or 5-pin unit, and it matters because they are wired differently.

How to wire a 4-pin relay

Pins 85 and 86 are the coil (85 = positive, 86 = negative). Pins 87 and 30 are the load contacts: 30 is COM, 87 is NO. There is no NC terminal. This is a simple on/off switch; when power to the coil is on, the 30–87 circuit closes.

How to wire a 5-pin relay

This configuration adds a fifth pin: 87a, which is the NC terminal. The 30–87a circuit is closed at rest and opens when the coil is energised. Use pin 87 for normally-open switching (load turns on with coil) and pin 87a for normally-closed switching (load turns off with coil). Both share pin 30 as the common.

Industrial panel relays use the A1/A2/COM/NO/NC convention described above, while 4-pin and 5-pin relays using the 30/85/86/87/87a numbering are more commonly found in automotive and 12V DC applications. The wiring logic is identical; only the pin labels differ.

Basic relay wiring: step by step

This covers a standard single-relay wiring for a switched load. Before starting, isolate both circuits and verify with a multimeter.

Control (coil) side

1. Connect your positive control voltage to A1.

2. Connect A2 to your switch output or MCU GPIO pin (through a transistor driver if necessary).

3. Place a 1N4007 flyback diode across A1/A2 with the cathode (stripe) to A1+. This suppresses the voltage spike when the coil de-energises.

Load (contact) side

1. Connect the live line of your load supply to COM.

2. Connect NO to one terminal of your load device for a normally-off circuit.

3. Complete the load circuit by running neutral/return from the other terminal of the device back to the supply.

TIP:  NC contacts are the right choice for fail-safe applications: a de-energised relay (power loss, blown fuse, broken wire) leaves the load active. Use NC for holding circuits or where the safe state is ‘on’.

Wiring a relay to a switch, 12V supply, fan, or solid state application

Now, let's break down common specific scenarios of wiring a relay in industrial conditions.

How to wire a relay to a switch

Connect one side of your switch to A1 (coil positive). Connect the other switch terminal to your control supply's positive terminal. A2 goes to supply a negative. When the switch closes, the coil energises, and the load contacts change state. This is the standard relay-to-switch configuration used in most manual control panels.

How to wire a 12V relay

The process is identical to the general steps above, except that your control supply is 12V DC. Connect 12V+ to A1 and 12V− (ground) to A2. The coil draws typically 150–200mA at 12V; verify this is within your switch or MCU output rating. On the load side, the contacts can switch AC or DC up to their rated current regardless of the coil voltage.

How to wire a fan relay

Fan motors are inductive loads, which means they draw higher current at startup than their running rating suggests. Use a relay rated at least 20–25% above the fan’s FLA (full load amps). Wire COM to the fan supply line and NO to the fan motor’s line terminal. The fan neutral connects directly to the supply, bypassing the relay. For electric fan relay wiring in enclosure cooling applications, ensure the relay is rated for the fan’s actual inrush current, not just its steady-state draw.

How to wire a solid state relay

A solid-state relay (SSR) has no moving parts and is wired differently from an electromechanical relay. The control side (typically terminals 3 and 4, or + and −) accepts a low-voltage DC signal — usually 3–32V DC — with no flyback diode needed. The load side (terminals 1 and 2) switches AC or DC depending on the SSR type. SSRs generate heat under load; always mount them on a heatsink and derate the current rating by 50% in enclosed panels. They switch faster and more quietly than mechanical relays, but cannot handle the same short-circuit fault currents.

Reading a relay ladder diagram

Ladder diagrams are the standard schematic language for industrial relay and PLC control circuits. They read left to right: power flows from the left rail through contacts to the coil on the right. Each horizontal line is a ‘rung’, explaining the name of the diagram.

Ladder diagram symbols at a glance

• Two vertical bars with a gap = Normally Open (NO) contact. Current passes when the coil is energised.

• Two vertical bars with a diagonal slash = Normally Closed (NC) contact. Current passes by default; and the coil breaks it.

• Circle on the right side of a rung = Output coil. Energised when the rung has continuity.

• Parallel branches = OR logic. Either path completing gives output.

• Series contacts = AND logic. All contacts in sequence must close.

  

Figure 2 — Relay ladder diagram: rung 1 (basic switch), rung 2 (latching/seal-in circuit with NC stop), rung 3 (output load)

The latching (seal-in) circuit

The most common ladder logic pattern in motor control is the latching circuit shown in rung 2 above. A momentary start button (NO) energises the coil. An auxiliary contact from that same relay is wired in parallel with the start button — once energised, the relay holds itself on. An NC stop button in series breaks the circuit when pressed.

This is why a motor keeps running after you release the start button. The K1 auxiliary contact is holding the circuit closed.

Notes on Wire Color:

 In North American industrial wiring, black = hot/line, white = neutral, green or bare = ground. Control wiring is often red. Always verify against your plant’s wiring standard before assuming.

Safety relay wiring and emergency stop circuits

A standard relay circuit stops a machine. A safety relay circuit guarantees it stops and ensures no single wiring fault can prevent it. These are categorically different design requirements.

What makes it a safety circuit

• Dual-channel inputs: the e-stop button has two independent NC contacts, wired to separate input channels on the safety relay module. Both must open on actuation.

• Cross-fault monitoring: the safety relay continuously checks that both channels agree. A wiring fault (short between channels, stuck contact) is detected and prevents reset.

• Monitored outputs (EDM): the safety relay checks that the downstream contactor actually opened before accepting a manual reset. This catches welded contacts.

• Forced manual reset: the machine cannot restart automatically after an e-stop. An operator must physically press a reset button once the hazard is cleared.

Standard safety relay connections

S11–S12 — Channel 1 input loop. E-stop NC contact 1 wired in series here.

S21–S22 — Channel 2 input loop. E-stop NC contact 2 wired in series here.

A1 / A2 — Safety relay supply voltage (typically 24V DC).

13–14, 23–24 — Safety (SF) output contacts. Wired in series with the contactor / drive enable. These open instantly on e-stop.

Y1–Y2 — Reset input and EDM (external device monitoring) feedback loop.

Common safety relay modules include the Pilz PNOZ series, Sick UE48, Schmersal SRB, and Allen-Bradley MSR. Each has slightly different terminal labelling — always reference the module’s wiring diagram.

STANDARD  Safety relay circuits for machinery guarding must be designed to the appropriate Performance Level (PL) per ISO 13849 or Safety Integrity Level (SIL) per IEC 62061. The wiring method alone doesn’t determine the PL — you also need fault response time, diagnostics coverage, and MTTFd calculations.

The reset sequence

1. E-stop is pressed — both NC contacts open, both channels drop.

2. The safety relay opens the SF output contacts instantly.

3. Operator clears the hazard and releases the e-stop (contacts return to NC).

4. The operator presses the manual reset button.

5. The safety relay verifies both channels and EDM feedback, then re-energises the SF outputs.

Common mistakes to avoid

• Skipping the flyback diode on DC coils. The inductive kickback when a relay de-energises can exceed 100V momentarily, which is enough to damage transistor outputs and PLC cards.

• Wiring NC and NO backwards. Always verify contact state with a multimeter in continuity mode before energising the circuit.

• Using a standard relay for a safety function. A standard relay has no cross-fault detection and no EDM monitoring. It cannot substitute for a certified safety relay module in a guarded machine application.

• Overloading contacts. Relay contacts are rated for resistive loads. Inductive loads (motors, solenoids) draw higher inrush current and cause arc erosion — derate accordingly or use contactors for inductive switching.

• Ignoring coil voltage tolerance. Most relay coils will pull in at 85–90% of rated voltage, but are not guaranteed to drop out above ~10–15% of rated. At the extremes of a system's voltage, relays can chatter or fail to release.

Cable and relay products from Nassau National Cable

The right cable selection matters as much as the wiring method. For control circuit wiring, NNC stocks:

• THHN / THWN-2 in 14 AWG for control panel wiring

• Belden and Southwire multiconductor control cable for multi-relay panel builds

• Shielded instrumentation tray cable for noise-sensitive control signals near VFDs and contactors

• DIN rail accessories, terminal blocks, and relay sockets for panel assembly

Questions about the right cable spec for your relay panel? Contact our technical team or browse the NNC catalog at nassaunationalcable.com.